Dr. Susanna Søberg: How to Use Cold & Heat Exposure to Improve Your Health

ANDREW HUBERMAN: Welcome to the Huberman Lab podcast, where we discuss science and science-based tools for everyday life. I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today, my guest is Dr. Susanna Søberg. Dr. Susanna Sjoberg completed her doctoral thesis work at the Center of Inflammation and Metabolism and the Center for Physical Activity Research at the University of Copenhagen in Denmark.

Her research has focused on how deliberate cold exposure and deliberate heat exposure can be used to enhance human metabolism. She is the first author of a seminal study which discovered the minimum thresholds for deliberate heat and deliberate cold exposure for increasing brown fat thermogenesis, which is essentially a mode of increasing heat production and metabolism in the body, and for establishing actionable protocols that can be used outside of the laboratory to improve metabolism and human health.

Dr. Søberg's research was published in the journal, Cell Reports Metabolism, in 2021, adding to a long and important history of research focusing on the role of cold and the role of heat in altering various aspects of the body's physiology, including hormone health, metabolism, and changes in neurotransmitters, such as dopamine and epinephrine.

In fact, today's discussion with Dr. Søberg focuses on the role of deliberate heat and deliberate cold exposure on metabolism, but it also includes discussion of the effects of cold and heat on things like neurotransmitter production, namely dopamine, and epinephrine, and norepinephrine, the so-called catecholamines, which strongly impact mood and metabolism.

In addition, Dr. Søberg answers many common questions about deliberate cold and deliberate heat exposure, including, for instance, the difference between cold showers versus cold immersion up to the neck versus total body cold immersion, including whether or not going back and forth between heat and cold changes fundamentally the way that heat and cold impact the metabolism, hormones, and neurotransmitter production.

And we talk about almost every single nuance and variation on deliberate cold and deliberate heat exposure protocols as it relates to the underlying science. In particular, how cold receptors at the level of the skin are impacted versus cold reception and perception at the level of the brain and how all of that impacts systems of the brain and body, relating to mental health, physical health, and performance.

Based on her scientific research and academic training, as well as her understanding and use of deliberate heat and deliberate cold exposure protocols, Dr. Søberg is considered one of the world's leading experts on these topics. In fact, she is the author of a recent book entitled "Winter Swimming," which is, I have to say, a terrific book because it breaks down chapter by chapter the different aspects of deliberate heat and deliberate cold into its various constituent parts, including cold acclimatization, the cold shock response, dangers and safeties of cold water, the impact of cold and impact of heat on various aspects of human health, as well as specifics relating to sauna, versus ice, versus cold swimming, showers, et cetera.

It's a very thorough read and a very easy and accessible read that if you're interested in deliberate cold or deliberate heat exposure or both, will allow you to embrace those protocols with the greatest degree of confidence that you're going to obtain the specific endpoints that you're interested in and to do so safely.

Before we begin, I'd like to emphasize that this podcast is separate from my teaching and research roles at Stanford. It is however part of my desire and effort to bring zero cost to consumer information about science and science-related tools to the general public. In keeping with that theme, I'd like to thank the sponsors of today's podcast.

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And now, for my discussion with Dr. Susanna Søberg. Dr. Susanna Søberg, welcome.

SUSANNA SØBERG: Thank you.

ANDREW HUBERMAN: So great to have you here. I feel like I should give a little bit of the backstory of how we got connected, which was that for many years, I've been interested in cold thermogenesis. It was the topic of my senior thesis in college. And I've of course followed the popularity of Wim Hof. And we've had Dr. Craig Heller, my colleague from biology department at Stanford, who works on cold and its impact on physiology and sports performance.

So, for a long time, I'd been interested in this area. But there's been a real lack of new, let's say, high profile, quality scientific information in terms of how, for instance, cold plunges and sauna, how that impacts human physiology. I know there's been some information out there, but it's been sort of scattered.

And then a little over a year ago, I see this paper in Cell Reports Medicine and was immediately struck. First of all, the fact that it was in Cell Reports Medicine. I've been on the Cell Press editorial board for a long time now. So, press journals are, of course, phenomenal journals.

And the title and the content of the paper was directly in line with the sorts of practices that people are very curious about and then are starting to emerge. Things like sauna, cold plunges, and there was your name first on the author list. And I reached out to you through social media. And we've done a little bit of live content there together.

And I've been tracking what you've been doing in the world in terms of your book, and talking about the results in your manuscript, and talking about the science and impact of deliberate cold exposure and sauna. And I have to say that it's been a wonderful and remarkable thing to see, and you're bringing so much quality information about this area that for a long time I think was kind of niche and is now becoming more and more mainstream.

So, I'm going to start off with a thank you for being here and a thank you for the work that you've done. And I'm looking forward to talking to you about it today. So, my first question, to get things started, is what is happening when we get into an uncomfortably cold environment?

So, for instance, if I'm really hot on a hot day, jumping into a cold pool feels really good. But if I'm already kind of at room temperature, I'm a little bit chilly, getting into that same temperature of water doesn't feel so good, right? There's a shock there. So, if you could just walk us through what happens when we get into uncomfortably cold water, whether or not it's by way of shower or cold plunge at the level of our physiology, and if you'd like, our psychology.

I think that's a good place for us to start because I think it will orient people to their own experience if they do that. And for those that haven't done it, it might start to peel back some of the layers as to what the underlying mechanisms of cold are.

SUSANNA SØBERG: Yeah. Thank you for that question. It's really good to just address what actually happens in our physiology when we get cold, and you can get cold in many ways. So, you can just head out for the one that gives you the most potent stressor, which is submerging into cold water. But you could also go in outside in the cold wind. That's also going to activate your sympathetic nervous system. So, get all these neurotransmitters going in your body and so your catecholamines.

Let's just address that we are taking a cold plunge, for example. So, if you are very hot, for example, before you go into the cold water. It's going to feel less stressful. But the temperature difference from your skin to the cold is definitely going to give you a shock, but your core temperature is warmer, and that's going to feel a little bit better.

So that's why when people go into a sauna, for example, and go out and into the cold water, they can do it easily, easier than if they were cold beforehand.

ANDREW HUBERMAN: Could I just ask you a few questions? So, you mentioned the sympathetic nervous system, which for people listening who aren't familiar with that, is the branch of our nervous system that's responsible for creating accelerations and heart rate, feelings of alertness, it's accompanied with stress and the stress response. But it's accompanied with waking up in the morning for that matter. So, it's not always about stress.

And then you mentioned the catecholamines, which are dopamine, epinephrine, and norepinephrine. So maybe a little bit later, we'll talk about those individual neurotransmitters. But you raise a really important point, which is something I get asked about a lot for people that are curious about using deliberate cold exposure, which is how cold should the water be.

And I know it's very hard to give a straight prescription for that because I think it boils down to what you just said, which is it's really the difference between your current temperature and really the temperature at the surface of your skin and the temperature of the water. So, if you're very warm, getting into cold feels good. If you're already cold, getting into more cold feels stressful.

Is there any way that we can start to gauge what is the best way to approach a deliberate cold exposure protocol? I mean, should it feel uncomfortable? And that leads into the question of how do we balance the discomfort with the amount of time that we spend in?

So, for instance, if it's just a little bit uncomfortable, will spending more time in the cold get us the same benefit as getting into very uncomfortably cold water for a very short period of time?

SUSANNA SØBERG: Yeah. It's a really good question. And I definitely think that this could be a future study on this as well to really unravel what kind of protocols are the best way or also for which outcomes, of course. So, if the temperature sure is very cold and you also feel very cold, then you should stay in the water a little bit longer. So, I think you should get uncomfortable cold. So as long as you get uncomfortable cold, it's cold enough, and you get this what we call the cold shock. So, the cold shock is activation of your sympathetic nervous system and these activation of the catecholamines, which you just mentioned before.

ANDREW HUBERMAN: Does the shock mean that I'm having trouble controlling my breathing? Is that a good gauge?

SUSANNA SØBERG: Yeah. You can say so because that's kind of like how we define it. So, you hyperventilate. So, you have a faster breathing rate. So that increases also because you activate your gasping reflex if you are new to this. But if you are adapted, it kind of subsides with time with adaptation.

So, what you can do is that you can train this cold exposure, and you can get adapted to it, so you don't have this hyperventilating response every time you go out in the cold water. So, this is like building up your resilience, building up your adaptation. It's going to make this shock like subside a bit. So, it's always harder in the beginning, but you should do hard things, right? It's not something that we-- you shouldn't think about cold water and cold-water immersion as something that is comfortable. It should be hard because that's the point of it, right? If you enjoy it, then yeah, then I'm thinking something is wrong, is not right. You should not enjoy it.

ANDREW HUBERMAN: Well, this is an important point that you're making because I think that many people shy away from deliberate cold exposure because it's uncomfortable in a way that at least from my experience is very different than the discomfort of exercise. Because with exercise, for instance, if running fast and breathing hard is uncomfortable, you can slow down or walk. If lifting weights is uncomfortable, you can remove some weight, or reduce the number of repetitions, or stop.

With deliberate cold exposure, I suppose you can be sort of halfway in halfway out of the water or partially underneath the cold shower, but it's very hard to titrate and adjust the level. It's kind of all or none. And I've seen-- actually, I can tell this by anecdote I've done some work with military special operations. I won't say which country. This was outside the US. And these are very tough individuals. They're used to going without sleep and doing hard, high consequence, high risk kind of work.

And they were asked to do some cold-water exposure training, and I was there that day, and it was remarkable. About a third of them just went straight in and just kind of grinded through it. They looked stoic anyway to me. There were a few whimpers, no cries. About a third talked a lot, and you could tell that they were agitated and anxious, but they made it through.

And then about a third of them just simply would not get in past their knees or thighs. It seemed like they were just dreading the whole experience. Some actually didn't actually go in completely. Which was really surprising to me in that you couldn't tell based on their physical appearance or anything else about them, they're all high performers, as to who would have this response.

So, it seems like people vary tremendously in terms of their ability to embrace the discomfort of the cold. From your studies, is that your experience as well, or are there these weird mutants who seem to just love going into the cold for the first time?

SUSANNA SØBERG: So, some people just feel better in the cold, and some people dread the cold even more. And you can say the more people are pushing the cold away, they might feel the cold pain even more. So, they would definitely-- people who are, maybe the soldiers you just talked about, some of them might be already adapted to the cold.

So, if they are not scared of the cold, they go out, and they embrace the cold in a better way. It could also be that some people have a more sensitive nervous system. And when you are a bit sensitive to the cold, you will, of course, try to get away from it, right? And you also feel the pain more if you avoid it. So, the more you avoid the cold, the more pain painful it will feel when you go into it.

ANDREW HUBERMAN: You mentioned being outside in a t-shirt, versus cold immersion up to the neck, versus shower. This is something a lot of people wonder about. What are the differences in terms of impact, short term and perhaps even long term, between cold showers, cold plunge to the neck, so that could be an ice water or just very cold water, immersion with dunking one's head and then coming up because obviously, people have to come up for air at some point, and then simply being outside on a cold day in shorts and a t-shirt or something of that sort.

SUSANNA SØBERG: So, there are different outcomes because they are very different exposures of the cold to your cold receptors in your skin. So, the more you can say you cover your body in the cold, which you would do in cold water because they're, of course, covered totally and the molecules are closer to your skin, you will have a more potent activation of all your cold receptors in the skin. So that one will definitely activate your autonomic nervous system more and rapid, compared to going out in a t-shirt in the cold wind, just go for a walk.

But that is also something that's going to activate your sympathetic nervous system. Meaning that you will have an increase in norepinephrine, and you will activate something called the brown fat. So, this is a healthy kind of fat tissue that we have in our body. And when you activate that, that's going to increase your metabolism.

ANDREW HUBERMAN: Before we talk about brown fat, and I'm so glad you brought it up because it's so much to talk about there, what about cold shower? I mean, obviously, cold shower is somewhere in between being out outside in the air cold air versus being immersed up to the neck.

SUSANNA SØBERG: If we had more studies on cold showers, we would learn more about how does that activate our metabolism, how does that increase our neurotransmitters in the brain, which could also have an impact on our mental balance. So, I think that would be interesting for the future.

But what we do know is from activating brown fat both from rodent studies but also in humans, is that as soon as we get cold on our skin, we will activate our brown fat. So, it is kind of like our first responder in the body to keep our temperature up. So, our muscles is like the second tissue in our body. We have two tissues, which can increase our thermogenesis. So, the brown fat, which is always like temperature regulating our body. And then we have the muscles, which will secondarily start to shiver, and that's going to increase our temperature in the body.

But as soon as you go into a cold shower, you will activate your brown fat also immediately. So, it could be good also for increasing metabolism in theory because we haven't really any studies showing how much does it actually activate the brown fat. So, if someone out there wants to do a study on that, that would be great.

ANDREW HUBERMAN: I've thought about why there are fewer studies of cold showers and cold immersion, and I think the answer to my mind is that from a methodological standpoint, it's just harder to do because if people are getting into cold water up to the neck, they're getting into cold water up to the neck. Whereas if people are getting into a cold shower, some people are larger or smaller some people are going to stand under the shower with it hitting their head. Some people the back of the neck. You could direct people to do it, but it's a little bit more difficult.

Also, I think for-- you and I are both research scientists. There's a little bit of a methodological challenge that might seem silly to people, but it's a real one, which is if people are in a cold shower also, the water is going to be-- I'm kind of pushing their clothing against their skin. There's a certain vulnerability and for most people coming to a laboratory in the first place, let alone being observed while they shower.

Whereas when you get into cold immersion, you're getting under the water. And some people might roll their eyes and say, OK, really? Is that the barrier? But science exists in these real-world contexts. And this will vary by culture and things of that sort. But we run human subjects in my lab. And I'll tell you just the process of getting people to the laboratory and having them park and find the lab. And it's a whole new environment with people in lab coats, and people moving around, and where's the restroom? I mean, there's a certain amount of stress just associated with taking part in a study for most human subjects.

So, I totally agree, however, we need more studies of cold showers. It's just a harder environment to control in my mind. So, it sounds like any form of cold to the skin that people register as what you call a cold shock or an uncomfortable, like this is kind of jarring, activates the brown fat. Do we know what the pathway is from cold receptors on the skin to the brown fat? I mean, how does the brown fat know that we're cold?

SUSANNA SØBERG: Yeah. Really good question. And it seems that I think that, of course, in the future, we will know much more about these pathways. But what we do know is that the cold receptors will send a signal to our temperature regulating center in the brain, so hypothalamus. And that's going to be taking in this message. And we have so many cold receptors in the skin. So, it's going to be very fast. You can say if you immerse the body into cold water, this is going to be so rapid. So, it will have a rapid increase in neurotransmitters in the brain. So, noradrenaline, adrenaline, and cortisol, which is not that much, it's but it's still there.

So, you have this increase in noradrenaline, which will then immediately activate the brown fat because you can say the activator is the most potent one, cold and noradrenaline, and that's going to activate the brown fat. But there is also a direct pathway from the cold receptors in the skin to the brown fat, which really shows that because of these different pathways, it shows that it could be that this tissue to keep us warm or was developed in our evolvement as humans to keep us warm and to save us whenever the temperature on our skin varies just a little bit to keep us in that right homeostatic balance, so we don't get hypothermic, but also so we don't get hypothermic.

But because it seems that the brown fat is also activated when we get warm on our skin. So, it's also maybe a temperature regulator in our body. But the pathway is different. I think it's also a third pathway directly from the muscles. So, the brown fat is also activated, even though the muscles are starting to shiver. So, there's an extra pathway, that way to keep our temperature up, so muscles and brown fat are working together to keep us warm, so we don't suffer too much in the cold water.

ANDREW HUBERMAN: That's super interesting. And here you're pointing to is the existence of three parallel pathways. And this notion of parallel pathways comes up over and over again in biology, as you and I know. And I think it's important for people to know about because as you said so eloquently, when something is very important to our survival and/or evolution, the brain and body install multiple mechanisms for it, not just one.

And so, it sounds like it's cold on the skin triggers a response in the hypothalamus, which then activates brown fat, cold receptors in the skin directly to the brown fat, and then shivering in the muscle to the brown fat. I want to talk about brown fat in depth and learn from you more about brown fat.

Before that however, I want to ask about shiver. I've heard that shiver causes the release of succinate, which then activates the brown fat. Is it known whether or not inducing shiver is important? And when should people shiver? I mean, I've gotten into cold plunges and shivered while I was in there. And then I've also had the experience of getting into a cold plunge or a cold shower then getting out or even standing outside on a warm day after swimming in a pool and then starting to shiver, so the shiver comes later. So how important is shiver, and does it matter when shiver happens?

SUSANNA SØBERG: Yeah. Shivering is good because that increases your metabolism, and that's going to burn some calories in your body. You shouldn't be so afraid of shivering I think because the shivering, as long as you don't get too hypothermic, so if you don't sit in the cold water for too long. And what you just said by shivering after you get up, that is because of the afterdrop. Something called the afterdrop is when your core temperature decreases even after you get out of the cold water. And it always does that, your body, because as soon as you get into the cold water, all your blood vessels is going to constrict because you need to keep your blood in your core and keep your vital organs warm. So as soon as you get up, those blood vessels will open again, and the warm blood will flow out and get colder and then flow back again into the core. And that's going to decrease the temperature in your core, of course.

ANDREW HUBERMAN: So that's the drop.

SUSANNA SØBERG: So that's the drop.

ANDREW HUBERMAN: I'm so glad you explained that. I've heard years ago, Wim Hof, I heard him talk about the drop, and I've heard colleagues of mine talk about the drop. But that's the first time I've ever heard it explained clearly. Let me make sure I understand this. So, I get into cold water. Obviously, I'm cold. Vessels constrict to keep blood near the center of my body keep me alive. I get out. The warming up of my body allows those vessels and capillaries to dilate again. The blood goes out to the surface, but the surface is still cold. And so that blood is cooled. And then my core body temperature drops. And that's what you're referring to as the drop, and that's what induces shiver.

SUSANNA SØBERG: Exactly.

ANDREW HUBERMAN: And then am I right in thinking that then the shiver activates brown fat, which then warms me up again?

SUSANNA SØBERG: Yes.

ANDREW HUBERMAN: Got it.

SUSANNA SØBERG: That's why you should end on the cold. We could get back to that.

ANDREW HUBERMAN: Yeah. Let's talk about it. Yes. Ending on cold, it's what I refer to as and what has now become known as the Sjoberg principle, which is a really important principle about the importance of ending on cold and not doing what I do, which is to get into a hot shower or back in the sauna, but we'll get back to that in a few minutes.

So that's wonderful that you can explain that so clearly because I think that shiver is something that a lot of people do avoid. People think, oh, I don't want the chattering of the teeth. And it feels like a loss of bodily control, which really, it is. It's an autonomic response.

SUSANNA SØBERG: Yeah. But I don't think that people should avoid it that much. It's just like seeing shivering as a way of your body, like it's training for all your cells. It's training for your muscles. It's training of your metabolism. And that's going to increase your what's called the insulin sensitivity.

So, if you can like in your mind get used to the thought of shivering is just like when you go exercising in the training center and get that feeling of like, oh, this is tough. Now it hurts a little bit. Yeah. It's going to hurt because that's what shivering also does. But it's just a different way of training yourself and your body. It's going to create what is healthy stress. It's called hormesis in the cells. And the more you expose your muscle cells or your brown fat cells to these kind of healthy stresses, exercise, cold, heat exposure, it's going to make them better at like activating and also keeping you healthy.

So as long as the cells get exposed to this, it's going to keep them on its toes you can say because it becomes more robust and increasing these heat shock proteins and cold shock proteins in the cells to make you more robust for the next time. And that is also what happens when you go to the training center. And I keep like drawing that parallel because people today know more about, we know more about exercise and what that is going to do to your muscle cells.

But the same kind of training is also what you do when you go out and into the cold water and submerge into cold water because that is just your cold training center you can say that and also your heat training center going into the sauna because the cells are getting stronger with hormetic stress. So, it's the same process, just different practices.

ANDREW HUBERMAN: I'm so glad that you brought up the fact that the discomfort or the embarrassment or both of shiver is still crucial actually to reach for and try and experience the same way that with exercise-- I think a lot of people don't realize this, but when we did our series with Dr. Andy Galpin, it became clear to me what should have already been clear to me and I think that most people don't realize, which is that if we were to measure heart rate, blood pressure, stress hormones, and inflammation in a human being during exercise, it would look as if they were ready to die.

Blood pressure would be high. Inflammation is through the roof. But all of that is setting in motion an adaptation or a set of adaptations that allow blood pressure to be lower at rest, that allow inflammation markers to be lower at rest. All the things that everybody is seeking with exercise, in addition to, of course, the aesthetic changes that people are seeking with exercise.

It sounds like the exact same things are happening with the cold. So, the redundant message here seems to be that the more discomfort, provided it's done safely, just like with exercise, the more shivering, the more cold shock provided, it's not to the extreme and stops somebody's heart. We can talk about thresholds for that a little bit later. It sounds like all of that is going to set in motion some long-term changes that will make people feel better and will improve health.

Could you just touch on a few of the longer-term changes that are known to occur? I mean, I'm well aware of the study showing that, I think it was the European Journal of Physiology showing long lasting increases in catecholamines, dopamine, norepinephrine, and epinephrine, for many hours after deliberate cold exposure. What are some of the other things that happen at the level of metabolism and brown fat in let's say the hours and day after deliberate cold exposure?

SUSANNA SØBERG: As soon as you go in, of course, there's an activation. But it seems like you're asking for the later outcomes, like blood pressure and stuff like that. Is that what you mean?

ANDREW HUBERMAN: Yeah, blood pressure but also in terms of metabolism. I know that in your study, and we'll talk about brown fat in depth here in a moment, but that there were changes to the brown fat that equate to changes in, for instance, people's ability to be comfortable in colder environments when they're not doing deliberate cold exposure. Or in the same way that I can exercise on an exercise bike or go for a hard run. But then if I go hiking with the family on Sunday and it's a steep climb, I can do that steep climb more easily because I'm, quote unquote, fit as a consequence of the exercise. What are some of the fitness adaptations of deliberate cold exposure?

SUSANNA SØBERG: Yeah. So, what happens is that you get adapted a little bit every time you go. So, you will like exercise, get a little bit stronger. So, every time you go into the cold water, for every time you will be more exposed to it, you will feel more comfortable in the cold. So, you're going to build your adaptation, which happens on a metabolic level, which is going to be the brown fat. So, you will have more activation of your brown fat. The mitochondria in the brown fat cells, you'll have more of those, and they will be more efficient at heating you up because it expects the body, expects you to do this again, so you are prepared in a way.

The capillaries in your skin will also become better at constricting. So, you will have a better shield of your body to prepare you for the next time. So, you will become better at going into the cold water in that way. So, the body makes these mechanisms and changes your body in a way, so you can expose yourself to the next time, right?

And also, your stress response will also subside a bit, so you will have less increase of your catecholamines with time. With time also, because of this activation of your brown fat or your muscles, you will have an increase in your metabolism, which will then make your insulin sensitivity better. And this is shown in studies. For example, there is this interesting study I found just before I started my, which was from Gibas-Dorna et al from 2016, where they measured metabolism not on brown fat, but they measured insulin sensitivity in middle aged men and women during one winter swimming season.

So, they were not very young like they were in my study, but they were middle aged. And I think this is very interesting. So, during these four, five months they were winter swimming, they saw that they had a lower blood pressure after the season, and they had a lower heart rate. And they also saw that they have a better insulin sensitivity.

And I think that is very interesting because if you can have a better insulin sensitivity, you can prevent lifestyle diseases and will lower blood pressure, which is a very strong outcome also for telling how much inflammation you have in the body. And because they didn't measure brown fat, I figured that it could be that was the missing link. That was one of the explanations to why we see this less inflammation in the body.

So, the long-term outcomes could be that you lower your blood pressure and have a lower heart rate, you also have a better insulin sensitivity and better glucose balance, but that is shown in my study.

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We'll get back to the insulin sensitivity and glucose balance. That's an impressive list of benefits. Blood pressure. Of course, most people are aware of, blood pressure and what it is. It's what they measure when we go to the doctor. And it's not very sexy nowadays. Blood pressure, people are, oh, blood pressure. People want to hear about the inflammatome and the microbiome, and all of that stuff is really interesting. But I think that blood pressure doesn't get enough attention.

And we have spoken to, on this podcast, to Dr. Peter Attia, who is an expert in longevity, and healthspan, and things of that sort. And I was surprised to learn again, I shouldn't have been surprised, that the number one reason people die worldwide is cerebral vascular disease and cardiovascular disease. And there are basically three things on the list of things to address. One is not smoking or vaping, by the way. There are a few other things related to blood markers, ApoB and things of that sort.

But then the big one is blood pressure. And so, it's interesting because we don't think about blood pressure that much anymore as the kind of people interested in health optimization and health. But blood pressure is so vital to control. So, it's wonderful to hear that deliberate cold exposure is one way to control blood pressure, I'm guessing in concert with other forms of exercise.

Let's talk about brown fat. And if you're willing, I'd love to drill into brown fat at a deep level. Again, my understanding of this is far more elementary than yours obviously. You're the expert. My understanding about Brown fat is that it's located in specific areas of our body, maybe more widespread than when I learned in school. I was taught it was just at the clavicles in the back of the neck and upper back, but who knows?

I learned that there's more of it when we're children, maybe more distributed throughout our body, and that it's rich in mitochondria. But what is so special about the brown fat? Like if we could just go into the biology of brown fat a little bit, what does it look like? You've measured it in human subjects. Where is it distributed really? Can it expand its distribution? Can we activate and expand the amount of brown fat as adults?

And for those of you that are cringing already thinking we're talking about getting fatter, it's quite the opposite. We're talking about not subcutaneous fat, but fat located around the organs. But please educate me, tell me where I'm wrong, and expand my knowledge on brown fat.

SUSANNA SØBERG: OK. Yeah. You're not wrong. But it's true that there are more locations of the brown fat than we previously thought. There's this very nice study from 2017 by Leitner et Al, where they had made these opacities overlays of their subjects, where they could see where in the body do we have brown fat and where can we grow more brown fat so to say.

So, the brown fat is very plastic. So, it means that it can grow, and it can decrease. And this is proven in studies where we have seen people with pheochryocytoma, is like a very specific cancer type where from the '70s where we can see that if they have this specific kind of cancer type, they have this tumor on their adrenal gland. So, they have a huge increase in noradrenaline.

And because of that, they have this continuous activation of the brown fat. And they have grown a lot of brown fat in whole body, abdomen or where it's located in these six different places, but it is less like very much compared to normal people. And what they then see, what we learned from this study is that brown fat can apparently grow if you have an increase in noradrenaline in the body. It's not like you want that because when that happens, you have a high blood pressure. You don't want it chronically, right? You just want it on like short amount of time. And then it can grow for a bit.

But you don't want it chronically, of course not. But because it activates also your sympathetic nervous system, so they also showed they have high blood pressure. They lost a lot of weight, of course, because this is activating your metabolism. So, they found luckily that when they removed this benign tumor, that the brown fat decreases again to normal size. And they gained weight again, and they had normal blood pressure. So, the story ends well.

But it's kind of like proof of concept of the brown fat can actually grow. So, it's plastic in its way of like it can grow, and it can decrease again. So that's very good. Good studies to see what the body is capable of. But we don't of course want all that brown fat. We just want to keep it actually and keep it activated because what we see in studies is also that after the age of 40, studies have shown that there is an association with having less brown fat but increase obesity.

So of course, we don't know yet whether brown fat decreases with age and therefore we get obese, or we get obese and therefore, we have less brown fat. But as Brown fat is an insulin-sensitive organ in our body and we get obese, just like the muscles get less insulin sensitive, the brown fat does as well. And therefore, it maybe decreases. It could be a theory that I think could be one of the reasons why we don't see that much Brown fat in elderly people. Some have a lot, especially people working outside. There are studies showing this.

ANDREW HUBERMAN: People who--

SUSANNA SØBERG: Work outside.

ANDREW HUBERMAN: Do physical work outside. Farmers.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: Interesting.

SUSANNA SØBERG: They expose themselves to it. So, they will just keep it in that way.

ANDREW HUBERMAN: And I suppose we should clarify for people in case they don't know that insulin sensitivity is a very good thing. You want that you want your cells to be sensitive to insulin. Insulin insensitivity is type 2 diabetes and is associated with obesity. So just a point of clarification there.

Yeah. It's interesting to me. I usually work out at home, but I go to a gym once or twice a week if I can because it's good if I see the outside world. And there are a few individuals at the gym who, they're not particularly large or muscular, but they are incredibly lean, and their posture is great, presumably from the musculoskeletal work. And they're in their 70s and 80s. I mean, it's remarkable.

And I know all the telltale signs of hormone augmentation. I'm very good at spotting that. There are a few telltale signs. I've talked about this on other podcast. And that's not why they're fit. They're clearly of that look, and you see this outside the gym too, of course. For people that look like they've done a lot of physical labor their whole life, they're just moving a lot. They have strong hands and features. And they're not necessarily excessively lean, but you can tell that they've been using their musculoskeletal system.

And I like to talk to these people and ask them like not what are you doing now for your workout, but what did you grow up doing? And I would say, and obviously, I haven't run statistics on this. But more than 75% of them respond that they grew up on a farm, or that they did some sort of manual labor, or a postman, or a postwoman, or doing something where they moved a lot for their early years and throughout middle age. And most of them are now in retirement, but some of them are still working, and they all still moving a lot.

So, the relationship between shiver and brown fat makes sense to me. But is it the case that as we're just moving around, and I've heard of NEAT, non-exercise induced thermogenesis. So, if we're just moving around that we are activating brown fat or does there need to be this stressor? Does there need to be shiver and a cold stimulus or a heat stimulus to activate the brown fat? In other words, is just staying active enough, or do we need to do some sort of temperature, shock type thing like deliberate cold exposure?

SUSANNA SØBERG: Yeah. I think that is a really good question because how-- also, why do we have this tissue then? If it has to be extreme, then you can question what do we need this tissue for? But it seems that you can activate the brown fat with just a little bit of exposure to cold. So cold is the most potent stressor activator of our brown fat because it's our temperature regulating organ in our body. So, first responder to that. So, the muscles will be a little bit too late, and therefore, we have maybe these two kind of tissues.

So actually, just exposing yourself or a hand actually just to cold water. So, studies have shown that if you just put your hand in cold water, not that you're going to do that all day, or every day, or anything. It's not something you have to do but. It just shows that you can activate your brown fat just by getting a temperature change on your skin.

So, you can go outside in a t-shirt. That's why also, we were just talking about, well, people who work outside, or move a lot, or get in and out of, like changing the temperature of their body all the time, they will have more brown fat. And activating that is going to keep your metabolism higher and your insulin sensitivity. Studies have also shown this.

So, the Brown fat can be activated as soon as you just change your temperature in the skin. So going outside in a t-shirt, wearing cooling vests, also studies have shown this. For 10 days, it's going to also grow your brown fat. So, you can get more brown fat if you expose yourself to the cold. You don't have to start in a cold shower. You don't have to start in a cold plunge if you're not really ready for that yet. But just exposing yourself to the cold wind has also shown to activate your brown fat.

Or if you don't want to be like in this awake state, then you can also just sleep in the cold. And you won't notice it that much maybe. But studies have shown that if you sleep in 19 degrees Celsius. Then you will activate your brown fat, and you will grow your brown fat, so you have more of it. So, this very nice studies from Hansen et al from 2017 showed that a group of subjects who slept in a room at 24 degrees. And then they made this PET CT scanning to see how much brown fat do they have from the beginning, so what we call baseline. Then they measured again after a month of sleeping in 19 degrees. And they saw, I think it's remarkable, just one month at 19 degrees sleeping there, they had a duvet on. And they still had clothes on when they're sleeping.

ANDREW HUBERMAN: So, they're under a cover, under a duvet.

SUSANNA SØBERG: Under a duvet. Yeah. The subjects were sleeping at 19 degrees for one month had increased its insulin sensitivity. The next month, they slept at 24 degrees. They measured this again, and they had decreased actually a little bit. And then they slept at the 27 degrees. So quite warm room actually for the fourth month. And they saw even less activation of the brown fat and also insulin sensitivity.

So, it seems that you can expose yourself, and pretty rapidly, the brown fat will respond to this because it's so sensitive to noradrenaline. So, if you keep exposing yourself to a little bit of cold, you will also get a little bit adapted to it, but that's because the brown fat has grown these more mitochondria in the cells. So, these small energy fabrics that's going to activate the cells, and that's going to take up glucose and fat from the fatty acids, from the bloodstream, to keep the thermogenesis up.

And that's going to clear up some sugar, and it's going to clear up so in the bloodstream and some fat as well. So, the brown fat can in that way decrease our unhealthy fat, which is the white fat. And the white fat is what we don't want too much of, but we still need some, of course. And it's our energy storage, so it's very important that it's there. We just don't need a lot of it. So, on our thighs and also around our inner organs, that's where it's located.

So, if we can have activation of the brown fat just by going out in the cold and just by sleeping in a cold room. Or if you have courage for it, you can go out and expose yourself in a cold plunge. Cold showers is also going to do the trick. So, you can do different variations of this. Just exposing yourself to various temperatures is going to activate the brown fat because it was involved to keep us in a perfect homeostatic balance regarding temperature. So, to keep us alive.

ANDREW HUBERMAN: Incredible. I just get a clarification around this 19 degrees Celsius room that they're sleeping in. So, they're under a comforter, a duvet. And you mentioned they have clothes on. The room is 19 degrees Celsius, but the temperature underneath their blanket might not be 19 degrees Celsius. So presumably, it's the cold on their face that's activating the increase in brown fat that was observed. Is that a reasonable expectation?

SUSANNA SØBERG: I think so, yeah, because you have so many receptors in your face. Actually, it's enough. And I think it corresponds very well with the studies showing that you can activate the brown fat just by putting a hand into a bucket of cold water. And I did this experiment myself in my studies just to see how well did they respond to cold water. So, it was 4 degrees Celsius cold water for 4 minutes. And then I just measured blood pressure and heart rate to see, do they have an activation of this?

I actually also measured the brown fat during this cold exposure for 4 minutes with an infrared thermography camera to see, can I see that the brown fat is activated? And just to go back to the location of the brown fat. So usually, you cannot really see activation of your brown fat because it's located centrally around your central nervous system.

And the biggest depot, as you mentioned before, is up here under the clavicular bones and very close to the skin surface. And because it's so close to the skin surface, I could measure it with this very expensive camera. And it's not very feasible for people to go home and do this don't because it takes a lot of practice, I can tell.

But we measured the brown fat with this. And I could see that after a few minutes that the activation was there, an increase in temperature arose from that activation, just 4 minutes. So, it's very rapid and I'm also measured in my study how deep was the Brown fat under your skin. So, it's very close to the surface, which also shows that it needs to be there to heat you up and heat your inner organs.

ANDREW HUBERMAN: Well, I'm delighted to hear all of this, and I'll tell you why. One is by way of anecdote. I mentioned a little bit earlier that as an undergraduate, I worked in a lab that studied thermogenesis. And we were doing that in animals. But we had this room that was very cold. The whole room was cold.

The guy who I worked for at the time, Harry Carlisle was a very accomplished physiologist. He came from this lineage. I don't know if this literature is still discussed much, but it's a beautiful literature from Rothwell and Stock. They were the ones who discovered non-exercise induced thermogenesis. The fact that people who bounce their legs a lot, and move around a lot, and have a lot of stochastic movement burn up to 1,800 calories more per day than people who sit more still.

SUSANNA SØBERG: Fascinating.

ANDREW HUBERMAN: Incredible. Just incredible. I don't think that work gets as much attention as it deserves. Published in journals like Nature, so very fine journals. But in any event, one of the things that I noticed when I started working in that laboratory was that I was cold because the room was cold.

And Dr. Carlisle, Harry, said, well, the key is to wear a t-shirt in here for about two or three days, and then you will cold-adapt. I thought, well, wouldn't I want to put on a hoodie and get warm in there, so I was comfortable? And he said, no. Actually, what you want to do is get yourself uncomfortably cold, activate your brown fat.

And indeed, when I did that, I think it was just two days of being in that cold environment. Then I could come back on the third day and be perfectly comfortable because the brown fat had expanded, or added mitochondria, or both. And I was perfectly comfortable in that environment. I also got very, very lean in those days and weeks.

Now I've never been somebody who's very lean, nor am I somebody who carries a lot of excess adipose tissue. I'm kind of somewhere in the middle. I'm sure I could adjust that with feeding if I want to. But it was striking what a powerful effect it had on my entire system of thermal regulation. And one of the things that I also delighted in when Cell Reports Medicine published your study is they had an accompanying press release that went out to those of us that receive press releases.

And it described a saying in Scandinavia which is essentially, I'm not going to attempt to speak Danish, even though I have much of my family is in Denmark, believe it or not, from Denmark. We have a lot of Danes in my family. I won't embarrass myself by trying to speak Danish as I did before the microphones were rolling.

That there's a saying that I think essentially translates to in the fall, when you're approaching winter, you want to actually wear fewer layers, not bundle up, when you go outside, so that you can prepare yourself for the cold of winter and be able to heat yourself up using your brown fat. And that in the spring, as the temperatures are warming, rather than removing layers, you want to wear more layers in order to be a little bit uncomfortably warm, so that in the heat of the summer, you're better at cooling your body. Do I have that right? And maybe do you know the saying, and would you be willing to share it? Only the Swedes and Danes will be able to understand. Maybe the Norwegians too. If you don't know it, that's OK.

SUSANNA SØBERG: Yeah. So, I know the concept of it because we say it. You should wear a less before winter and more before summer.

ANDREW HUBERMAN: So, there it is in English. So, it doesn't have to be esoteric.

SUSANNA SØBERG: Yeah. And you're completely right. And I think this is just something that we know in the Scandinavian countries, I think that we intuitively know this. But if we just go back a little bit in history, I think that around the 1950s, the Russian government went out and said, well, we should do something about tuberculosis pandemic or epidemic there was at this time. So, they wanted to have the people be more resilient to the cold and also increase our immune system.

So, in Scandinavia, and actually also in Russia, we put our babies outside to sleep in the prom. And that is like to also to get more resistant to the cold but also to increase our immune system. And we still do that in Denmark.

ANDREW HUBERMAN: Do you really?

SUSANNA SØBERG: Yeah, we do.

ANDREW HUBERMAN: Babies are taken out in the cold?

SUSANNA SØBERG: In the snow, in frosty rain, everything. My two boys have been sleeping out in winter, or at least their first many three, four, five years because it's very good for them, and they get a better immune system and get resilient to the cold, so they will have less colds. And also, they run around in a t-shirt when it's super cold because they have activated all their brown fat. I didn't understand at that time, I must say. But I kind of intuitively also knew because we have inherited this way of doing things with our culture.

And I have heard people coming from the US saying Danes are crazy. They put their babies outside in proms and leave them there. And then they go inside and drink coffee on the cafes.

ANDREW HUBERMAN: Well, I don't think Danes are crazy. I adore the Danes, their amazing culture and people. I'm so fortunate to have family members from Denmark. But I did notice. So, when we were in Copenhagen, and I know we saw you there not long ago. That was June. The water in the harbor was cold, even though the Pacific is close to here, which is very cold. I felt pretty cold. But it was summertime-ish. So, people were in summertime mode. T-shirts, and shorts, and things of that sort.

But it did strike me that people in Copenhagen are dramatically fitter than they are in the United States. I mean, first of all, everyone's bicycling everywhere. Not many people wearing sunglasses. So, trying to extract as much photon energy from the sun as possible, which I support. As everyone knows, I'm a big fan of getting sun.

But also, when we did see swimmers, they were swimming in this cold water like it was nothing. And the range in age of the swimmers was remarkable. You saw the kind of fit, triathlete-looking types but also young kids, like really young kids. And then people probably in their again their 70s, 80s, maybe even 90s. Really remarkable. Vastly different than what you she have to go to the ocean here in Los Angeles or elsewhere. So yeah, you Scandinavians are on to something with this.

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I'd like to talk about your study. If you could give us a little bit of the backdrop about what motivated that study. And then walk us through what you did. Who the subjects were, what you had them do, what you measured, in as much detail as you would like to share because I think it's such an important, even fair to say, landmark study because it also explored not just cold but sauna and the co-use of cold and sauna as a way to probe metabolism and brown fat and other markers as well.

And as you do this, I'm hoping at some point that you might tell us some of the observations that you might have made that interested you that perhaps were not in the paper because that's one of the great benefits of sitting across from somebody who did the work in detail. So yeah, if you could tell us about your study, and what you did, and what you discovered.

SUSANNA SØBERG: Thank you for that question, Andrew. I'd love to also explain a little bit what did we do because when people read this kind of paper, they just see the numbers. They don't see what happened before that. And human studies are very different from my study. My studies, you can do a knockout of something, and then everything is like perfectly matched and controlled. Doing human studies is very different from that because people are different, even in the groups. So yeah.

When I started this research in 2016, I did not really know what the brown fat was. So, I started reading up on all this. And I was very interested in preventive medicine. And also, the studies that I did before, brown fat was also very much in the preventive side, like how can we-- that was about something else, but the sweet tooth, and how can we lower our sweet tooth, and stuff like that. But after that I, wanted to do something new.

So, I looked into the brown fat, got hired in this fantastic research group, where it's the cell group. So, they mostly did cell studies, and they didn't have anyone to do a human study yet. But they really wanted me to do that. So, I read up on a lot of research about how does the brown fat get activated? What have been done already? And I mentioned the paper before with sleeping in the cold. I found that particular paper very fascinating.

And that was also where at that time, I was like, OK. So cold exposure as an intervention of sleeping in the cold could be a good thing to go out and say, well, people do this. But on the other hand, first of all, it was already done. That was one thing. But the other thing was like, well, I wanted to see if we can do some kind of activity. So, we can have people move also, go and do something. Do something together or whatever.

And the cold made us think about, well, what about winter swimming? It was kind of like a bit of a joke in the beginning. I was like, winter swimming. Yeah, it's going to activate the brown fat, right? But when we read the literature, we couldn't really find anything about activation of the brown fat with cold water besides hand in a bucket of cold water. That was already there.

So, we were just thinking, OK. So, it should be very potent activation of the brown fat if it's cold water but very different from cold air. So, it was kind of also a new thing we were going into. And we knew that we were going to do more of a proof-of-concept study at the beginning of it because it was like winter swimmers must in theory activate the brown fat, right? But we kind of didn't really was this kind of stressor too much, too little, or what will happen actually?

But we had this idea about, well, we always say that cold water winter swimming will activate your metabolism. But do we know if it does that? No, we don't. And while this idea was a little bit fun at the beginning, we kind of accepted. It was like, OK, let's just try this out. But because we didn't have the funding for it, we was like, OK, let's do a proof-of-concept study. Let's go with a small number but enough to see a difference between the groups. So, the power calculation of that study is done on what we know from PET CT scannings of the brown fat. So that's the main outcome, of course.

And we wanted to go a little bit smaller on the numbers of participants because we wanted to dig a little bit deeper into the different mechanisms and also redo some of the days. So, I really wanted to do that to see if I can replicate also the findings. And that's going to take a lot of funding, but it's also going to take a lot of time to do it.

So, the proof of concept was just going small but looking at different mechanisms. We also took fat biopsies, for example, and looked at the white fat to see if there was any differences between the groups before and after and stuff like that. So that's kind of like how it started. And the first year was like a field study for me. So, I was not a winter swimmer when I started this.

ANDREW HUBERMAN: Oh, really?

SUSANNA SØBERG: No. I wasn't at all. And I will say I was a bit afraid of the cold myself. A bit of a cold sissy. Always cold, having big socks on and sweaters and stuff like that. So, I was like, I am so comfortable. I'm just like everybody else. Very comfortable. I like being completely temperature neutral.

But I started like playing with this thought, like, well, if this is so healthy, in theory, I should not pack myself up. I should start not doing that. But the first year, observation of winter swimmers on the jetty. They kind of joked about it and say, come on, Susanna. You need to try this. You cannot study this unless you have tried it. And I was like ha, ha. Very funny. Of course, I can do that. But I couldn't.

I read the literature. I understood in theory what happens when you go into cold water. But I completely understood it when I first tried it. The first few times, not so funny. It felt painful. It was just like running too long after a long break, and your muscles hurt the day after, right? You completely regret that you took that extra mile.

ANDREW HUBERMAN: What about, when you say uncomfortable, you mean uncomfortable when you got in and when you were in, or uncomfortable afterwards? Because I find that on rare occasions-- I should just, full disclosure, I do deliberate cold exposure every morning for about a minute to 2 minutes in a cold plunge. There are days that I miss. But when I'm at home, I do that. And when I travel, I do a cold shower. I do finish with a warm shower. And we'll talk about why that's probably not the best idea.

And I've been doing it for some years now on and off. So just full disclosure, I'm a devotee, and I have family members that hate the cold but have gotten into it and are starting to like it. And I don't necessarily like the experience in the cold water, but I love the way I feel when I get out.

And I'm 100% on that statement about loving it when I get out. Occasionally, it feels good to be in there. It feels invigorating. And I think I've learned to control the gasp reflex and the hyperventilation, and I just have told myself what we know, which is that the forebrain struggles to engage for the first 20 or 30 seconds. But if you can get past that wall, it's far easier to push through. But when you say that it was really uncomfortable, do you mean the experience of getting in, or you also felt lousy afterward?

SUSANNA SØBERG: Yeah. And very important to clear that out. I only felt very uncomfortable doing it at the moment. But afterwards, the first time I went with a group, and actually, my husband was there as well because I really wanted someone I knew coming along. Because it's very normal if you haven't done this before, you feel a little bit anxious about it. This is shown in studies as well because blood pressure and heart rate goes up in those who are new to this kind of activity.

So, I was a little bit anxious about it. So, it was really uncomfortable just doing it. But afterwards, as soon as I got up, I felt fantastic. And we went into the sauna. And I did three rounds because I just loved it. I loved the feeling afterwards because you have all these neurotransmitters going in your brain, and you feel more positive. I feel invigorated. I had so much energy. And like I could totally see why people would do this to get energy throughout the day because I definitely had that. I didn't have to do three dips to get that. I think one would be enough.

And I often do that also now today. I do one dip. Sometimes, I do two or three dips in one round, you can say in one day. But often, it's just one or two times a week. For me, that is enough to get that energy and to get that positive feeling. And I think that is also why I put up my study in that way. I wanted to study the lowest dose, you can say, the lowest amount that we can get away with but still see health benefits.

So, what I observed there on the jetty was that some did it a long time. They were in the water for a very long time. And to me it seemed maybe a bit extreme.

ANDREW HUBERMAN: Could you give me an example of a long time?

SUSANNA SØBERG: Well, so maybe they were really swimming, and they could be 20 minutes or half an hour.

ANDREW HUBERMAN: That's a long time.

SUSANNA SØBERG: That's a long time. And there was like ice. And people who came up-- I mean, I just didn't really feel that this is something that I wanted to go out and recommend to people after my Ph.D.--

ANDREW HUBERMAN: You don't want any research subjects dying either because if you're not adapted. I mean, people can do that. Also, a 20-minute cold shower or 20-minute cold plunge, I know people do it, but it's probably not a good idea.

SUSANNA SØBERG: No. Probably not. It's going to exhaust your cells and make them age too fast. So exactly. When you pass that hormetic stress, the healthy stress level, that's what is happening. The quite opposite is almost chronic stress actually in the cells.

Well, what happened then was that I found out that if I want to have this protocol get through ethical committee, I really needed to go very sleek with the not too long and make sure that they were also very healthy and to get approval, of course, of this study.

But what I did was to recruit winter swimmers who already had been swimming for two or three seasons. And I just observed them. I said I'm not going to do an intervention study yet. I did that after. But I wanted to do like a proof of concept where they were already adapted to the cold and then compare them to a matched control group who were matched on, you can say diet. So were they vegetarian or not. And one of them was in each group.

ANDREW HUBERMAN: They weren't all vegetarians?

SUSANNA SØBERG: No, no, no. Just one in each group. Yeah.

ANDREW HUBERMAN: I'm going to say, with all the amazing fish and meat in Denmark, I'd have a hard time being a vegetarian. The breads are amazing, the fruits and vegetables too. OK. So, there were a couple of vegetarians in each group.

SUSANNA SØBERG: Yeah. One in each. Yeah. OK.

ANDREW HUBERMAN: Token vegetarian. I have family members who are vegetarian. So, I'm just poking fun.

SUSANNA SØBERG: Yeah. But they were matched on different things. So, what we usually match them on is also BMI. And we chose one gender in this study. And we would always choose both men and women normally. But we do see that there are different brown fat levels depending on gender. So, women have more brown fat than men.

ANDREW HUBERMAN: Really? Interesting.

SUSANNA SØBERG: Yeah. I think it's interesting.

ANDREW HUBERMAN: That deserves a study.

SUSANNA SØBERG: Yeah. Why, actually? I think it's interesting because women are also smaller in size and mass, right? But they also have a lower peripheral temperature, especially on hands and ears.

ANDREW HUBERMAN: Is that right? That's documented that women do run colder than men?

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: Physiologically. I didn't say psychologically.

SUSANNA SØBERG: No, no. No.

ANDREW HUBERMAN: We won't go to the psychological cold-heat.

SUSANNA SØBERG: That's different. Something else.

ANDREW HUBERMAN: That's a different podcast.

SUSANNA SØBERG: Yeah. Another time. So, women are just colder physically. So, on hands and ears, it's measured on that. And feet as well. So compared to men. And men have bigger hearts than women. And they can pump out more blood peripheral than in a woman's body. So that could be an explanation for the colder hands, for example. Thermocomfortable state is also different between genders. So, men are more comfortable at 22 degrees Celsius, and women are thermocomfortable at 24 degrees Celsius.

ANDREW HUBERMAN: So, the thermostat wars of home have been now validated.

SUSANNA SØBERG: Yes.

ANDREW HUBERMAN: 2 degrees Celsius. By the way, prior to starting recording, I made the executive decision that we were going to go with Celsius throughout the podcast because the majority of the world uses Celsius. So, for those of you that think in Fahrenheit, the internet is your friend in making those conversions. So, we're sticking with Celsius. So, men tend to be thermocomfortable at 22 degrees Celsius, women at 24. OK, interesting.

SUSANNA SØBERG: Explains a lot about also some arguments in the homes where men are turning down the heater, and women are turning up the heater, and they cannot really-- I'm on both sides here. I understand the men. We understand the women, but there is a difference there. Which was also one of the reasons why we in this proof-of-concept study chose one gender. So, it is not like only because we wanted to study men. It was just to eliminate all the confounding factors which could have an impact on our results. So that was one of the reasons.

But also, because yeah, so women have more brown fat than men. And otherwise, we would have to do four groups or something like that. And not having funding yet, we were like, OK, we need to do like just one group, just a control group, and a group who were always the winter swimmers.

So recruited winter swimmers who have been swimming for two to three seasons because I wanted them to be already adapted. But not going too long in the water. So, they told me, I did a lot of screening here, of course, beforehand and interviews to ask them how much do you do and how much do you how long do you stay in the water. And I monitored how long did they didn't stay in the water and recruited based on that they only did like two to three times per week. It seems reasonable for Denmark at least to do that.

And they stayed only in the water for 1 to 2 minutes. So, the cold shock subsides very quickly, and you will get this activation of your rest and digest system, which is your parasympathetic nervous system. So, the other branch of your autonomous nervous system. And you get that activation because you submerge into cold water. And when you do that, you have an activation of your diving response. And that's going to slow down you can say the consumption of oxygen also in your body. And that's going to slow down your heart rate.

ANDREW HUBERMAN: Could I pause you on this because I've heard this before that when we get into cold water, shower, or immersion, we get this sympathetic autonomic response. So increased blood pressure, increased heart rate, release of norepinephrine from the locus coeruleus in the brain, release of adrenaline, dopamine, adrenaline from the adrenals, dopamine presumably within the brain.

But that the parasympathetic response is activated when we put our face into cold water or go underwater. And that's a calming relaxation response. So, this brings us back to, I don't want to take us off track from you describing the study, but this brings us back to the first question, which is if I go completely underwater for a moment when I start my cold plunge, does that change the physiological outcome as compared to if I just submerge myself up to the neck?

And actually, nowadays, there seems to be a little bit of a movement online of people putting a bowl of ice water on their countertop and submerging their face into it. Did you see this? As a start, I've seen more and more posts about this. So, could you just touch on what the dive reflex is, and why it perhaps activates the parasympathetic response, this calming response?

SUSANNA SØBERG: Well, so the diving reflex is activated when you submerge into cold water.

ANDREW HUBERMAN: Even just to the neck?

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: Or I thought you had to get your face under. I'm not arguing different. You're the expert.

SUSANNA SØBERG: I haven't read that. I just seen that you can activate your diving response as soon as you go underwater with your body. So, you don't have to do it with your face, as far as I understand. I could be wrong. Yeah. So, when you activate your diving response, you will slow down your oxygen consumption in your body. And that is because the body tries to preserve oxygen, so you will not get hypothermic too fast. So, it's kind of like a survival system in your body.

So, this survival system is very important for us, of course. So that would be activated. And because of that, you will have to, maybe 1 minute or so. I can't be precise on that because maybe it also varies a bit in humans. So, 1 to 2 minutes, you will have full activation of the sympathetic nervous system but also the parasympathetic nervous system.

And that's going to activate, for example, something like serotonin in your brain, which is also good for mental balance and people feeling in mental balance afterwards after they go up. So that is like measured on a questionnaire and also measured like on anecdotes. Of course, people tell all the time that they feel good afterwards. We need studies on this. So, if anyone's sitting out there thinking that's interesting, then please do some studies on that to get more out on that.

ANDREW HUBERMAN: So, you observed these winter swimmers who have done this for a few seasons.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: They're coming around for a new season of winter swimming, and you've decided to recruit them as subjects. They are getting into cold water climbing down a ladder or jumping into the water up to their neck.

SUSANNA SØBERG: Yeah. Climbing. Yeah.

ANDREW HUBERMAN: OK. Climbing down a ladder because this is done outdoors. What a fun study to do. My graduate thesis was done under fluorescent lights with no windows in a building that-- I mean, I had a ton of fun as a Ph.D. student. I actually lived in the laboratory as a Ph.D. student. I loved it so much. But not something required to do a Ph.D., by the way. But they're climbing down the ladder, getting in up to their necks, staying in for 1 to 2 minutes, and then getting out. And how many times a week are they doing this?

SUSANNA SØBERG: So, they do this two to three times per week. And for each time they go, each day they go, they take three rounds, so three dips and two sauna sessions. So, they start in the cold, and they end in the cold water.

ANDREW HUBERMAN: OK. So, it's get in for 1 to 2 minutes. Then get out and get into the sauna.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: What is the temperature of the sauna?

SUSANNA SØBERG: About 80 degrees Celsius.

ANDREW HUBERMAN: OK. Then how long are they in the sauna?

SUSANNA SØBERG: So, they stayed there for 10 to 15 minutes. So, depending on if they went two times per week or three times per week.

ANDREW HUBERMAN: OK. And then they get back into the cold for a few minutes? 2 minutes.

SUSANNA SØBERG: Up to 2 minutes. Yeah.

ANDREW HUBERMAN: OK. Then back into the sauna 15 minutes or so.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: Then back into the cold for a third round.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: Back into the sauna. And then they're--

SUSANNA SØBERG: Ending on the cold.

ANDREW HUBERMAN: And then back into the cold again and then ending on cold. And we will talk about why it's important to end on cold, the so-called Sober principle. How cold was the water in this particular, given the average, because I realize it's outdoor winter swimming, so it's going to vary depending on wind chill and things as well.

SUSANNA SØBERG: Of course. So, it's a very uncontrolled environment to do this kind of study. But I wanted to do something that was also very close to something people could do for free, going out in nature and use that, and also have the nature. It's very healthy impact on us. It lowers our stress level as well. So, by doing so, I also measured the temperature every time they went.

So, I have this graph. And it's actually in the "Winter Swimming" book. It shows the temperature in Denmark going like from October to April. And it's like that's at 12 degrees. I think it's around 12 degrees Celsius, the water. And then it goes down to 2 degrees on average in January and then up again. So, it's within the spectrum of very cold water, I would say, from around 15 degrees Celsius and down. But it was actually not colder than like two to 4 degrees on average when it was the coldest. So, it doesn't have to be that cold to be good enough and enough to activate our metabolism.

ANDREW HUBERMAN: And what time of day are the participants doing this cold sauna alternation?

SUSANNA SØBERG: So, I think they did this throughout the day. So, I didn't control whether they wanted to go in the morning, in the afternoon, or in the evening. At that time where I set up this study, I was not controlling it in that way. I wanted them to go whenever they had time. And I also think that is the most important message to give to people, is do it when you have time.

If doing it when you get home from work, and it's 6:00 in the evening, and this is the time where you can do it, then try out if it's going to impact your sleep or not. If it doesn't impact your sleep, then fine. But you have to try for yourself and find out what works for you. It's the same for coffee, for example, right? Some people can drink coffee in the evening and go to bed, and they can sleep. I can't.

ANDREW HUBERMAN: Or exercise.

SUSANNA SØBERG: Or exercise. Exactly. So, I can't do that. And that's because the coffee, exercise, cold water immersion is going to activate your sympathetic nervous system. You have an increase in stress response in your body. And that's going to make it really hard to fall asleep for some people at least. Maybe you are super exhausted anyways, and then you will just crash anyways. But yeah. But that's the only thing. So, I just told them to do this if they can during the daytime. And that's primarily what they also did.

ANDREW HUBERMAN: And then all along, you're measuring brown fat by way of this infrared camera, right? So, what did you observe in terms of changes in brown fat? How quickly did that occur? And then I'd like to ask also about sauna a bit more because earlier, you mentioned that you can activate brown fat with sauna as well, with heat on the surface of the skin.

How long did it take before you observed significant increases in brown fat, and was it increased density of brown fat or distribution? Was it showing expansion to different regions throughout the body? And maybe you could also touch on some of the changes in insulin sensitivity and metabolism.

SUSANNA SØBERG: Yeah. Very good question. And I didn't mention this before. But besides measuring temperature as an outcome for brown fat activity, we also did PET MRI scanning of the brown fat. So, this is like the golden standard for measuring brown fat. And it's not very feasible for normal people to get a CT or PET MRI scanning of the brown fat. It's super expensive.

So, we had both to see if we could have a continuous measure of brown fat in humans because that was already not out there. So, I wanted to see during both the experimental days but also during day and night what kind of like circadian rhythm do we have in our brown fat activity. So that's why I wanted to have that as well.

So, the PET CT scanning or the PET MRI scanning was to see upon cold activation stimulation for some hours. Do we have activation? Can we see the brown fat in this subject. And also, during thermoneutrality or thermocomfortable state, how is that activated in each of the group, of course?

ANDREW HUBERMAN: So, you want to see how comfortable people were away from the cold water and sauna, just at different temperature environments. Is that right?

SUSANNA SØBERG: Yeah. So, I also measured that how comfortable are you. I made this scale, like visual analog scale, and asked them, how comfortable do you feel with this temperature? And throughout the study days, during cold exposure and thermocomfortable day. I had a whole day where I just kept them thermocomfortable to see do they activate the brown fat if they are just completely thermocomfortable as good as we could get with that because we were asking people on a scale from 1 to 10, and 5 being thermocomfortable, where are you on this scale? So, 1 would be very cold, and 10 would be super burning hot. Yeah. And so that was a way to try to figure out how do they actually feel also during this study. I also measured in electromyography, so of muscles, to see, do they shiver during the cooling day? Sometimes, people shiver before they know they're really shivering.

ANDREW HUBERMAN: Oh, interesting.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: So, our conscious perception of shivering might not be the best readout of shiver.

SUSANNA SØBERG: Yeah. Well, if you also get adapted to the cold water, you will have less shivering. There will be less vigorous. There will be very small. So, you wouldn't probably know that you are shivering because the shivering is so small, and the mitochondria in the muscle cells will be so dense that it doesn't need to shiver maybe that much to get that thermogenesis going, compared to when you're completely new to cold water exposure, and you're not adapted. Then the body needs to create these mitochondria, these energy fabrics to keep you warm. And that's also what the exercise is in the beginning.

But when we measured this, we did see that the winter swimmers were shivering less or having less vigorous shivering when they said I'm cold. So even though their perception of the cold was pretty similar in the groups, we could see that the activation of the muscles that we measured on were different and more vigorous in the control group.

ANDREW HUBERMAN: Were the subjects incentivized to be in the study? Were they paid or anything of that sort? Do they just happen to like doing cold and sauna, and so that's why they did the study?

SUSANNA SØBERG: Well, they got paid a little bit for it, but not much. And that's how we do this study--

ANDREW HUBERMAN: I was just curious. I was just curious. There might be some folks that wonder. So, what did you discover in terms of changes in brown fat insulin resistance, or insulin sensitivity rather, and metabolism?

SUSANNA SØBERG: So, what we saw was we had this kind of different measures to see what to try and unravel what's actually going on when they are already adapted to the cold water, compared to a control group who was matched on various parameters.

We did see that the winter swimmers had an increased insulin sensitivity. They produced less insulin on all the experimental days. So besides from just cooling them and measuring the brown fat on each of these cooling days. There were two cooling days and one thermocomfortable day. So, I wanted to measure insulin when they were fasting, meaning that they hadn't eaten in 8 hours before the study day.

And they were completely lying still, not moving, just in a bed. And we measured insulin during the experimental day just to see what level are they on. And we could see that the winter swimmer had lower production of insulin. And they also, when they had a glucose drink, so we give them that to test before we enroll them in studies to see if they have diabetes, for example, and not knowing, for example. That would ruin maybe the study. So, we test for that and see if they have a normal curve. So, what we did see in that the winter swimmers had a faster glucose clearance in the bloodstream. So, after two hours, we could see that they had a lower level, and the curve went down faster than in the control group.

ANDREW HUBERMAN: So, despite having lower insulin release, they have better blood glucose clearance, which is really what we all seek. Excessive insulin is bad. Insulin being a more or less a chaperone for blood glucose can do all sorts of other things as well, of course. And having high blood glucose, obviously terrible for cells, especially brain cells. I don't think people realize how toxic high blood glucose is, having high glucose is.

If you want to kill neurons, you put them in an environment where there's too much sugar. Oh, yeah. Very neurotoxic. And there are mechanisms like insulin that buffer that, keeping blood glucose in reasonable range, so that doesn't happen. I mean, I think that's why people will go into insulinemic shock. Hypoglycemic shock is also possible. So that range in which neurons are happy is not a tremendously large range.

Incidentally, the range in which neurons are happy and surviving is much greater as one gets colder than when you heat up. I mean, you can basically destroy brain cells by getting too hot for too long.

SUSANNA SØBERG: Oh, yeah.

ANDREW HUBERMAN: You can definitely destroy brain cells permanently by getting too cold for too long, but you have to get really, really cold for a really long time.

SUSANNA SØBERG: Yeah. Yeah. Very interesting.

ANDREW HUBERMAN: Yeah. We're thinking about doing an episode on sort of survival of the brain after death kind of things, which actually happens. You hear about these people who are declared dead and then come back. And there's actually now a lot of cryopreservation-type approaches for that. Anyway, we risk going into the esoteric now. So I'll steer us back to our discussion about your study.

So, if I do the math, these subjects are in the cold. Let's say they're doing three rounds of cold for 1 to 2 minutes, two or three times a week, what were the thresholds that you discovered were important for getting these positive changes, such as reduced blood sugar or clearance of blood sugar being more efficient, reduced insulin, improved brown fat distribution and density. How much cold exposure do people need, how much heat exposure do people need in order to extract these benefits?

SUSANNA SØBERG: Yeah. So, when we then calculated the numbers together, we could see that this ended up being 11 minutes in total per week. So not in one session, of course. But they had two to three visits to the water and the sauna per week. So, when we divide that out, it corresponds to being in cold water 1 to 2 minutes at a time. But also, in the sauna 10 to 15 minutes at a time.

And I think this is very like also similar to what we see in other studies when we look, for example, to the observational studies from the Finnish cohort study from Laukkanen et al, for example. They published this very amazing paper in 2015. Some results from this long cohort study where they show that up to 30 minutes in the sauna was healthy, and you lower your risk of cardiovascular disease. And that's like the threshold. And if you go further than that, then there is not more healthy benefits to gain from that.

And before that, it's like 90 minutes, then you will have this dose response relationship up to 90 minutes that's really decreasing your risk of cardiovascular diseases.

ANDREW HUBERMAN: That's per week. 90 minutes per week.

SUSANNA SØBERG: 90 minutes per session now.

ANDREW HUBERMAN: Per session.

SUSANNA SØBERG: Yeah. Per session. If we then compare that with my study, which was 10 to 15 minutes per session, then I think it fits very well with what we call the hermetic stress or healthy stress that you expose the cells to this kind of like potent, very stressful situation, where they increase heat shock proteins in the cells. And that will repair the cells. But if you then overdo it and you go beyond the maybe 30 minutes in the sauna, this observational study from Finland with more than up to 2,000 sauna bathers where they followed these for 20 years, they see that 30 minutes per session is like enough. And if you go above that, you don't get more health benefits out of it. So, I think there's a window where we can say the healthy stress corresponds to 10 minutes.

ANDREW HUBERMAN: Per session.

SUSANNA SØBERG: Per session. And it's not much actually. So, it shows that you don't have to expose yourself very much to the heat or very much actually to the cold to get this healthy benefits from going into cold, going to heat, and have healthy benefits on your cardiovascular system.

So, I think this is very important also message to get out that you don't have to go extreme. You don't have to swim for half an hour in the cold water. You can go in the water for 1 to 2 minutes per session but go up to 11 minutes per week in total. And for the sauna, my study showed 57 minutes in total per week. And if we also then divide it out on these two to three days and two sessions each day, corresponds to 10 to 15 minutes.

So, it's a low threshold, but I think it's good to have that too. Maybe we can aim for that if people need to have something to aim for. And I think it's really good to have that because then you don't overdo it. And if you overdo it, you exhaust the cells. And that will increase your risk of cardiovascular disease also.

ANDREW HUBERMAN: I get a lot of questions about this, and I did solicit for questions for this podcast on Twitter. And one of the questions that I got was as one becomes more cold adapted, do the benefits start to wear off? Or can people do too much cold exposure? Of course, the answer to that is, yes, you can become hypothermic.

But I'm sensing a different answer now, which is if I understand correctly, the threshold is 11 minutes total per week of deliberate cold exposure divided into two or three sessions of maybe 1 to 3 minutes, depending on how long somebody stays in. And then 57 minutes. I want to be careful not to round up to an hour but divide it into maybe three 20-minute sessions or so. So, one doesn't have to be perfect as long as you get beyond that threshold.

But I wonder something, which is it the case that if somebody said, oh, you know I'm just going to do one 11-minute session per week. That might actually not be as beneficial as dividing it up because what you told us earlier is that the hermetic response depends on having that cold shock. You actually don't want to become too cold-adapted. I mean, once the blood pressure response drops down, so in minute 4, 5, and 6, you're getting very cold, and you're shivering. But one is not getting the autonomic stimulus that they want.

I guess I could liken this to if exercise worked in a way where it was only the first few minutes of exercise that really triggered the adaptation. Of course, this is not how it works. But in fact, probably quite the opposite. But if that were the case, then it's not simply the total amount of exercise but dividing up the sessions into little bouts where every single time it acts as a stimulus. That seems to be the key here.

This is very important because having watched the landscape of this on social media but also in books and generally-- I think you're the first person to really touch on this, that the goal is not to get so cold-adapted that you can sit in for the full 11 minutes in one session, where the goal isn't to be able to do an hour of very hot sauna. If you want to, I suppose people could do it for other reasons. But if the goal is to improve these health metrics, then the idea is to keep the stimulus a stimulus.

SUSANNA SØBERG: Short. Exactly. Yeah.

ANDREW HUBERMAN: Great. Well, this also, I think there's practical feasibility as you pointed out because getting into a cold shower, or cold immersion, or natural body of water for a couple of minutes is far less challenging to most people than finding a full morning to go spend there. But I've never really heard it articulated that the longer sessions might not be beneficial and might actually be detrimental.

That's very important. Were there any other observations that you made that did not make it into the paper or that were kind of in the margin notes in terms of psychological benefits or anything of that sort? There was this recent study on soldiers that talked about weight loss as sort of a controversial study for a lot of reasons.

But one of the things they remarked in the paper was that there were a lot of psychological changes. Improved buffering against anxiety. The men and women in that study reported one of the significant effects was significantly improved sexual satisfaction. They didn't tell us what that meant for these subjects. So, we won't go there. But a number of subjective improvements. Was there anything that you observed or took note of in your study that perhaps didn't make the main abstract but that we should be aware of?

SUSANNA SØBERG: Yeah. There were some. And today, I regret that I didn't measure on sleep, for example. I frankly didn't really think about that when I designed the study. So, we were very much occupied with the metabolism and kind of had the thought maybe this could impact sleep quality.

And I wish I had the thought that why don't you just ask them in a questionnaire? But I ask them every morning-- or every morning, it was not many mornings. Just two mornings actually we measured on. But the winter swimmers told us before I wrote them that they had a really good sleep quality. The control group also had that. But they told me on the day where we measured the brown fat on a day and a night. So, for actually two days and two nights.

They told me that they had a good night's sleep, but they also woke up. So, it's just telling me that they also had a quick wake-up, and then they fell asleep again. And the winter swimmers, they have a really good sleep. So, it's like in general, they also say we sleep very well. I sleep very well. So, it's anecdotally general corresponds to what I heard in my study. But nothing that I measured on, which could be fun to do in the future, but we didn't measure on sleep quality. That would have been a really good idea to do.

They also told me that they were very comfortable when they were cold. They don't mind, winter swimmers, they don't mind going out, for example, in the cold with a t-shirt. They were also less scared of showing their skin. That was also one observation.

ANDREW HUBERMAN: Interesting. So, kind of a reduced social anxiety.

SUSANNA SØBERG: Yeah. They were just so comfortable in the lab. As you just mentioned before, coats on, and everybody's running around. It was very busy. And all the other scientists out in the hallway. And also, my supervisor had her office down the hallway. And one of the winter swimmers one day just got out of bed after had been in the study for 8 hours. It was a long day, right? He jumped out of the bed and had his clothes in the bathroom, and he went out completely naked. He didn't care. He just went out. It was like, oh.

ANDREW HUBERMAN: So that's a side effect perhaps of getting too comfortable with the cold. We're not recommending that. Although in your book, you dedicated some-- let me start that again. Although in your book, you dedicated some pages to naked winter swimming, or I should say naked cold-water exposure, as opposed to with bathing suit. Are there any data on this? I'm sorry, chuckling. But I think in most places in the United States, skinny dipping is not legal most public beaches. There are few.

In fact, my laboratory before moving to Stanford was in San Diego at the Salk Institute for Biological Studies. Beautiful building. Incredible science is done there. The beach right below that is called Black's Beach. And it's a known nude beach. And so, whenever tourists were heading down the stairway there, I would sort of let them know, especially if they had kids, I'd let them know.

And it's a nude beach of a particular genre. So, I'd give them a little Warning about what they could expect down below. In any event, those beaches are quite rare in the United States maybe compared to Europe, I don't know. So, is there anything special about clothed versus clothed exposure?

SUSANNA SØBERG: Yeah. I think in that sense, we are a bit more free with this kind of like-- but remember, we also had this winter swimming culture for hundreds of years in Denmark. And the oldest winter swimming clubs that we have, especially the one we have in Copenhagen where did my next study, which we haven't talked about, and it's also not published yet. But in that winter swimming club, it's the oldest one we have, and it's huge. And they swim naked at this facility.

ANDREW HUBERMAN: Men and women.

SUSANNA SØBERG: Men and women. And they have sauna where they can go in together. And they also have separate saunas. But it's very much a Danish thing. And I think I think it's good if people want that. And I had it in my book because people want to know if they have to swim with their bathing suit on, or if they can take it off, or what's the difference. Is there any difference in this? And if you ask me, there is no difference.

If you have your little skinny bikini on, it's not going to do any difference to your cold exposure or your adaptation. It's not going to do any difference for your benefits, of course. But I think that it has something else. It has something to do with how you also observe yourself, how you observe your surroundings. And there's some sense of freedom in skinny dipping.

So, I think people in Denmark who does this is they do the winter swimming because they feel free when they do it. They come home from work. They go to this club, and they skinny dip, and they feel like in touch with nature. And they have maybe done this their whole life. So, this is an old tradition in Denmark in some of the clubs. But the newer clubs that are coming, they don't skinny dip. So, everyone has bathing suits. I never skinny dip because there are people around. People with phones and taking pictures all the time.

ANDREW HUBERMAN: Different nowadays. Everything's recorded.

SUSANNA SØBERG: Yeah. And also, this old tradition is also fading away because of that.

ANDREW HUBERMAN: Yeah. I use sauna and cold at home. But when I travel there, there's a banya, so Russian banya, has hot sauna and cold plunge. There's one in San Francisco called Archimedes Banya. And that one is clothing optional. So, some people are clothed, such as myself. And then other people are not. And it's co-ed most of the time. I think they have female separated evenings or something like that.

And then the other banya is Spa 88, which is on Wall Street in New York, is an amazing banya as well. And these are starting to crop up in different cities. Or maybe they've been there for a long time, and as deliberate cold exposure and sauna gets more popular, more people are using them. The one in New York, that Spa 88 is always closed.

And it's interesting because people hear naked or skinny dipping, and they might get certain ideas in mind. All these places are very well lit, and they all have a tone of health. It's about the kind of health and wellness. I guess the point being that there's no requirement to do one thing or the other. Although in the studies that you did obviously, people were clothed. But I did I did pay attention to those pages in your book. I thought it was interesting that you put some dedicated passages in your book related to this.

SUSANNA SØBERG: My publisher wanted that.

ANDREW HUBERMAN: Oh, your publisher wanted that. Interesting.

SUSANNA SØBERG: Yeah. It was not me. It was like my publisher really wanted to have a little discussion about that. So, I was like, OK.

ANDREW HUBERMAN: Well, I think it points to a larger theme, which is I think for a lot of people who already do these practices, there's no shock there. For people that do not do deliberate cold exposure or sauna, I think that there is this idea perhaps that, oh, you know, these traditions that are kind of fringe or that they're--

And I just I want to cue that point because there are so many things that are happening right now in biomedical research and medicine. Serious, quality, peer-reviewed studies published in excellent journals like your paper on things like deliberate cold exposure, sauna, the use of particular supplements. Natural herbs and supplements. I mean, there's an entire branch of the National Institutes of Health in the United States dedicated just to the study of supplements and behavioral interventions for health, like meditation and breathwork. Really incredible. It's really incredible.

And psychedelics, of course, being something that for a long time was part of a certain community and feel and now is being frankly adopted by mainstream medicine, even pharma. So, the times are changing. And so yes. I think it's important to know that it's perfectly acceptable and encouraged to wear clothing.

SUSANNA SØBERG: Absolutely absolutely. Yeah. And one other thing that I wanted to mention going back to your questions around if there were any observations in the studies which we really maybe haven't discussed yet, and maybe it's in the back of the paper and not mentioned that much was one of the winter swimmers didn't have any brown fat when we measured him.

ANDREW HUBERMAN: Zero.

SUSANNA SØBERG: Zero. And we do see this in previous studies as well that some humans don't have any brown fat.

ANDREW HUBERMAN: Did he carry a lot of white fat adipose tissue? Was he obese?

SUSANNA SØBERG: No. He wasn't. No. He was not obese because he would not have been in the study then.

ANDREW HUBERMAN: Oh, right. Yes. You mentioned this earlier. Forgive me.

SUSANNA SØBERG: No, no. But what I did observe before I knew that he didn't have any brown fat was that during the cooling experiment where I cooled him for 2 hours before they go into the PET CT scanner, he was not able to control his shivering like the winter swimmers could. So, he got he got very cold very easily compared to the others.

And I didn't know what was different about him. But all or me and the three others were working on the experiment, we were like, OK, what's going on because we turned down the temperature. But he started like shivering and then we had to turn it up again. And it was just all over the place. The temperature is not, it wasn't that controlled like the others. It was pretty similar protocol. I could just do pretty much the same because they're the same size and also same gender.

So, it was easier to foresee what was going to happen and when will they start shivering. I quickly learned that. But with this subject, with this volunteer, it was just very much different. And then when we scanned him and didn't find any brown fat, I didn't even think about it. So, when we scanned him and we didn't see anything, I told the PET CT people, like, oh, you put up the wrong scanning.

ANDREW HUBERMAN: Right. Blame the technology.

SUSANNA SØBERG: Yeah. The technology. It was like this scanning look like the thermoneutral day, the thermocomfortable day where we also scanned them to see if they have any brown fat. So, you have made a mistake I was pretty sure. And the reanalysis analyzed this scanning, and they just concluded, well, the scanning was fine. The experiment went well. It was just that he didn't have any brown fat.

So, he was like what we just in the paper called a brown fat negative. So, he didn't have any. And in my studies, it would be called a knockout. So, he didn't have any brown fat. So, what the observation with him and I think that's interesting is that he both shivered very early on and didn't regulate his temperature as well. He also told me that then he was like a 5 on the scale of how comfortable he felt with the cold.

ANDREW HUBERMAN: Out of?

SUSANNA SØBERG: So, it was from 1 to 10, and 5 being thermocomfortable, and 10 being very cold, and 1 very hot. So, on this like scale up and down. And he was like more up and down on this scale than any of the others. It was an observation that I did. But we did see in his blood samples also that his blood samples looked a bit more like the control group. And also, his insulin levels were like the control group. So, a little bit higher than the other winter swimmers. And his blood glucose clearance was not as fast as the other winter swimmers.

So, he was like an outlier what we call it. And in the analysis, we also had to take him out of the analysis because he was an outlier. So, the results showing that the brown fat is more efficiently activated in the winter swimmers is without having him in that group. But it didn't ruin the study if I tried to put him in as well, and it didn't ruin the results or anything. But just to keep it more clear, we took him out of the analysis.

ANDREW HUBERMAN: Yeah. So, he was a mutant, a knockout. Yeah. And I'm sure they're out there. Very interesting. So, if you shiver early, then perhaps you have less Brown fat to begin with, although it's hard to conclude from one person. That's sort of the implication there.

SUSANNA SØBERG: Or you haven't adapted to the cold. So, you should build that up. Yeah.

ANDREW HUBERMAN: So, in addition to looking at regulation of blood sugar, brown fat, metabolism, and so on, were there any markers that you examined in the deliberate cold exposure group as compared to controls that revealed to you that deliberate, cold exposure could have additional benefits, say, for immune system function or for any function for that matter?

SUSANNA SØBERG: Yeah. So, we looked at inflammation. Of course, we measure the outcome of blood pressure and so on. But we also measured the IL-6 in this study, just to see also inflammatory and inflammatory markers. So, IL-6 went up, and it also follows with the IL-10. So that is like also very known in the literature.

So, we measured that and I think it's very important to think about the cold exposure and the heat exposure as something that then lowers the inflammation in the body. And if we can do that, we will have an open door for preventing lifestyle diseases, right? So, for type 2 diabetes but actually also for some mental diseases as well. So as known as depression and anxiety and also Alzheimer's disease, which are all associated in research, also newer research showing that inflammation increases the risk of depression, anxiety, and Alzheimer's disease, neurological diseases.

So, if we can decrease inflammation in the body, we will decrease our modern lifestyle diseases, but also these increasing mental diseases that we see in these modern lifestyle times. So, I think it's very interesting that we can go out in nature, and we can use these natural stressors. And I don't want to have it sound very romantic or anything. It's just exposure to temperature actually, just to cold or to heat, that is going to trick our body into a natural state again and reset it where the homeostasis, the balance is lost a bit.

So, the body is going to repair itself in that way. And I think it's beautiful that we can do that just by changing the temperature of our body. And although people are very scared of doing this because in our times, we have been away from cold, away from heat temperature for decades now since we isolated our houses better, and we are more sedentary. We also sit more indoor. We don't move as much. So, this very modern sedentary lifestyle has made us more temperature comfortable or just neutral. So, no wonder, I mean, that obesity is increasing.

We don't expose ourselves to the natural stresses that we did earlier on in our environment but up until maybe the 70s the 60s, where we started having more like comfortable lifestyles, right? And obesity increases in the 80s. We can see that from statistics. So, I think that if we can take in cold and heat, and you mentioned other things also before. But of course, exercise is very important here. And also, a bit of fasting actually because it all increases the hermetic stress in the body. So, it doesn't have to be other than natural stresses to the body, which then could keep us in that natural balance again.

ANDREW HUBERMAN: Could we talk about what I refer to as the Søberg principle, which is to end on cold? And the reason I called it the Søberg principle is because in reviewing-- Oh, by the way. I wasn't a official reviewer of your paper. But I mean in reading and reviewing your paper for its after published contents, I noticed that you had people end on cold. And this has been a long-standing debate in the deliberate cold exposure community. Should you warm up with a warm shower afterwards or get back in the sauna? What should you end on? End on cold or end on heat?

And the Søberg principle says end on cold, as I understand it, in order to force your body to heat itself back up and thereby increase metabolism further still. Is that right?

SUSANNA SØBERG: Yes. So, when you end on the cold, you force your body to heat up by itself. And that will require that you keep your brown fat activated and also your muscles, which is a good thing. It's a good collaboration to keep your thermogenesis up. And that's like an exercise, even when you go home. So, in that way, you don't have to think about your cold exposure or dipping in your plunge or open sea or what it is as just an exercise that you do for 1 to 2 minutes, and then it's over.

If you end on the cold, you have an exercise for your body going on for hours afterwards. And that's not only on your metabolism, but it's also going to keep your neurotransmitters activated as well and increase that because your body is still cold. So, you need those neurotransmitters to activate the brown fat as well. So that's going to make your brown fat cells more efficient and also your muscle cells more efficient. So increasing mitochondria in the cells, which will then generate heat very fast.

So, if you have done this for a few times, so maybe three, four, or five times, being new to this but have tried it a few times, you will notice a switch where you like feel that you get easily warmer, and you can keep yourself warmer. And that is also what was shown in my study, is that the winter swimmers were physically warmer on the skin compared to the control group.

ANDREW HUBERMAN: When they are out of the cold.

SUSANNA SØBERG: When they are out of the cold. Just relaxing. And we tested this in on the days where they were sleeping in the lab. So, we could see that they had a more activation of the brown fat, higher temperature. So probably because they also lose heat, they have a higher heat loss to the body compared to the control group because they have a more vascular skin because of the contrast of cold and heat. So, they lose heat faster from the body during the day but is that a bad thing. No. Probably not because that's going to keep your brown fat and your muscles a little bit activated. So, it has to work to keep you warm.

ANDREW HUBERMAN: And I would hypothesize that it also might lead to some of the subjectively reported improvements in sleep because in order to fall asleep, you need your core body temperature to drop by about 1 to 3 degrees. So, it's not just sufficient to be sleeping in a cold room and under the blanket. You also need your body temperature to drop.

SUSANNA SØBERG: Yes.

ANDREW HUBERMAN: And so, what you're saying, if I understand correctly, is that by ending on cold and forcing oneself to heat up naturally, that increases the brown fat stores, which I sort of see as kind of like the oil in the candle of the furnace that is thermogenesis. And that in turn leads to increased heat loss, which people might think, oh, I don't want to lose heat from the body. But there are times when you want to lose heat from the body.

Basically, it sounds like what we want is to be a very efficient heating and cooling system. That it's not about being cold or being hot. It's really about keeping the system tuned well. Keeping the oil in the candle, this brown fat, functioning.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: What could I ask one question about fed or fasted? Are there any known benefits of doing deliberate cold exposure and/or sauna fasted, versus after a meal, say within the last hour or something of that sort? I do my deliberate cold exposure first thing in the morning. So, in general, I'm fasted because I don't eat until a little bit later in the day. But what's known about that, and was that looked at in your study? I know you measured glucose, but that was as a separate test away from the cold.

SUSANNA SØBERG: Away from the cold. Yeah. But I also tested glucose on the days on the cold. So, we measured that as well on the cooling days. Specifically, on fasting and fat, I don't know. I don't think that I have seen studies specifically on this.

ANDREW HUBERMAN: OK. More science needed. A number of people ask about the use of deliberate, cold exposure to offset some of the symptoms of various diseases. Now here we're not talking about curing disease. We're talking about offsetting symptoms. One question I've seen quite often is whether or not people with Raynaud's syndrome. This is a syndrome-- my high school girlfriend had this syndrome, and I'll never forget we were at a school dance together. And this was when we first started dating. And she had Raynaud's, which leads to very poor blood flow to the extremities. And she was very cold.

So, she left to go to the bathroom and warm up her hands in the warm water. And I was left standing there at the dance. And people came up to me and asked you why I was there and who I was there with. And I kept telling them who I was with. And they didn't believe me because they couldn't believe that she would be with me. It made total sense if you knew me at the time. I was way out of my league with her at the time. I like to think eventually, I caught up.

But in any case, she was in the bathroom for about an hour. So, at one point, I did consider the possibility that she had just left. But indeed, she hadn't. She warmed her hands back up. But people with Raynaud's suffer from this thing of very, very cold extremities. Their fingertips will even turn blue as if they were starting to get frostbitten. It's quite dramatic.

And that question gets asked whether or not there's any use of cold to try and increase the elasticity, the plasticity of the small capillaries and vessels. But everything you've described up until now, it seems like that would be a logical thing to do.

And in addition to that, whether or not people with autoimmune, conditions people with any other types of conditions are known to benefit from deliberate cold exposure. I'm not aware of any studies, but I get asked about this a lot, and there were a lot of questions about this for you in the Twitter feed.

SUSANNA SØBERG: Yeah. Thank you for those questions, and I get them as well on social media. And I have to say that I haven't seen any studies directed on this outcome in measuring Raynaud's syndrome. I do know that it's not that rare actually a problem. And I know that many women or more women than men suffer from this.

But logically, it would help them if they exposed their hands to cold and also heat to make the vessel more vascular. And I have heard from people saying that it had helped them but also hurt for some others saying it didn't help them. So, studies are needed on this specific topic, I think. I hurt my hands when I go into the cold, and I don't have this syndrome at all. But I keep my hands above the water.

ANDREW HUBERMAN: You do?

SUSANNA SØBERG: Yeah. And I do that. Often, I take a little bit of a swim. And then of course, I have to have my hands in the water. But it helps me when I then get back to the jetty and then take my hands up because then I can stand there for a little bit and get my 1 to 2 minutes exposure. And then I can go up because then otherwise, that would stop me from being in the water enough time as long as I would like to.

So, if people suffer from having this pain in the fingers, and it can be very intense. So just take the hands up a bit from the water. And that's going to help you. Also boots, neoprene boots. It's going to help on the feet. Some people feel the pain in the feet and in the ankles. And that's going to help them also a little bit.

ANDREW HUBERMAN: OK. So, there is no problem with keeping hands out or feet in neoprene booties if people feel the need to do that. If pain of the hands or feet is a barrier for people doing deliberate cold exposure, then it would be to keep hands out or to keep your feet in booties.

SUSANNA SØBERG: Yeah. Because you do get the exposure. But of course, hands and feet are very potent places in your body to get a fast activation of your nervous system, of course. But you can also just dip them and then take them off. It's still going to activate that. But your full body is covered in the cold receptors. You have a full activation anyways.

ANDREW HUBERMAN: You are providing very reassuring information to people because I know a number of people that do not like to put their hands in. I find that the more of my body I get in, the more comfortable I am. So, I don't know if it's psychologically and/or physiologically. I find that where there's an interface between the water and the cold, it's most uncomfortable. So, I prefer to just get everything under. I keep my head out.

Although these days, I've been dunking all the way in and then coming out and then dunking once more with my head under before I get out after the plunge. That raises a different question. Now we're getting into the practicalities of deliberate cold exposure, which I think are important. Sometimes, I'll experience, and I hear from a lot of people that they'll get a kind of back of the head headache at the interface of the water when they're in doing cold immersion to the neck.

I assume this has to do with blood flow, that there's vasoconstriction right up until the neck and in the region surrounding it, but that maybe there's still blood flow to the head. But do we know what the origin of these headaches is? And again, this doesn't happen for everybody, but some people do experience them.

SUSANNA SØBERG: OK. Yeah. I haven't really heard about that one specifically. But I would say that there are different reasons for maybe keeping your head out of the water, but it seems like maybe for some, that could be a reason for like just getting like a quick head dunk to--

ANDREW HUBERMAN: Going all the way in once.

SUSANNA SØBERG: Yeah.

ANDREW HUBERMAN: That's what I've started doing to eliminate-- I wasn't getting headaches, but I noticed that interface. And I wasn't in the rest of the experience of it so much. So, I started dunking all the way in. I noticed in some of the photos that you've put out and, in your book, that you'll sometimes wear a cap while you go in.

SUSANNA SØBERG: Yeah. Well, it comes from different reasons. So, let's talk about some of the physiological reasons. So, when you submerge in cold water up to the neck, studies have shown, and this is from Denmark studies from Bispebjerg Hospital that when you submerge into cold water up to the neck at 0 degrees, so 0 degrees Celsius, very cold, you have a decreased blood flow to the brain by around 30% to 40%. And makes sense because you activate the sympathetic nervous system. And therefore, you will have a less blood flow to the brain. Makes you maybe a little bit dizzier.

ANDREW HUBERMAN: Proof again that you need a heart more than a brain because when the sympathetic nervous system gets activated blood flow is maintained to the heart to keep you alive but obviously taking away from the brain to keep you from thinking. That's why it's hard to think when you're stressed.

SUSANNA SØBERG: Yeah. Well, the muscles and your vital organs need to-- you have to be able to run away from that tiger, right?

ANDREW HUBERMAN: The rationale, it makes total sense. And who am I to disagree with mother nature?

SUSANNA SØBERG: But yeah. So, one of the reasons being that you should keep your head out of the water is that you could decrease in blood flow to the brain further if you dunked the head. So, there's this very nice paper from a research group in Canada, where they have collectively looked at different papers where they compared heat loss in a group in the papers where they dunked the head and compared it to heat loss submerging up to the neck to see how much extra heat do we lose from our core when we dunk the head.

And I think it's very interesting that if you submerge up to the neck, you have a heat loss of 11% from the body core. And when you then also dunk the head, you increase that heat loss rate by 36%. I'm not here to say what is right and what is wrong. I just think that people should know the information, so they can for themselves evaluate what is best for them.

But if you increase your heat loss rate by 36% from your core, that's going to increase your afterdrop, which we touched upon a little bit earlier, even further. So that's meaning that you are closer to hypothermia than you are if you just submerge up to the neck. So, you should really think about whether this is like something that you want to do or if it's just better for you not to get that cold in your core.

The beanie is also because I have a little bit of sensitive ears. So, meaning that if there's wind, and because we swim in the open sea in Denmark, we have a lot of wind. Our conditions are just very rainy, very windy. And when the temperature is also freezing, you could get this-- what is that called? So very cold and lightheaded just from wind. So, if you also submerge into cold water and you then get up, you would get a brain freeze immediately.

So, it is enough to just go up to the neck wear a beanie to just not get dizzy also because the heat loss is increased of course, but also, the blood flow to the brain has decreased. So, the beanie will keep you a little bit warmer, so you can stay for 1 to 2 minutes. So, it's just a way of getting around some of the conditions also. So, people can choose that if they feel that, but it's quite normal to do in Scandinavia, wear a beanie.

ANDREW HUBERMAN: Love it. And so, for those of you afraid of doing a 2-minute cold shower, what Dr. Søberg just described. You see how she and others are capable of doing things far harder than that. The way you describe it with the cold wind and Scandinavia is quite striking.

Along the lines of covering the head, there's this seemingly paradoxical thing of people going into hot saunas and wearing wool caps. If you go to a banya or you go to a sauna, and there are people who are, well, from Eastern Europe typically, or Finland, or Russia, or Ukraine, or elsewhere. What you'll see is that many of them are wearing wool caps in the sauna, which many people think is to make it hotter, but it's actually not the case. It actually insulates you from the heat environment.

The sense of urgency to get out of the hot sauna is a brain-driven mechanism. And so, the reason that people wear wool hats in the sauna is it actually lets you stay in the sauna longer because it takes a lot of heat to the skin before you feel that you, quote unquote, have to get out. Whereas so when you insulate the brain, you don't get that signal. It's pretty interesting. I've tried this before just by putting a towel over my head in the sauna, and you can stay in there much more easily and for much longer.

As we talk about these different stimuli for the hermetic response, the adaptation to stress, it occurs to me that the big ones in our evolutionary history have been light. I mean, you were talking about seasonal changes. We know, especially as you go up to Nordic countries, there are seasonal changes in the amount of light by time of year. Dramatic ones, in fact. Less so at the equator, of course.

Light, temperature, food, movement. And it's sort of interesting. And at the same time, perhaps it should have been obvious to us that there are stimuli that our bodies have evolved to adapt to in very powerful ways. And so, the idea that temperature, heat and cold, could evoke these tremendous physiological changes that are beneficial for us probably shouldn't surprise us at all. I mean, these are not esoteric mechanisms. They're actually the foundational mechanisms by which our body and the bodies of other animals adapt.

So, I do have a question about the different ways that people could approach deliberate cold exposure. So, for instance, children. I've been to banyas where there are kids 6 or 7 years old with their parents at the banya. And so, they're in hot sauna. I'm not suggesting people do this if they're not adapted to it. And talk to your parents, kids. And talk to your kids, parents. Talk to your doctors. But it is remarkable.

I mean, children doing sauna from a young age or deliberate cold exposure. Are there any data on this, and is it safe assuming that, obviously, that they can swim or they're doing this in a tub or shower? And then I'd also like to ask you about are there any additional male-female differences? I know your study focused on men. But other studies have focused on both. And you, of course, are a woman and can attest to your own experience with this. So, children, men, women, differences there in terms of protocols? Is there anything that people should build into the structure of their deliberate cold exposure that's unique to that?

SUSANNA SØBERG: Yeah. So, this was on cold exposure. So yeah. I think that starting with the question about children. I think that it's important to think about as children are smaller than adults, so we cannot really completely transfer all the information and the benefits and also protocols for how long and stuff like that to children. We cannot do that because they are just smaller in mass.

And one study that actually proves this is a study where they have compared heat loss in children, boys who were 12 years old, compared to adults, men, and looked at heat loss of the core temperature. And exposed them to a 1- or 2-minutes cold exposure. So, immersion up to the neck.

And what they saw was that the boys in this study could actually defend their core temperature in the same way as the adults could, but they had to use their muscles way faster. So, it means that they couldn't stay for as long, and they use more energy to defend their core temperature compared to the adults. But for 1 minute, it seems that they could actually, but they will be colder when they then come out because they are smaller in their mass to their ratio, right?

So, it means that if the surface is so large on children and their mass and muscles being smaller to that ratio, it means that they can be in the water less time before they get hypothermic. So just think about that. They are just smaller. They can't defend their temperature for a very long time. But in this study, they saw that for up to, I think it was a minute or so.

ANDREW HUBERMAN: 1 minute.

SUSANNA SØBERG: 1 minute. Yeah.

ANDREW HUBERMAN: I'm glad you mentioned hypothermia in smaller bodied people, children. I used to do some Pacific Ocean swims in the morning without wetsuits, and I adapted to it pretty quickly. And these are fairly long swims, and we brought an excellent swimmer with us that was interning with me for a while, is 16 years old at the time and very lean. And he wasn't small for his age, but he was smaller than us. It was all guys on the swim that day. And sometimes, women join us.

And he got hypothermic. And he's an excellent swimmer. And he didn't report feeling overly cold. But fortunately, we got him to shore and heated him up again. So, he lived. I don't think his mother is going to ever let him go swimming with us again. He's thriving in the world. He's a university student now. And he recalls that swim. I mean, this is why you always want to ocean swim with a buddy, with people. Yeah. He became hypothermic. His teeth turned yellow. He was slurring his words. He wasn't making sense. We got him on to shore, and he was kind of drooling and a little bit semi-euphoric. Hypothermia is no joke.

SUSANNA SØBERG: It's no joke.

ANDREW HUBERMAN: Yeah. So, I'm really glad that this is coming up because the cold is a powerful stimulus, and kids and smaller bodied people are at a greater risk of hypothermia. So, a good reason to approach it with caution. Maybe start with cold showers. Get then cold immersion in still water. Natural water and open bodies of water of course are always going to be more dangerous for other reasons, currents and things of that sort.

SUSANNA SØBERG: Yeah, exactly. And drowning. Yeah.

ANDREW HUBERMAN: So important note there. What about any additional male-female differences or similarities that we should be aware of? And this comes up all the time on social media. Any time I post anything about a study, it's what about women? Because oftentimes, there are differences.

SUSANNA SØBERG: Yeah. And we also just talked about the difference in temperature in men and women. So, it means that if we replicated my study in women, it could be that they would have enough, you can say, cold exposure with just 9 minutes per week. It could be because they apparently are also just colder, and they have increased metabolism, and their brown fat is just they have more brown fat. It could be, but this is just something that I frankly don't know.

But women also do cold exposure, winter swimming with the 11 minutes protocol. I do it myself and feel good about it. So, I would say that women, also regarding activation of the brown fat, it should be the same in theory. But I don't know if women actually do need to have another protocol when it comes to this rapid cold exposure.

I think that it's another question if we are talking about ice swimming when it comes to how far can you be in the cold water without getting hypothermic. Then there will be differences in men and female. But if you do this cold exposure for a very brief amount of time, which is what I try to talk about, what we call also microstressing the body to increase the hermetic stress, the healthy stress. Then this is such a short amount of exposure that it's fairly the same. I think women can look at this as a fairly good protocol for them as well.

ANDREW HUBERMAN: I always say that if you really dread the cold and don't like the cold, then you are a perfect candidate for using deliberate cold exposure because the sympathetic, aka the stress response, will be greater. And thereby, the adaptation to that shorter 1 or 2 minutes is going to be much greater.

For people that are perfectly comfortable in the cold, it's harder to get an adaptation response. The same way that if somebody is very strong and they can lift a very heavy weight, that that very heavy weight is unlikely to evoke the same kind of or same degree of adaptive responses if somebody is not quite as strong. So, another reason to keep these exposures relatively short and more frequent than to do longer duration exposures frequently.

However, let's say somebody only had two days a week to do deliberate cold exposure. Maybe they don't have access to a sauna. Maybe they do. Would you suggest that they get in for 1 or 2 minutes, then get out, then get back in for another couple of minutes, then get out, and call that for 4.5 minutes to try and get to that 11 minutes total per week, as opposed to getting in for a full 5 minutes, and then getting out, and coming back a second time that week?

I know this is getting down into the weeds, but these are the sorts of things that I think people really want to know because a lot of people either don't live close to a body of water or don't have a cold plunge that they can do this with, although cold shower apparently works too. So, most people live close to a shower.

SUSANNA SØBERG: Yeah. Definitely. I think the changes in temperature is what is strengthening your cells in the body. So, if you can do the short amount of exposure and then get out and get back in, that is going to, you can say, strengthen your cells because you are challenging them to adapt to changing temperatures.

So, during one session, you can change this, right? You can do it if you are able to go to cold water but also a sauna. Then you just do that automatically, and you will have a change in temperature. But you could also do it while variating the temperature. In your cold plunge if you have a plunge, or if you have an open sea, or you have seasons even. We have that in Denmark. So, we have four seasons. And the temperature is going to vary with that.

So, we have nature who can just change this for us, and we don't have to think about it. But if you have a cold plunge, well, then I would say that changing the temperature is what is going to create this hermetic stress and also keep your cells on its toes, you can say, because right the body will still be stressed to try to adapt to the new temperature as it's seen as something actually toxic to the body, right? It's a small piece of toxicity that you are exposing yourself to. You don't have to swallow it, but it's enough that you touch it actually.

ANDREW HUBERMAN: Yeah. A great way to frame it. That brings me back to this idea of circadian time. In your study, you didn't control for specific time of day. And now I'm realizing that may be a great asset to the whole thing. So, we know, for instance, that our bodies go through pretty dramatic shifts in temperature from the time we wake up. Our body starts heating up as we wake up and continues to heat until the afternoon. And then starts to drop in the later afternoon. And then assuming all things are working correctly, that body temperature drops, and we sleep.

So, I could imagine that doing deliberate cold exposure at different times just by way of convenience or by way of intention could be very beneficial because my body temperature is going to be quite a bit warmer at one time of day versus another. And in that way, keeping the system tuned.

And that's really what I keep hearing coming through as you explain these data and all these beautiful studies, yours and others, is that it's not really about getting cold. It's about going from warm to cold and from cold to warm. And I love this idea because I probably said this 100 times on my podcast and a million times in my life, and I'll continue to, which is that biology is not an event. It's a process. Like these metabolic and thermoregulatory processes are indeed like the turning of a knob. It's a verb as opposed to a noun.

And so, I think if people can internalize that idea, that they're going to have a lot more flexibility, a lot more fun, and get a lot more benefit as opposed to thinking, OK, I need to get into x degrees of water for x amount of time on x number of days in a very rigid way.

SUSANNA SØBERG: I get this question all the time. How much and how cold? And I mean, it's just like, well, because we also don't have studies showing exactly if you just keep 5 degrees in your water and you do that for a month, then what happens? Maybe in the future, we will know much more about this, and I'm sure it's going to come, and I really hope so. But I just think by logically changing that temperature up and down, up and down, and you also do that in your water, it's not that important what temperature you will have your water then. They'll just keep changing, going up and down. It could be all up to 12 degrees Celsius.

You're going to activate your brown fat anyways. I mean, 12, 19 degrees cold air is enough to activate your brown fat. So maybe we don't have to go as cold as I think many people think and putting ice even all the time. You don't have to. I don't think it's necessary to expose yourself to that cold temperature all the time but vary it a bit.

ANDREW HUBERMAN: So, keep the system off balance. It's the way to keep it tuned. You mentioned a study that is more recent and ongoing that's not published. If you're willing, could you share maybe some of the data from that findings from that study with, of course, the cue to everybody that these are not yet published data, so the conclusions could change. The data could change for that matter.

SUSANNA SØBERG: Yeah. So, we haven't analyzed all the data yet, and I know from the study that we did publish that we would need to look more at the data. So, I don't really have any results yet that I can share because we are still in very preliminary analysis of this. So, I wouldn't know yet what to exactly say about it. But what we looked at was both men and women. So that's coming.

ANDREW HUBERMAN: Oh, that's fantastic. That answer is going to please a great number of people and intrigue everybody. Well, I want to really thank you for coming here today to talk about your work and the incredible direction that it points to because I think that no one study is definitive, but your study really again stands as a landmark in the landscape of exploring deliberate cold exposure and heat, how it can impact and potentially impact our health.

Because frankly, there just haven't been that many high resolution detailed modern studies of this. There have been studies of sauna. There have been some studies of cold. There are a lot of groups in physiology that work on hypothermia and very cold exposure. But most of the temperatures used in those days just aren't practical.

So first of all, I just want to thank you for viewing the work that you've done and for the work that you continue to do. I'm waiting with bated breath as I say to hear the results of this study that's ongoing on both men and women. So, we'll have to have you back to inform us about that soon.

And I want to thank you for the incredible public education efforts that you've been doing on social media with respect to your book. And we, of course, will put links to all of those things in the show note captions, so people can learn from you and can continue to learn from you. We certainly need more scientists who are both experienced with doing hard core research, as it's called, and who also do the practices. I think that's a wonderful additional asset. You're not just behind a lab coat or bundled up in a down feather jacket as everyone else is getting into the cold. You do these things. And that you are so open and generous in the way that you share knowledge, which includes coming here today to share knowledge with me and our audience. So, thank you ever so much.

SUSANNA SØBERG: You're very welcome. I am so pleased to be here. And thank you so much for inviting me. And I could explain my study, and I can share some of my insights from doing that. So, I'm very grateful for being here.

ANDREW HUBERMAN: Delighted, and we'll have to have you back again.

Thank you for joining me for today's discussion all about deliberate cold and deliberate heat exposure science and protocols with Dr. Susanna Søberg. If you'd like to learn more about Dr. Søberg's research or you would like to learn about the research of her institute, the Søberg Institute, please see the links in the show note caption.

Also, in the show note caption, you can find a link to Dr. Søberg's excellent book, "Winter Swimming." If you're learning from and/or enjoying this podcast, please subscribe to our YouTube channel. That's a terrific zero cost way to support us. In addition, please subscribe to the podcast on Spotify and Apple. And in addition, on both Spotify and Apple, you can leave us up to a five-star review.

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