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

Andrew Huberman 0:00 (music plays) 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 Søberg 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 catacholamines 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 acclimization, the cold shock response, dangers and safeties of cold water, the impact of cold and the 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 are 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

Andrew Huberman 3:23 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. Our first sponsor is Plunge. Plunge makes what I believe is the most versatile at-home self-cooling cold plunge for deliberate cold exposure. I've talked numerous times on this podcast about the many benefits of deliberate cold exposure, and indeed, today's episode is focused entirely on the benefits and the science of deliberate cold exposure. Plunge uses a powerful cooling filtration and sanitation unit to give you access to deliberate cold exposure in clean water whenever you want. As we will discuss during today's episode with Dr. Susanna Søberg, deliberate cold exposure, especially deliberate cold exposure done up to the neck in water, can be used to achieve a number of important endpoints related to mental health, physical health, and performance. I've been using a Plunge for more than two years now, and I can tell you that it makes it very easy to get your deliberate cold exposure at home. It doesn't require much cleaning. In fact, it's very easy to keep clean, which is essential. You don't want bacteria and other things growing in your cold plunge. Basically, everything about the Plunge is made easy so that anyone, including myself, can get their deliberate cold exposure on a regular basis at home. If you're interested in getting a Plunge, you can go to Plunge, spell P-L-U-N-G-E, .com/huberman and get $150 off your cold Plunge. Again, that's plunge.com/huberman for $150 off. Today's episode is also brought to us by Maui Nui Venison, which I can confidently say is the most nutrient-dense and delicious red meat available. Maui Nui spent nearly a decade building a USDA-certified wild harvesting system to help balance deer populations on the island of Maui. The solution they built turns the proliferation of an invasive species into a wide range of nutrients-dense products, from butcher cuts and organ meats to bone broth and jerky. Their bone broth has an unmatched 25 grams of protein per 100 calories. Several guests on this podcast who are experts in nutrition have pointed to the value of getting at least one gram of quality protein per pound of body weight each day. With Maui Nui, that's very easy to do while eating delicious meals and getting it from a sustainable source. If you would like to try Maui Nui Venison, go to maui nui venison.com/huberman and get 20% off your first order. Again, that's maui nui venison.com/huberman to get 20% off. Today's episode is also brought to us by Thesis. Thesis makes custom nootropics, and nootropics is not a word that I like because it means smart drugs, and the brain doesn't have neural circuits for being smart, rather has neural circuits for focus, neural circuits for task switching, neural circuits for creativity, and on and on. Thesis understands this and designs custom nootropics designed to get your brain and body into a specific state in order to do the mental and/or physical work that's important to you, such as creativity or focus or clarity. If you'd like to try Thesis Nootropics, you simply go to their website...You fill out a brief quiz and they will design a custom starter pack so that you can assess which things work for you more or less well, and then they'll iterate with you over the course of the next few weeks or months to come up with the ideal nootropic kit for your needs. To get your own personalized nootropic starter kit, go online to takethesis.com/huberman. You can take that three-minute quiz, and they'll send you four different formulas to try in your first month. Again, that's takethesis.com/huberman and use the code Huberman at checkout to get 10% off your first

Andrew Huberman 6:49 box. I'm pleased to announce that I will be hosting two live events in September of 2023. The first live event will take place in Toronto on September 12th. The second live event will take place in Chicago on September 28th. Both live events will include a lecture and a question and answer period and are entitled The Brain Body Contract, during which I will discuss tools and science related to mental health, physical health, and performance, and I should mention that a lot of that content will have absolutely no overlap with content covered previously on the Huberman Lab Podcast or elsewhere. If you're interested in attending either or both of these events, please go to hubermanlab.com/tour and enter the code Huberman to get early access to tickets. Once again, that's hubermanlab.com/tour and use the code Huberman to access tickets. I hope to see you there. And now for my discussion with Dr. Susanna Soberg.

Andrew Huberman 7:40 Dr. Susanna Soberg, welcome.

Susanna Søberg 7:42 Thank you.

Andrew Huberman 7:43 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, uh, 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've been interested in this area, but there's been a real, uh, 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, uh, the, first of all, the fact that it was in Cell Reports Medicine. I've been on the Cell Press edi- editorial board for a long time now, so press journals are, of course, phenomenal journals-

Susanna Søberg 8:49 Mm-hmm.

Andrew Huberman 8:50 ... 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 that 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 gonna 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, um, 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, uh, at room temperature or I'm a little bit chilly, getting into that same temperature of water doesn't feel so good, right?

Susanna Søberg 10:07 Mm-hmm.

Andrew Huberman 10:07 There's a shock there.

Susanna Søberg 10:08 Mm-hmm.

Andrew Huberman 10:08 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.

Susanna Søberg 10:24 Yeah.

Andrew Huberman 10:24 And for those that haven't done it, um, might start to peel back some of the, the layers as to what the underlying mechanisms of cold are.

Susanna Søberg 10:32 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, uh, stressor, which is submerging into cold water, and, but you could also go in, outside in the cold wind. That's also gonna activate your, um, your sympathetic nervous system. So get all these neurotransmitter going in your body and so your, your catecholamines. Um, let's just address that we are taking a cold plunge, for example. So if you are very hot, for example, um, before you go into the cold water, it's gonna feel less... it's gonna feel less stressful, but the, the temperature, uh, difference from your skin to the cold is definitely s- gonna give you a, a shock, but your core temperature is warmer and that's gonna feel a little bit better. So that's why when people go into a sauna, for example, and go out then in, into the cold water, they, they, they can do it easily, uh, easier than if they were cold beforehand. So-

Andrew Huberman 11:35 Mm-hmm. Could I just, um, ask you a few questions? So you, you mentioned the sympathetic nervous system, which for, um, people listening who aren't familiar with that is the, the branch of our nervous system that's responsible for creating accelerations in heart rate, um, feelings of alertness. It's accompanied with stress and the stress response, but it's accompanied with, uh, waking up in the morning for that matter. So it's not always about stress. And then you mention the catecholamines, which, um, are dopamine, epinephrine, and norepinephrine. So maybe a little bit later we'll talk about those individual neurotransmitters,

Andrew Huberman 12:05 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 of 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. Um, 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 13:02 Yeah. That is really good question, and I definitely think that this could be, um, future studies on this as well to really unravel, uh, what kind of protocols, uh, are the, are the best way or also for which outcomes, of course. So if the temperature is, uh, very cold and you feel that, uh, you also feel very cold, then you c- you should stay in the water a little bit longer. So I think it's just 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, uh, activation of the, the catecholamines, which you just mentioned before.

Andrew Huberman 13:42 Does the shock mean that I'm having trouble controlling my breathing? Is that a good gauge?

Susanna Søberg 13:47 Uh, yeah, you can say so because that's kinda like how we define it. So you hyperventilate, so you have a faster, um, uh, breathing rate. Um, so that increases also because you activate your gasping reflex if you are new to this. Um, but if you are adapted, it, it kind of subsides with time, a- with a- adaptation. So what you can do is that you can train this cold exposure, and you can kinda like 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, building up your adaptation is gonna make the shock like subside a bit. So it's, 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, I'm thinking something is wrong, is not right (laughs) . You should not enjoy it.

Andrew Huberman 14:47 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, um, if running hard, you know, running fast and breathing hard is uncomfortable, you can slow down or walk. If, um, you know, 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-

Susanna Søberg 15:26 Mm-hmm.

Andrew Huberman 15:27 ... uh, and adjust the level to kind... It's kind of all or none. And I've seen, um... I should just, uh, I can tell this by anecdote. I, I've done some work with military special operations. Um, I won't say which country. This was outside the US. Um, 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-

Susanna Søberg 15:58 Mm-hmm.

Andrew Huberman 15:58 ... and just kind of grinded through it, you know? Like, they look stoic anyway, to me.

Susanna Søberg 16:03 (laughs)

Andrew Huberman 16:03 Um, there were a few whimpers, no cries.

Susanna Søberg 16:05 (laughs)

Andrew Huberman 16:05 About a third, um, talked a lot and got really c... You could tell that they were agitated and anxious, but they-

Susanna Søberg 16:11 Yeah.

Andrew Huberman 16:11 ... made it through, and then about a third of them just simply would not get in past their knees or thighs, were just, uh, it seemed like they were just dreading the whole experience. Some actually, didn't actually go in completely, um, which was really surprising to me and 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. Is that... From your studies, is that your experience as well, or, or are there these weird mutants who seem to just love going into the cold for the first time?

Susanna Søberg 16:47 So some people just feel better in the cold, and some people, uh, 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, they, they would definitely... People who are maybe the soldiers you, you just talked about, they, some of them might be already adapted to the cold, 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 a- away from it, right? And you also have the cold pain more, um, feel the cold pain more if the, if, if you avoid it. So the more you avoid the cold, the, the more pain, painful it will feel, uh, when you go into it,

Susanna Søberg 17:37 so yeah.

Andrew Huberman 17:37 Mm-hmm. You mentioned being outside in a T-shirt versus, um, cold immersion up to the neck versus shower. I think, um, 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 in ice water or just very cold water, um, immersion with dunking one's head and then coming up, because obviously people have to come up for air at some point-

Susanna Søberg 18:04 (laughs)

Andrew Huberman 18:05 ... and then simply being outside on a cold day in shorts and a T-shirt or something of that sort?

Susanna Søberg 18:11 So there are different outcomes, um, because the, they are very different, uh, 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 there are, of course-... covered totally and, and the molecules are te- uh, closer to your skin, you have a more potent, um, activation of all your cold, uh, receptors in the skin. So that one will definitely activate your auto- no- enormous, uh, nervous system more and rapid-

Andrew Huberman 18:45 Mm-hmm.

Susanna Søberg 18:45 ... uh, compared to going out in a T-shirt in the cold wind, just go for a walk. Um, but that is also something that's gonna activate your, uh, sympathetic nervous system.

Andrew Huberman 18:55 Mm-hmm.

Susanna Søberg 18:55 Meaning then that you have an increase in norepinephrine, uh, and you will activate something called, uh, the, the brown fat. So this is a healthy kind of, uh, fat tissue that we have in our body. And when you activate that, that's gonna, um, increase your metabolism.

Andrew Huberman 19:11 Before we talk about brown fat-

Susanna Søberg 19:12 Mm-hmm.

Andrew Huberman 19:12 ... and I'm so glad you brought it up-

Susanna Søberg 19:14 Mm-hmm.

Andrew Huberman 19:14 ... um, because so much to talk about there, uh, what about cold shower? I mean, obviously cold showers somewhere in between-

Susanna Søberg 19:20 Yeah.

Andrew Huberman 19:21 ... um, being out, outside in the air, cold air versus, uh, being immersed up to the neck.

Susanna Søberg 19:27 If we had more studies on, on cold showers, we would learn more about how does that activate our metabolism? How does that increase our, uh, neurotransmitters in the brain, which could also have an impact our, on our mental, uh, balance. So I think that will be interesting for the future. Um, but what we do know is from, from, uh, from activating brown fat, and, uh, both from rodent sto- uh, studies, but also in, 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, in the body to keep, uh, our, um, temperature up. So our muscles is like the second tissue in our body. We have two tissues which can increase our, uh, thermogenesis. So the brown fat, which is like always, like, temperature regulating our body, and then we have, uh, the, the muscles which will secondarily, uh, start to shiver, and that's going to increase our, um, temperature in the body. But as soon as you go into a cold shower, you will activate your brown fat, uh, 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, (laughs) it would be great.

Andrew Huberman 20:43 I think I, I thought about why there are fewer studies of cold showers than 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-

Susanna Søberg 21:08 Yeah.

Andrew Huberman 21:08 ... but it's a little bit, um, more difficult. Also, I think, uh, for, uh, you and I are both research scientists, there's a little bit of a, um, 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, um, kind of pushing their clothing against their skin. There's a certain vulnerability in, uh, for most people, um, coming to a laboratory in the first place, let alone being observed while they shower.

Susanna Søberg 21:32 Yeah.

Andrew Huberman 21:32 Whereas when you get into cold immerge- uh, cold immersion, you're getting under the water, and, uh, you know, some people might roll their eyes and say, "Okay, really? Is that the barrier?" But, you know, 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, um, the process of getting people to the laboratory and having them park and find the lab and, you know, it's a whole new environment with people in lab coats and people moving around and, "Where's the restroom?" I mean, it's a, there's a, there's a certain amount of stress just associated with taking part in a study for most human subjects. So, um, I, uh, totally agree, however, we need more studies of, of cold showers. It's just a harder environment to control-

Susanna Søberg 22:09 Yeah.

Andrew Huberman 22:10 ... in, in my, in my mind.

Andrew Huberman 22:11 So it sounds like any form of cold to the skin that people register as what you call the cold shock or uncomfortable, like, "Oh, this is kind of, um, 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 22:32 (laughs) .

Andrew Huberman 22:32 (laughs) .

Susanna Søberg 22:32 Yeah, really good question, and it seems that I, I think that, of course, in the future, we will know much more about these pathways. But what we do know is that, uh, the cold receptors will send a signal to our temperature regulating center in, in the brain, so hypothalamus, um, and that's going to be, um, uh, taking in this message. And we have so many cold receptors in the skin, so it's going to be very fast as you can say, if you immerse the body into cold water, this is going to be so rapid. Uh, so it will have a rapid increase in, uh, neurotransmitters in the brain, so noradrenaline, adrenaline, and cortisol, and, uh, which is not that much, but it's, but it's still there. So you have this increase in, uh, noradrenaline, which will then immediately activate the brown fat, um, because the, you can say the activator is the most potent one, cold and noradrenaline, that's going to activate the brown fat. But there's also a direct pathway from the cold receptors in the skin to the, to the brown fat, which really s- shows that if... because of these different pathways, it shows that, that it could be that this tissue to keep us warm was, was, uh, developed in, in our evo- evolvement, uh, as humans to keep us warm and to save us whenever the temperature o- on our skin varies just a little bit to keep us in that right homeostatic balance so we don't get hypothermic, um, but also so we don't get hyperthermic, because it seems that the brown fat is also activated when we get warmer in our skin. So it's also, um, maybe a temperature regulator in our, in our body, but the pathways is different. I think it's also a third pathway from directly from the muscles. So the brown fat is also activated, um, even though the, the muscles are starting to shiver. So there's a... an extra pathway that way to keep our, uh, our temperature up. So muscles and brown fat are working together to, to keep us warm so we don't, uh, suffer too much in the, in the cold water.

Andrew Huberman 24:34 It's super interesting and w-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, um, I think it's important for people to know about because, um, as you, uh, said so, uh, so eloquently, the, when something is very important to our survival or, and/or evolution, the brain and body, uh, install multiple mechanisms for it, not just one. And, um, and so, it sounds like it's cold, skin, cold on the skin triggers an, 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. Um, I want to talk about brown fat in depth and learn from you more about brown fat.

Andrew Huberman 25:22 Um, before that, however, I want to ask about shiver. Um, I've heard that shiver causes the release of succinate, um, which then activates the brown fat. Is it known whether or not inducing shiver is important and when should pe- 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 plunger 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 26:02 Yeah, well, shivering is, 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, uh, as long as you don't get too hypothermic, so if you don't s- if you don't sit in the cold water for too long.

Andrew Huberman 26:17 Sure.

Susanna Søberg 26:17 Um, and what you just said by shivering after you get out, that is because of the after drop. Something called the after drop is when your core temperature, uh, decreases even after you get out of the cold water, and it always does that, um, your body, because it, w- as soon as you get into the cold water, eh, all the, eh, your blood vessels is gonna constrict because you need to keep your, uh, blood in your core and, and keep your, uh, vital organs warm. So as soon as you get out, that, 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 26:56 So that's the drop.

Susanna Søberg 26:57 So that's the drop, yeah, after drop-

Andrew Huberman 26:58 Got it. Oh, I'm so glad you explained that because I've heard, um, 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- let me make sure I understand this. So, um, I get into cold water. Obviously, I'm cold, vessels constrict to keep w- 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 27:38 Exactly.

Andrew Huberman 27:38 Great. And then am I right in thinking that then the shiver activates brown fat, which then warms me up again?

Susanna Søberg 27:44 Yes.

Andrew Huberman 27:45 Got it.

Susanna Søberg 27:45 That's why you should end on the cold, but we can get back to that. (laughs)

Andrew Huberman 27:48 Yeah, let's talk about... Yes. Ending on cold is, um, uh, you know, it's what I refer to as and what has now become known as the Søberg principle, which is, um, a really important principle about the importance of ending on cold, um, and not doing what I do, which is to get into a hot shower or- or back in the sauna, but we'll- we'll get back to that in a, in a few minutes. So, um, that's wonderful, um, 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 that..." (knuckles rapping)

Susanna Søberg 28:17 Yeah.

Andrew Huberman 28:17 You know, the chattering of the teeth and-

Susanna Søberg 28:18 Yeah.

Andrew Huberman 28:18 Um, and it, it feels like a loss of bodily control, which really it is. It's, it's an autonomic response.

Susanna Søberg 28:24 Yeah, but I don't think that people should, should avoid it that much. It's just like seeing shivering as a way of your body in a, in a, like, it's training. It's training for your, 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 gonna hurt because that's what shivering also does, but it's just a different way of training your cells in your body. It's going to create what is healthy stress. It's called homeostasis in the cells. And the more you expose your s- your muscle cells or your brown fat cells to these kind of like healthy stresses, exercise, cold and heat exposure, it's gonna make them, uh, better at, like, activating and also, um, at, uh, keeping you healthy. So as long as the he- the cells get exposed to this, it, it's gonna keep them on its toes, you can say, because it becomes more robust, um, increasing these heat shock proteins and cold shock proteins in the cells to, um, make you, uh, more robust for the next time and that is also what happens when you go to the training center, and I keep, like, d- drawing that parallel because p- people today know more about, we know more about exercise and what that's, uh, is, is gonna do to your muscle cells, um, and but the same kind of like training is also what you do when you go out and, and into the cold water and submerge into cold water because that is just, uh, your cold training center, you can say that, and, and also your heat training center going into the sauna because the cells are getting stronger with hermetic stress, so it's the same process, just different practice.

Andrew Huberman 30:11 Mm-hmm. I'm so glad that you brought up the fact that the discomfort or the embarrassment or both of shiver is still crucial to, uh, actually to reach for and try and experience the same way that with exercise. Um, 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, um, heart rate, blood pressure, um, stress hormones, and inflammation in a human being during exercise...... it would look as if they were ready to die.

Susanna Søberg 30:46 (laughs)

Andrew Huberman 30:46 Blood pressure would be high, inflammation is through the roof-

Susanna Søberg 30:48 Yeah.

Andrew Huberman 30:48 ... but all of that is setting in motion an a- adaptation or set of adaptations that allow blood pressure to be lower at rest, that allow, um, inflammation markers to be lower at rest, all the things that everybody is seeking with exercise, in, in addition to, of course, the aesthetic changes that people are seeking with exercise. Sounds like the exact same things are happening with the cold. So,

Andrew Huberman 31:08 um, the redundant message here seems to be that the more discomfort, provided it's done safely just like with exercise, the more shivering, the, um, the more cold shock, provided it's not to the extreme and stops somebody's heart, right? 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, uh, I think it was European Journal of Physiology, it was, uh, the European Journal of Physiology, showing long-lasting increases in catecholamines, dopamine, norepinephrine, and, um, 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, um, day after deliberate cold exposure?

Susanna Søberg 32:05 As soon as you go in, of course, there's an activation, but it seems like, no, you're asking for the, the later outcomes, like blood pressure and stuff like that. Is that what you mean?

Andrew Huberman 32:13 Yeah. Blood pressure, but also in terms of metabolism. I know that, you know, in your study, you shou- and we'll talk about brown fat in depth here in a moment, but-

Susanna Søberg 32:20 Yeah.

Andrew Huberman 32:20 ... that there were changes to the brown fat that equate to changes in, for instance, people's ability to be comfortable in colder-

Susanna Søberg 32:29 Oh, yeah.

Andrew Huberman 32:29 ... environments when they're not doing deliberate cold exposure-

Susanna Søberg 32:32 Yeah.

Andrew Huberman 32:32 ... or, uh, in the same way that I can, um, exercise on an exercise bike or go out for a hard run, but then if I go hiking, uh, with the family on Sunday and where it's a steep climb, I can do that steep climb more easily because I'm, quote unquote, "fit"-

Susanna Søberg 32:44 Yeah.

Andrew Huberman 32:44 ... as a consequence of the, of the exercise. What are s- uh, what are some of the fitness adaptations of deliberate cold exposure?

Susanna Søberg 32:51 Yeah. So what happens is that you, 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, you will feel more comfortable in the cold. So you're gonna, you're gonna build your adaptation, which happens on a metabolic level, which is gonna be the brown fat, so you will have more activation of your brown fat. The mitochondria in the, the brown fat cells are gonna be, um, you have more of those, and they will be more efficient at heating you up because it expects, the body expects you to, to do this again, so you are prepared in a way. The capillaries in your skin is also ge- b- will also become better at, like, constricting, so you will have a better shield of your body to, uh, prepare you for the next time. So you will be, become better at, uh, going into the cold water in that way. So the, the body makes this mechanism and t- changes your body in a way so you can expose yourself to the next time, right? And, uh, and also, you will have, um, also, uh, your, um, stress response will also be, uh, subside a bit, so you have a l- less increase of your, uh, catecholamines, um, with time. With time, also, you have, because of this activation of your, uh, brown fat or your muscles, you have an increase in, um, uh, in, in your metabolism, which will then, uh, make your insulin sensitivity, uh, better. And this is shown in, in studies. For example, um, there was this interesting study I found just before I, I started my PhD, which was from, um, Guibers-Dormer, um, et al from 2016, where they measured, um, metabolism, uh, not in, uh, not on brown fat, but they measured insulin sensitivity in middle-aged men and women, uh, during one winter swimming season. So they were not very young like they were in my study, uh, but they were, they were middle-aged, and I think this is very interesting. So they, during these four or 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, uh, better insulin sensitivity. And I think that is very interesting because if you can have a better insulin sensitivity, you can prevent lifestyle diseases, so, um, and with lower blood pressure, which is, uh, a very strong outcome also for telling how much inflammation you have in the body. Mm-hmm.

Andrew Huberman 35:19 And because they didn't measure, uh, brown fat, um, I figured that it could be that was the missing link, that was the, one of the explanations to why we see this, um, less inflammation in the body. So, um, the longer outcomes, uh, the long-term outcomes, could be that you lower your blood pressure and have a lower heart rate. Um, you also, um, have a l- better insulin sensitivity and, uh, better glucose balance, but that was shown, that is shown in my study.

Susanna Søberg 35:48 I'd like to take a quick break and acknowledge one of our sponsors, Athletic Greens. Athletic Greens, now called AG1, is a vitamin mineral probiotic drink that covers all of your foundational nutritional needs. I've been taking Athletic Greens since 2012, so I'm delighted that they're sponsoring the podcast. The reason I started taking Athletic Greens and the reason I still take Athletic Greens once or usually twice a day is that it gets me the probiotics that I need for gut health. Our gut is very important. It's populated by, uh, gut microbiota that communicate with the brain, the immune system, and basically all the biological systems of our body to strongly impact our immediate and long-term health. And those probiotics in Athletic Greens are optimal and vital for microbiotic health. In addition, Athletic Greens contains a number of adaptogens, vitamins, and minerals that make sure that all of my foundational nutritional needs are met and it tastes great. If you'd like to try Athletic Greens, you can go to athleticgreens.com/huberman and they'll give you five free travel packs that make it really easy to mix up Athletic Greens while you're on the road, in the car, on the plane, et cetera, and they'll give you a year's supply of vitamin D3 K2. Again, that's athleticgreens.com/huberman to get the five free travel packs and the year's supply of vitamin D3

Susanna Søberg 37:02 K2.

Andrew Huberman 37:03 And we'll get back to the, uh, insulin sensitivity and glucose balance. Um, that's a, an, an impressive list of benefits. Um, you know, 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, um, it's not very sexy nowadays. You know, blood pressure, people go, "Oh, it's, you know, blood pressure." It's not... You know, people want to hear about the inflammatome and the microbiome and all of that stuff is really interesting, but, um, I think that blood pressure doesn't get enough attention. Um, and we have spoken to, on this podcast, to Dr. Peter Attia, who, um, is an expert in longevity and health span and things of that sort, and, uh, 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.

Susanna Søberg 37:51 Yeah.

Andrew Huberman 37:51 And there are basically three things on the list of things to address. Uh, one is not smoking-

Susanna Søberg 37:57 Mm-hmm.

Andrew Huberman 37:57 ... um, or vaping, by the way (laughs) .

Susanna Søberg 37:59 (laughs)

Andrew Huberman 37:59 Uh, not smoking and th- there are a few other things, uh, related to blood markers, apo b and things of that sort, but then the big one is blood pressure. And so it's, it's interesting because we don't think about blood pressure that much anymore, um, uh, as a, 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, I'm guessing in concert with other forms of exercise.

Susanna Søberg 38:25 Yeah.

Andrew Huberman 38:26 Um, let's talk about brown fat, and, um, if you, if you're willing, I'd love to drill into brown fat at a, at a deep level. Um, again, eh, my understanding of this is, 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, uh, maybe more widespread than when I learned in school. I thought it was, uh, I was taught it was just at the clavicles and 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, what... If we could just go into the biology of brown fat a little bit, what does it look like? Uh, 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, um, 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, the organs. But please educate me, um, tell me where I'm wrong, and, um, expand my knowledge on brown fat.

Susanna Søberg 39:38 (laughs) Okay. Yeah. You are not wrong, but, um, it's definite- it's true that there are more locations, uh, of the brown fat, the, than we previously thought. Um, there is this very nice l- study from 2017 by Leitner et al where they, eh, had made these ............................, uh, overlays of, um, s- their subjects where they could see where in the body do we have brown fat and where can we grow more brown fat, um, so to say. Um, so the brown fat is ca- is very plastic, so it means that it can, it can grow and it can decrease, and this is proven in, in studies where we have seen, um, um, people with a phaeocryocytoma is like a very specific cancer type where pe- wh- where from the '70s where we can see that if you, they have this specific kind of, eh, cancer type, um, they have a ti- they have this tumor on the, eh, adrenal gland, so they have like an, a huge increase in noradrenaline, and because of that, they ha- have this continuous activation of the brown fat, and they ha- have grown a lot of brown fat in the whole body, eh, ab- abdominal where it's located in these six different places, but it is less, like ve- very much compared to, like, normal people. Um, and what they then see, eh, what we learned from this study is that brown fat can e- apparently grow if you have, um, 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, you just want it on, like, a 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, it activates also your sympathetic nervous system, so they also showed they had high blood pressure, they had... Eh, they lost a lot of weight, of course, because this is activating your metabolism. So they, they found luckily that when they, eh, removed this be- uh, benign, eh, tumor, eh, that, eh, the brown fat, um, decreases again to normal size and they gained weight again and they had, eh, normal blood pressure. So the story ends well (laughs) -

Andrew Huberman 41:44 (laughs)

Susanna Søberg 41:44 ... but it's kind of like proof of concept of the brown fat can actually grow, so it's plastic in its, in its way of like it can, it can grow and it can decrease again. So that's very good. Good studies to, to see what i- what the body's capable of. But we don't, of course, want all that brown fat. We just want it to be, um... We just want to keep it actually and, and keep it activated because what we see in studies is also that after the age of 40, um, people, um, eh, studies have shown that there is an association with having less brown fat but increased obesity. So of course, we, we don't know yet whether, eh, 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, eh, just like the muscles get less sensitive, um, insulin sensitive, the brown fat does as well, and therefore it maybe decreases. It could be a theory. Eh.

Andrew Huberman 42:46 Mm-hmm.

Susanna Søberg 42:46 That I think could be one of the reasons why we, we don't see that much brown fat in, in elderly, eh, people. Some have a lot. Especially people working outside. There, there are studies showing this.

Andrew Huberman 42:57 Who, uh, people who, um s-

Susanna Søberg 42:59 Who work outside.

Andrew Huberman 42:59 ... do physical work outside?

Susanna Søberg 43:01 Outside.

Andrew Huberman 43:01 Farmers and, um-

Susanna Søberg 43:03 Yeah. Yeah. Yeah.

Andrew Huberman 43:03 Interesting.

Susanna Søberg 43:04 Yeah.

Andrew Huberman 43:04 Yeah. It's-

Susanna Søberg 43:04 They expose themself to it, so it w- it they'll, they'll just keep it in their way.

Andrew Huberman 43:08 Mm-hmm. It's, um... And I suppose we should, um, clarify for people in case they don't know that insulin sensitivity......is a very good thing.

Susanna Søberg 43:17 Yeah. Yeah.

Andrew Huberman 43:17 You want that. You want your cells to be sensitive to insulin. Um, insulin insensitivity is type 2 diabetes, um, and is associated with obesity. Um, so just a point of clarification there. Uh, yeah, it's interesting to me. I, I, I usually work out at home, but I, I go to a gym once or twice a week if I can because it's good if I see, uh, the outside world.

Susanna Søberg 43:38 (laughs)

Andrew Huberman 43:39 Um, and there are a few individuals at the, the gym who are, they're not particularly large or muscular, um, but they are incredibly, um, lean and their posture is great, presumably from the musculoskeletal work. Um, and they, uh, they're in their 70s and 80s. I mean, it's remarkable, right?

Susanna Søberg 44:01 Yes, yes.

Andrew Huberman 44:01 And, um, and I know all the telltale signs of hormone augmentation. I'm very good at spotting that.

Susanna Søberg 44:05 (laughs)

Andrew Huberman 44:05 There are a few telltale signs. I've talked about this on other podcasts and-

Susanna Søberg 44:08 (laughs)

Andrew Huberman 44:08 ...they're not, they're, that's not why they're, they're, uh, they're, um, they're fit. Uh, they're, they're, they're clearly of that look, and you see this for, outside the gym too, of course, for people that look like they've done a lot of physical labor their whole life.

Susanna Søberg 44:21 Yeah.

Andrew Huberman 44:21 They're just moving a lot. They, they have strong hands and features and they, they are, um, and they're not necessarily excessively lean, but you can tell that they've been using their musculoskeletal-

Susanna Søberg 44:32 Yeah.

Andrew Huberman 44:34 ...system. And I like to talk to these people and ask them, like, not, "What are you doing now for your workout?" But, w- what, "What did you grow up doing?" You know, 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 were 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

Andrew Huberman 45:08 the relationship between shiver and brown fat makes sense to me. Uh, but is it the case that as we're just moving around, um, I've heard of NEAT, non, um, exercise-induced thermogenesis, so if we're just moving around, um, that we are activating brown fat, or do, 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, um, is just staying active enough or do we need to do some sort of temperature, uh, shock type thing, like deliberate cold ex- uh, exposure?

Susanna Søberg 45:39 Yeah, I think that is really good question because how, how ... also why do we have this tissue then if it's... if it has to be extreme, then you can question what, 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, to cold. So cold is the, the most potent stressor, activator of, uh, our brown fat because it's, our temperature regulating, eh, organ in our body is a first responder to that. So the muscles will be a little bit too late, eh, and therefore we have maybe these two kind of tissues. So actually just, eh, exposing yourself or a hand actually just to cold water. So studies have shown, um, that if you just put your hand in cold water, not that you're going to, gonna do that all day or, or every day or anything, it's not, eh, it's, 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 T-shirt. That's why also we were just talking about, well, people who works outside or move a lot or get ou- in and out of, uh, like changing the temperature of their body all the time, they will have more brown fat and, eh, activating that is gonna keep your metabolism higher and your insulin sensitivity. Uh, study 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 gonna also, (laughs) eh, grow your, uh, 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, uh, shower. You don't have to start in a cold, uh, 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 wanna be, like, eh, in this, uh, awake state, um, e- 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, um, then you activate your, uh, brown fat and you will grow your brown fat so you have more of it. Mm-hmm. So this, uh, very nice studies, um, from Hansen et al. from 2017 showed that, eh, a group of subjects who slept in a room at 24 degrees and then they made this PET/CT scannings to see how much brown fat do they have from the beginning, so what we call baseline. Then they measured again after, um, 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 were, still had clothes on when they're sleeping.

Andrew Huberman 48:16 So they're under a cover, under a duvet.

Susanna Søberg 48:17 Yeah. Under, under a duvet.

Andrew Huberman 48:18 Uh-huh. Yeah.

Susanna Søberg 48:19 Yeah. Yeah.

Andrew Huberman 48:19 Okay.

Susanna Søberg 48:19 The subjects were sleeping at 19 degrees from one month, had increased his insulin sensitivity. The next month, they slept at 24 degrees, they measured this again and then they had (laughs) decreased actually a little bit, and then they slept at, uh, 27 degrees, so quite warm room actually for, for the f- uh, fourth month, um, and they saw, eh, even less, eh, 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, right? 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, um, has grown these more mitochondria in the cells. So these small energy fabrics that's gonna activate the cells and that's gonna take up glucose and fat from the... fatty acids from the bloodstream to keep, eh, the thermogenesis up and that's gonna clear up some sugar and it's gonna clear g- uh, so in the, in the bloodstream and some, some fat as well. So the brown fat can in that way decrease, eh, our unhealthy fat which is the white fat. Um, and the white fat is what we don't want too much of but we still need some of course, e-... and it's our energy storage, so it's very important that it's there. We just don't need, uh, a lot of it. So on our thighs and also around our inner organs, that's where it's, 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 are, have courage for it, you can go out and expose yourself in a cold plunge. Um, cold showers is also gonna do the trick. So you can do different variations of this. Just exposing yourself to various temperatures is gonna activate the brown fat because it was evolved to keep us in a perfect homeostatic balance regarding temperature, so to keep us alive.

Andrew Huberman 50:12 Incredible. Uh, I want to just get a clarification around this 19 degrees Celsius, um, room that they're sleeping in. So they're under a comforter, a duvet, and, um, and you mentioned they had clothes on. Uh, 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, uh, the, the increase in, uh, brown fat that was observed. Is that, is that a reasonable, uh, expectation?

Susanna Søberg 50:40 I, I think so, yeah, because it's, you have so many, uh, cold receptors in your face a- uh, so it's, actually it's enough, and it, I think it corresponds very well with the studies showing that you can activate the brown fat just by putting a hand into a, a bucket of cold water.

Andrew Huberman 50:54 Mm-hmm.

Susanna Søberg 50:55 Uh, and I did this experiment myself in, in my studies just to see how well did they, uh, respond to cold water. So it was, uh, four degrees Celsius cold water for four minutes, and then I just measured blood pressure and heart rate to see do they have like an activation of this. I actually also measured the brown fat during this, uh, uh, cold exposure for four minutes with an infrared thermography camera to see can I see that the brown fat is activated, and just be- just to te- uh, go back to the location of the brown fat. So usually, you cannot really see activation of your brown fat (laughs) because it's located, uh, centrally in your, uh, uh, around your central nervous system. Um, and, and the biggest depot, as you mentioned before, is up here, uh, under the, the clavicular bones.

Andrew Huberman 51:43 Mm-hmm.

Susanna Søberg 51:43 So, um, and very close to the skin's surface, and because it's so close to the skin's surface, I could measure it with this very expensive camera, you know, and it's not very feasible for people to go home and do this at (laughs) a dome because it takes a lot of practice, I can tell. Um, but we measured the brown fat with this, um, and, and I could see that after a few minutes that the activation was there and increase in temperature, uh, arose from that, um, activation, just four 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, it needs to be there p- to, to heat you up and heat, i- heat your inner organs.

Andrew Huberman 52:25 Well,

Andrew Huberman 52:26 I'm delighted to hear all of this, and I'll t- I'll tell you why. Uh, 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.

Susanna Søberg 52:38 Uh-huh.

Andrew Huberman 52:38 The whole room was cold. Uh, the guy who I worked for at the time, and Harry Carlile was a very accomplished physiologist, he came from this lineage, I don't know if this literature is still, um, discussed much, but it's a beautiful literature, um, uh, from Rothwell and Stock. They were the ones who discovered, um, non-exercise induced thermogenesis. The fact that people bounce, who bounce their legs a lot and move around a lot and-

Susanna Søberg 52:59 Oh, yeah.

Andrew Huberman 52:59 ... have a lot of kind of-

Susanna Søberg 53:00 Yeah.

Andrew Huberman 53:00 ... um, stochastic movement, um, burn up to 18- 1,800 calories more per day than people who sit more still.

Susanna Søberg 53:09 Fascinating.

Andrew Huberman 53:09 Incredible.

Susanna Søberg 53:10 Yeah.

Andrew Huberman 53:10 It just incredible.

Susanna Søberg 53:12 Mm-hmm.

Andrew Huberman 53:12 Um, I don't think that work deser- uh, gets as much attention as, as it deserves. Published in journals like Nature, so very, uh, fine journals. But in any event, uh, 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, um, Dr. Carlile, Harry, um, said, "Well, the key is to wear a T-shirt in here for about two, three days-"

Susanna Søberg 53:35 Yeah.

Andrew Huberman 53:35 "... and then you will cold adapt." I thought, "Well, wouldn't I wanna 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-

Susanna Søberg 53:53 Yeah.

Andrew Huberman 53:53 ... because the brown fat had expanded, um, or, uh, or added mitochondria or both, and I was perfectly comfortable in that environment. I also got very, very lean (laughs) -

Susanna Søberg 54:04 (laughs)

Andrew Huberman 54:04 ... in those, um, in those days and weeks.

Susanna Søberg 54:05 (laughs) Yeah.

Andrew Huberman 54:05 Now I've, 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.

Susanna Søberg 54:14 (laughs)

Andrew Huberman 54:14 But it was, it was striking, uh, what a powerful effect it had on my entire system of thermal regulation. And one of the things that I, uh, also delighted in when Cell Reports, um, Medicine published your study is they had an accompanying, um, press release that went out to, to those of us that receive press releases, and it described a, um, a saying in Scandinavia which is, um, essentially tr- I'm not gonna attempt to, uh, speak, uh, Danish.

Susanna Søberg 54:41 (laughs)

Andrew Huberman 54:42 Even though I have, uh, much of my family is in Denmark, uh, believe it or not, from Denmark. Um, we have a lot of Danes in my family. Um, I won't embarrass myself by trying to speak Danish as I did before the, the, uh-

Susanna Søberg 54:51 (laughs) .

Andrew Huberman 54:51 ... microphones were rolling. But, um, wha- that there's a saying that I, I think essentially, uh, translates to in the fall when wha- when you're approaching winter-

Susanna Søberg 55:02 Mm-hmm.

Andrew Huberman 55:03 ... 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-

Susanna Søberg 55:20 Yeah. (laughs)

Andrew Huberman 55:20 ... in order to be a little bit uncomfortably warm so that in this h- heat of the summer, you're better at cooling your body. Do I have that right?

Susanna Søberg 55:28 Mm-hmm.

Andrew Huberman 55:28 And maybe, do you know the saying and would you be willing to share it? Yeah, only the Swedes and, um, uh, and Danes will be able to understand. Um, maybe the Norwegians too. If you don't know it, that's okay.

Susanna Søberg 55:38 Yeah. So I know the, I know the, the concept of it because we say it, you should, you should wear less before winter and, uh, and more before summer, so yeah, so- (laughs)

Andrew Huberman 55:48 Oh, well, there it is in English, so, uh, it doesn't have to be esoteric, but, okay.

Susanna Søberg 55:51 (laughs) Yeah, and, and you're completely right, and I think this is the, th- this is just something that we know in the Scandinavian countries. I think that we, we, we, in- intuitively know this. But if we just go back a little bit in history, I think that, um, around the 1950s, the, uh, the Russian government went out and said, "Well, we should do something about the to- tuberculosis, uh, um, pandemic, uh, or epidemic that was at that, this time." So they, they wanted to have the, the people, um, be more resilient to the cold and also increase our immune s- uh, system. So in Scandinavia and actually also Russia, we, uh, put our babies outside to sleep, uh, in the prom. And that is like to, uh, also to, to get more resistant to the cold but also to increase our i- immune system. And we still do that in Denmark. So we-

Andrew Huberman 56:44 Do you really?

Susanna Søberg 56:45 Yeah, we do. Babies are taken out in the cold? In the snow, in frosty rain, everything. My two boys have been sleeping out in winter or at least the fir- their first many, three, four, five years because it's, like, very good for them and they get, uh, a better, uh, 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, (laughs) I must say, but I kind of, like, intuitively also knew because we have inherited this, uh, way of doing things with our culture. So, and I have heard people coming from the US saying, uh, "Danes are crazy. They put their babies outside in proms and leave them there and then they go inside and drink coffee (laughs) on the cafes." (laughs)

Andrew Huberman 57:33 (laughs) Well, I don't think Danes are crazy. I, I, I, I adore the Danes.

Susanna Søberg 57:37 No. (laughs) Yeah.

Andrew Huberman 57:37 Um, they're, they're amazing, uh, culture and people. I'm so fortunate to have family members, uh, from Denmark. But I did notice, so when, when we were in Copenhagen, uh, and I know, um, we, we saw you there, uh, uh, not long ago. It was June. Um, the water in the harbor was c- was cold-

Susanna Søberg 57:54 Yeah.

Andrew Huberman 57:54 ... for, for even though the Pacific is close to here, uh, which is very cold,, uh, felt pretty cold. But I, it was summertime-ish, um, so people were in summertime mode, right? T-shirts and shorts and things that, 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.

Susanna Søberg 58:21 Yeah.

Andrew Huberman 58:22 Um, not many people wearing sunglasses, so trying to extract as much photon energy from the sun as possible, which I support. Uh, as everyone knows that's a, I'm a big fan of getting sun. But also, um, when we did see swimmers, um, they were swimming in this cold water, uh, and like it was nothing, and their, the range in age of the swimmers was what 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, their 70s, 80s, maybe even 90s.

Susanna Søberg 58:55 70s, yeah, yeah.

Andrew Huberman 58:55 Really, uh, remarkable. Uh, vastly different than what you see if you go to the ocean here in, uh, Los Angeles or, or elsewhere. So, um, yeah, you Scandinavians are onto something with this.

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Andrew Huberman 1:00:17 I'd like to talk about your study. Um, if you could give us a little bit of the backdrop about what motivated that study and then, um, and then walk us through what you did. You know, who the subjects were, um, what you had them do, what you measured, um, in as much detail as you would like to share because I think it's such an important, you know, even, uh, fair to say landmark study because it also explored not just cold but sauna and the, 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, uh, I'm hoping at some point that you might, um, tell us some of the observations that you might have made that interested you that perhaps were not in the paper-

Susanna Søberg 1:01:02 Mm-hmm.

Andrew Huberman 1:01:02 ... 'cause that's one of the great benefits of sitting across from somebody who, who did the work in detail. So, um, yeah, if you could tell us about your study, um, and, uh, what you did and what you discovered.

Susanna Søberg 1:01:12 Thank you for that question, A- Andrew, I, I love to, like, 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, what happened before that, and human studies are very different from, from my study. My study, you can do a knockout of something and then everything is, like, perfectly matched and controlled. Doing human studies is very far different from that because people are different even in the groups. So yeah, but what we, um-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 m- medicine. And also, the studies that I did before brow- uh, brown fat was also, like, very much, uh, in, in the preventive side, like, how can we... That was about something else, but, uh, the sweet tooth and how can we lower our sweet tooth and stuff like that. So but after that, I wanted to, to do something new, so I looked into brown fat, got hired in this, uh, fantastic research group where they... It's a cell group, so they mostly did cell studies, and they didn't have anyone to do a human study yet. Um, and, um, but they really wanted me to, to do that. So I read upon 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, um, at that time, I was like, "Okay, 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 is, first of all, it was already done. (laughs) That was one thing, but the other thing was like, well, I wanted to, to see if we can do an, like ki- some kind of activity so we can have people move also, go and do something, do something together or whatever. And, and, and the cold, uh, made us think about, "Well, what about winter swimming?" And it was kind of like a bit of a joke in the beginning. I was like, "Winter swimming? Yeah, it's gonna activate the brown fat, right?" (laughs) But, 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 really... That was already there. So we were just thinking, "Okay, so it should be very potent, um, 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, uh, we knew that we were gonna do, like, a more of a, um, a, a proof of concept study, um, at the beginning of it because it was like, winter swimmers, um, must, in theory, activate the brown fat, right? But we kind of didn't really know was, was this kind of stress or too much, too little or what will happen actually. But we had this idea about, well, we always say that cold water and winter swimming will activate your metabolism, but do, but do we know if it do- does that? No, we don't. So, um, and while this idea was a little bit f- fun, fun at the beginning, um, we kind of accepted. It was like, "Okay, let's just try this out." But because we didn't have the funding for it, we was like, "Okay, let's do a proof of concept study. Um, let's go with a small number, but enough to see, um, a difference between the groups." So the power calculation of that study is done on, uh, what we know, uh, from PET/CT scannings of the brown fat. So that's the main outcome of that, of course. So, um, and we wanted to go a little bit small on the numbers of, of participants because we wanted to dig a little bit deeper into the different mechanisms and also redo some of the, the, the days. So I really wanted to do that to see if I can replicate also the findings. And that's gonna take a lot of, a lot of, uh, funding, but it's also gonna take a lot of time to do it. Um, so the proof of concept was just going small, but looking at different mechanisms. We also took, um, uh, 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, and the first year was like a field study for me, so I was not a winter swimmer when I started this. It was just- Oh, really? No, I wasn't, um, at all, and I would say I was a bit afraid of the cold myself. (laughs) Um, bit of a cold sissy, uh, always cold, having big socks on and sweaters and stuff like that. So I was like, "I am so comfortable. I'm just everybody else, very comfortable. I like being com- completely temperature neutral." Um, 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." Yeah, but the first year, observation of winter swimmers on the jetty. They kind of joked about it and say, "Come on, Susanne, you need to try this. You cannot study this, um, unless you have tried it." And I was like, "Ha ha, very funny. Of course I can do that." (laughs) But I couldn't. I, 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, uh, after a long break and you, and your muscles hurt the day after, right? You, you completely regret that you took that extra mile. (laughs)

Andrew Huberman 1:06:36 What about... When you say uncomfortable, you mean uncomfortable when you got in and when you were in, or uncomfortable afterwards? Because-

Susanna Søberg 1:06:42 Mm-hmm.

Andrew Huberman 1:06:43 ... I find that, um, on rare occasions... Well, I should just, uh, full disclosure, I, I do deliberate cold exposure every morning-

Susanna Søberg 1:06:52 Mm-hmm.

Andrew Huberman 1:06:52 ... for about a minute to two 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. (laughs) So, um, and we'll talk about why that's probably not the best idea, but, um... And I've been doing it for some years now, um, on and off. Uh, but, uh, so just full disclosure, I'm a devotee, um, and I have family members that, uh, hate the, the cold but have gotten into it and, and are starting to like it. But they don't, and I don't necessarily like the experience in the cold water, but I love the way I feel when I get out.

Susanna Søberg 1:07:29 After.

Andrew Huberman 1:07:29 And I have... 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, y- it's, it's far easier to, to push through. Um, 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 1:08:00 Yeah, it's, and very important to clear that out. I only felt very uncomfortable doing it at the moment, but afterwards, the first time I went, uh, with, uh, with a group, and actually my husband was a- as well, because I, I really wanted someone I knew, um, coming along. Because it's very normal if you haven't done this before, you feel a little bit anxious about it and... This is shown in studies as well because blood pressure and heart rate goes up in, in those who are new to this kind of activity. So, um, 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.

Andrew Huberman 1:08:36 Mm-hmm.

Susanna Søberg 1:08:37 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, you feel, y- I feel invigorated, I had so much energy. Uh, and that, like, I, 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, 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, uh, dips, um, in, in one round, you can say in one day, but, uh, often it's, like, just one or two times a week. For me, that is enough to, to get that energy and to get that positive feeling. And, and I think that that is also why I st- 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, um, 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 little bit extreme.

Andrew Huberman 1:09:44 Could you give me an example of a long time?

Susanna Søberg 1:09:46 Well, so maybe they were, like, really swimming-

Andrew Huberman 1:09:48 Mm-hmm.

Susanna Søberg 1:09:48 ... and they could be 20 minutes or half an hour.

Andrew Huberman 1:09:50 That's a long time.

Susanna Søberg 1:09:51 That's a long time, and there was, like, ice, and people who came up... I, I mean, I just didn't really feel that this is so- something that I wanted to go out and recommend to people after my PhD. (laughs)

Andrew Huberman 1:10:02 Yeah, you don't want any research subjects dying either.

Susanna Søberg 1:10:05 (laughs)

Andrew Huberman 1:10:05 Because if you're not adapted, I mean the people-

Susanna Søberg 1:10:07 No.

Andrew Huberman 1:10:07 ... p- you know, people can do that. Also, a 20-minute cold shower or a 20-minute cold plunge, I know people do it, but it's probably not a good idea.

Susanna Søberg 1:10:15 No, probably not.

Andrew Huberman 1:10:15 Yeah.

Susanna Søberg 1:10:15 It's gonna exhaust your cells and age, they'll make them age too fast.

Andrew Huberman 1:10:19 Mm-hmm.

Susanna Søberg 1:10:19 So exactly, that's, uh, when you pass that hermetic stress, this, the healthy stress level, that's what is happening, the quite opposite is, is,

Susanna Søberg 1:10:28 uh, almost chronic stress actually in the cells. Well, what happened then was that, um, I found out if, if I want to have this protocol get through ethical committee, I, I really needed to go, like, very, like, sleek with it, not too long, and, and make sure that they were also very healthy, and, and, uh, to get approval, of course, of this study. Um, but what I did was to, to recruit winter swimmers who already have been swimming for two or three seasons, and I just observed them. I said, "I'm not gonna do an intervention study yet." I did that after, but I, I wanted to do, like, a proof of concept where they were already adapted to the cold, and then compare them to a match control group, who were matched on, um, on you can say diet. So were they vegetarian or not, um, and one of them was in each group. Uh, also being-

Andrew Huberman 1:11:20 They weren't all vegetarians?

Susanna Søberg 1:11:22 No, no, no.

Andrew Huberman 1:11:22 Okay.

Susanna Søberg 1:11:22 Just one in each group, yeah.

Andrew Huberman 1:11:23 I was gonna say, with all the amazing fish and meat in, uh, in Denmark-

Susanna Søberg 1:11:27 (laughs)

Andrew Huberman 1:11:27 ... I'd have a hard time (laughs) being a vegetarian.

Susanna Søberg 1:11:29 (laughs) No, no.

Andrew Huberman 1:11:29 The breads are amazing, the fruits and vegetables too, but... okay.

Susanna Søberg 1:11:31 Yeah.

Andrew Huberman 1:11:31 So there were a couple of vegetarians in each group.

Susanna Søberg 1:11:33 Yeah. One-

Andrew Huberman 1:11:34 Okay.

Susanna Søberg 1:11:34 ... one in each, yeah.

Andrew Huberman 1:11:35 Okay.

Susanna Søberg 1:11:35 And they were matched on-

Andrew Huberman 1:11:36 Token vegetarian.

Susanna Søberg 1:11:37 (laughs)

Andrew Huberman 1:11:37 I have family members who are vegetarian, so I'm just poking fun.

Susanna Søberg 1:11:40 (laughs)

Andrew Huberman 1:11:41 But, uh...

Susanna Søberg 1:11:42 Yeah, but they were, they were matched on different things. So what we usually match them on is also BMI. Um, 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, uh, brown fat levels, uh, depending on gender. So women have more brown fat than men.

Andrew Huberman 1:12:03 Really?

Susanna Søberg 1:12:03 Mm-hmm.

Andrew Huberman 1:12:04 Hmm.

Susanna Søberg 1:12:05 Yeah.

Andrew Huberman 1:12:05 Interesting.

Susanna Søberg 1:12:05 Yeah. I think it's interesting. Uh...

Andrew Huberman 1:12:08 A deserved study.

Susanna Søberg 1:12:09 Yeah.

Andrew Huberman 1:12:10 Yeah.

Susanna Søberg 1:12:10 Why, actually? I think it's interesting because women are also smaller, so in size, mass, right? But they also have, um, a lower peripheral temperature, especially on hands and ears and, and-

Andrew Huberman 1:12:22 Is that right? That's, that's documented-

Susanna Søberg 1:12:24 Yeah.

Andrew Huberman 1:12:24 ... that w- women do run colder than men?

Susanna Søberg 1:12:26 Yeah. And there is also-

Andrew Huberman 1:12:27 Physiologically. I didn't say psychologically.

Susanna Søberg 1:12:29 (laughs) No, no, no.

Andrew Huberman 1:12:30 We won't, we won't go to the psychological cold, cold/heat... yeah.

Susanna Søberg 1:12:34 (laughs) That's different.

Andrew Huberman 1:12:35 Yeah.

Susanna Søberg 1:12:35 Something else.

Andrew Huberman 1:12:35 That's a different podcast. (laughs)

Susanna Søberg 1:12:37 Yeah. Another time. So women are just colder physically, uh, so on hands and ears, it's measured on that. Um, and, and, and feet as well, so compared to men. And men have bigger hearts than women, and they can pump out more blood, uh, peripheral, uh, than in a woman's body. So they... that could be an explanation for the colder hands, for example. Thermo-comfortable state is also different between gen- genders. So men are more comfortable at 22 degrees Celsius, and women are thermo-comfortable at 24 degrees Celsius. And this is-

Andrew Huberman 1:13:14 Okay, so the thermostat wars of home have been now validated.

Susanna Søberg 1:13:18 (laughs) Yes. Yes, there is a study

Narrator 1:13:19 Okay.

Susanna Søberg 1:13:19 ... that says this, yes.

Andrew Huberman 1:13:19 Two degrees Celsius. By the way-

Susanna Søberg 1:13:21 (laughs)

Andrew Huberman 1:13:21 ... prior to, um, starting recording, uh, I made the executive decision that we were gonna go with Celsius throughout the podcast, um, because the majority of the world uses Celsius. So for those of you that think in Fahrenheit, um, th- the internet is your friend in making those conversions. Uh, so we're sticking with Celsius. So men tend to be thermo-comfortable at 22 degrees Celsius, women at 24.... okay, interesting.

Susanna Søberg 1:13:47 It explains a lot about like also some arguments in the homes where men are turning down the heater and women are turning (laughs) up the heater, and they cannot really... So it's, it's really... it's... I'm on both sides here. I understand the men, we understand the women, but it... There is a difference there, which was also one of the reasons why we w- had... 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 see, to eliminate, eh, all the confounding factors which could have an impact on, on our results. So, um, that was one of the reasons. Um, but also because we... Yeah, so women have, have more brown fat than men, and otherwise, we would have to, like, do four groups or something like that. And not having funding yet, we were like, "Okay, we need to do, like, just one, eh, group, just a control group and then, uh, and a group who were already, uh, winter swimmers."

Andrew Huberman 1:14:42 Mm-hmm.

Susanna Søberg 1:14:42 So I recruited winter swimmers who have been swimming for two to three seasons, um, because I wanted them to be already adapted, but not going too long in the water. So they told me... Eh, I did a lot of screening here, of course, beforehand and interviews to see, to ask them, "How much do you do?" And, um, "How much do you s-... how long do you stay in the water?" And I, um, monitored how long did they then stay in the water and recruited based on, um, that they only did, like, two to three times per week. It seems reasonable, uh, for, for, for Denmark, at least, (laughs) to do that. And they stayed only in the water for one to two minutes. So

Susanna Søberg 1:15:21 the cold sub- subsides very quickly, and you will get this activation of your rest and digest system, which is your parasympathetic nervous system, so the, the other branch of your autonomous, uh, 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, uh, that's gonna slow down the, you can say, the, uh, the consumption of oxygen also in your body, and that's gonna slow down your heart rate. And-

Andrew Huberman 1:15:51 Could I pause you on this?

Susanna Søberg 1:15:51 Yeah.

Andrew Huberman 1:15:51 'Cause I've heard this before that when we get into cold water, shower or immersion-

Susanna Søberg 1:15:56 Mm-hmm.

Andrew Huberman 1:15:56 ... we get this sympathetic autonomic response.

Susanna Søberg 1:15:59 Yeah.

Andrew Huberman 1:15:59 So increased blood pressure, increased heart rate, release of norepinephrine from the locus coeruleus in the brain, release of, um, adrenaline, dopamine. Adrenaline from the, 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 that... and actually nowadays there s- seems to be a little bit of a movement online of people putting a bowl of ice water on their countertop and submerging-

Susanna Søberg 1:16:49 Yeah.

Andrew Huberman 1:16:49 ... their face into it.

Susanna Søberg 1:16:50 Yeah.

Andrew Huberman 1:16:50 Did you see this? There's a star-

Susanna Søberg 1:16:51 Yeah, yeah.

Andrew Huberman 1:16:51 ... obviously more and more posts about this.

Susanna Søberg 1:16:53 Yeah.

Andrew Huberman 1:16:53 So, um, could you just touch on the what the dive reflex is and why it acti- perhaps activates the parasympathetic response, this calming response?

Susanna Søberg 1:17:02 Well, so the diving reflex is activated when you