Updated: Oct 24
Cold thermogenesis keeps you young
Loss of brown adipose tissue (BAT, or brown fat) is associated with ageing & mitochondrial dysfunction. Targeting brown fat may increase healthspan.
Since tumor cell metabolism radiography accidentally detected brown fat in adult humans back in 2002, research related to the benefits of brown fat has grown exponentially.
Increased levels of brown fat are associated with reduced risk of metabolic disorders & related diseases, improved body composition, and better mental health.
The best way to activate, maintain, and recruit new brown fat in adult humans is deliberate cold exposure, such as whole body cold water immersion in an ice bath.
To increase brown fat, choose a temperature that makes you gasp and a duration that makes you shiver. Don't be surprised when your body fat scanner is fooled by your results.
Ice baths boost brown fat
Babies have it best
One of the unique characteristics of human evolution is that our infants are born more immature and helpless than any other mammalian creature. The complexity of our enormous brains, and the pelvic constraints demanded by walking upright, make it impossible for human Mothers to birth anything but the most vulnerable of little, underdeveloped creatures.
As babies, we require extraordinary levels of care and protection from the dangerous environment during the long period of maturation from childhood to maturity. As I wrote in Cold Water Immersion Shaped Our Evolution, our ancient Grandmother's likely eased the pain and risks of childbirth by delivering their babies while immersed in cold, glacial-melt waters. That would explain two unusual things about human babies:
Why they have the instincts and the motor control at birth to swim, but not crawl, and
Why human babies are born with copious quantities of brown fat (i.e., brown adipose tissue).
Brown fat performs several critical functions, including the production of thyroid and other hormones. However, its most important purpose is non-shivering cold thermogenesis (Nedergaard et al. 2017)
When babies are exposed to cold, they don't shiver like older children and adults do. They don't have sufficient muscle mass. Instead, they have brown fat to burn up glucose and fats and generate the heat necessary to keep them warm.
Researchers in Spain confirmed the activation of brown fat in newborns by immersing one foot in cool (66F) water while measuring the skin temperature between the baby shoulder blades. Thermographic imaging showed increased heat production in the areas where brown fat is expected to be active, leading researchers to conclude that even mild cold exposure was sufficient to stimulate cold thermogenesis in newborns. What's more, they discovered that new born babies who had been cold exposed on their birthdays exhibited superior thermogenesis during cold exposure the next day, compared to babies that had never been exposed. They suggested that:
Cold exposure at the very early stages of postnatal life promotes brown adipose tissue (BAT, also called brown fat) thermogenic responses leading to a better defense of body temperature in subsequent cold events. - Urisarri et al. (2021).
Brown fat shrinks with age
Not only are babies born with a large quantity of brown fat to defend their core body temperature, but their even their little newborn bodies will acclimate to cold exposure by strengthening thermogenic response mechanisms. From an evolutionary biological perspective, it probably makes sense that the human body is equipped with the capacity to adapt to its environment.
But what happens to the human body that is not exposed to cold?
As the body ages and muscles mature, it builds capacity for shivering thermogenesis -- e.g., chattering teeth. Without regular exposure to cold, brown fat depots shrink, even as the rest of the body grows.
In industrialized populations that are housed in comfortable, climate-controlled conditions and accustomed to warm baths and hot showers, brown fat shrinks far below levels typical of post-pubescent muscle maturity. For example, a study of over 15,000 PET scans from the Netherlands indicated that the likelihood of identifying brown fat in human subjects in their 20's exceeded 15%, but declined to less than 4% among subjects in their 50's (Steinburg et al. 2017). Similar results were obtained in a separate examination of 5,000 scans at the Sloan Kettering Cancer Center in New York City (Becher et al. 2020).
By age 50, 95% of Americans have no detectable brown fat at all.
For decades, doctors believed that human adults just didn't have any brown fat at all. They were wrong, but in their defense brown fat is difficult to detect. Although autopsies did reveal brown fat in children and young adults (e.g., Heaton 1972), it was so rare in middle-aged adults that most medical researchers believed that all human beings eventually grew out of it. Even when brown fat was positively identified in seven out of eleven outdoor workers in northern Finland (compared to zero positive detections from autopsies of indoor workers, Huttunen et al. 1981), those results were largely ignored.
It wasn't until the 1980's, when positron emission tomography (PET) for imaging metastatic cancers was adopted, that clinicians began to question the assumption that human adults held no capacity for brown fat. Finally, in 2007, three Swedish researchers published a paper claiming that PET images conducted for cancer imaging in cold hospital instrument rooms consistently showed activation of brown fat between the neck and collarbones, and along the spine between vertebrae (Nedergaard et al. 2007).
Their assertion ignited a fifteen years of furious new research activity related to human brown fat, and a series of discoveries regarding the role it plays in cold thermogenesis, regulation of metabolism and body composition, and as an essential secretory organ for production of hormones that modulate the maintenance of brain and bone cells.
Benefits of brown fat
Given the association of increased levels of brown fat with metabolic health, and concomitant reduction in risks of metabolic, thyroid, insulin (Type 2 diabetes), cardiovascular, and health disorders, understanding the reason for the loss of brown fat over time may be critical to prescriptions for health and longevity among older Americans. For example, brown fat does more than keep us warm in the cold. Brown fat produces more thyroid hormone than any other organ in the body. According to Ben Bikman, PhD and author of Why We Get Sick (Bikman 2020) "over half of all the thyroid hormone in the body is coming from brown adipose tissue."
Brown fat secretes other hormones, too. One of them is called FGF21, and it has been shown to reverse cognitive deficits in rodents. As I wrote in Brown Fat For Brain Health, increased levels of brown fat may protect against Alzheimer's dementia.
However, most research related to the benefits of brown fat relate to metabolism and body composition. The most important findings are that increased levels of brown fat are associated with lower body mass index, improved insulin sensitivity, and reduced levels of white belly (i.e., visceral) fat.
Belly fat is accumulated around the internal organs of the body. It is characterized by increased waist size, a pot belly, and associated with a myriad of adverse health outcomes, including increased risk of heart attack. Increased levels of brown fat are associated with a healthier distribution of white fat, reducing the portion of white fat around the belly in favor of fat beneath the skin. The latter, called subcutaneous white fat, is comparatively benign.
Even more remarkable is the finding that increased brown fat is associated with increased liver density, due to reduced fatty deposits in the liver (Wibmer et al. 2021). The health risks of non-alcoholic fatty liver disease can be life-threatening. They include: abdominal swelling (ascites), jaundice, cirrhosis, and liver failure. This relationship, and the fact that activation of brown fat has been shown to inhibit tumor growth in mice (Seki et al 2021) explain how author AJ Kay shrunk her inoperable liver tumor.
These findings suggest that maintaining levels of brown fat typically associated with young adulthood offers protections from diseases associated with ageing, including: insulin resistance, Alzheimer's, obesity, heart disease, cancer, and liver failure. In one study of 5000 patients, those with brown fat were found to have lower prevalence of Type 2 diabetes, lower triglycerides levels, lower incidence of heart disease, and reduced risk of high blood pressure (i.e., hypertension -- Becher et al. (2020).
The association between brown fat dysfunction and the chronic diseases of ageing is so strong, that several researchers have suggested targeting brown fat has the potential to promote lifespan and healthspan (e.g., Darcy & Tseng 2019).
How do ice baths activate brown fat?
The mechanism by which brown fat is signaled for non-shivering cold thermogenesis has only recently been revealed. Thermoreceptors do not communicate directly with brown fat. Instead, they send signals to the hypothalamus, that subsequently activates brown fat via the sympathetic nervous system.
The sympathetic (compared to parasympathetic) is the division of the autonomic nervous system responsible for the fight-or-flight response to threats. It typically increases heart rate, blood pressure, and prepares the body and the metabolism for physical action. For example, sympathetic activation causes the liver to release glycogen to spike blood sugar, and white fat cells to release triglycerides into the blood stream in a process called lipolysis. These dual substrates are instrumental to fuel rapid action, should the body need energy fast.
Sympathetic activation also produces norepinephrine, which can double or triple during acute cold exposure. It is the norepinephrine that activates brown fat, where the increased glucose and lipids are converted into heat. This phenomenon explains the data from continuous glucose monitoring devices that I showed in Deliberate Cold Exposure for Diabetes. Healthy subjects experience an immediate boost in blood glucose upon entering the ice bath, caused by the sympathetic nervous system activation of glycogen release from the liver, only to see levels come rapidly back down as their brown fat clears the glucose and converts it into heat. However, subjects with Type 2 diabetes who begin with elevated blood sugars do not experience a further increase. It is as if their livers sense that blood sugar levels are already high enough to support action, and no further boost is required. In those hyperglycemic subjects, blood glucose drops fast and stays down for hours, because activation of brown fat is an effective mechanism for increasing insulin sensitivity.
While glucose provides fast-acting energy, the total body reserves of glucose are limited. The body can store much larger quantities of energy in white fat depots than in liver glycogen. While ordinarily the liver can store enough carbohydrates for about one whole day of fasting, the consumption of glucose during non-shivering cold thermogenesis will accelerate depletion. When liver stores eventually run out, energy metabolism must rely exclusively on fat, resulting in a condition called ketosis. To avoid low blood sugars (hypoglycemia) during ketosis, the liver synthesizes the minimum glucose necessary from non-carbohydrate substrates in a process called gluconeogenesis. However, brown fat metabolizes both glucose and triglycerides immediately upon activation, without waiting for depletion of glucose stores. In this way, an ice bath is the fastest route to endogenous ketone production.
Given the extraordinary benefits of increased brown fat, pharmaceutical companies have made brown fat activation and recruitment a target of drug development. For example, it has long been known that foods like hot peppers and herb extracts like berberine will activate brown fat in a phenomenon called diet induced thermogenesis (Wu et al. 2019, Suchaki & Stimson 2021), but several pharmacological trials have so far failed to improve upon their effects.
Cold exposure is undoubtedly the most effective regimen to activate and recruit BAT. - Saito et al. (2020).
Compensatory metabolic mechanisms prevent weight loss
The immediate increase in energy consumption, as well as the correlation between higher brown fat and leaner body composition, has led several researchers to hypothesize that deliberate cold exposure might be an effective therapy to manage obesity. Given that more than 50% of the adult American population is now either obese or severely obese, any safe mechanism for boosting metabolism and increasing energy expenditure warrants further attention. Moreover, managing the health implications of adult obesity likely costs more than $173 billion/year as recently as 2016 (Ward et al. 2021).
The problem is that decades of research have yet to discover mechanisms by which the added energy expenditure induced by acute cold exposure will lead to reliable weight loss. As I wrote in Calories & Cold Exposure, a regular ice bath practice will correct metabolic disorders, and may improve your body composition, but it will not lead to weight loss from increased calorie expenditure.
This counter-intuitive conclusion merits some explanation, because there is no dispute that whole body cold water immersion results in increased metabolism for thermogenesis. The problem is that compensatory metabolic mechanism likely reduce energy expenditure after rewarming. For example, researchers at University of Colorado Medical School studied the effects of intermittent cold exposure on obese mice placed on a calorie-restricted diet. Both cold-exposed and a control group of mice lost 20% of their body mass during their diets, and careful measurement showed that cold-exposure increased metabolism via thermogenesis, and increased brown fat depots. Nonetheless, total energy expenditure in the cold-exposed mice was no greater than in the control group, because basal metabolism in the cold-exposed group dropped at night, conserving enough calories to compensate for the increased consumption due to cold exposure during the day (Presby et al 2019).
Personal experience suggests that the benefits to body composition from deliberate cold exposure are indirect. For example, Adrienne Jezick cured her Hashimoto's and lost nearly 30lbs after starting a practice of regular ice baths. Nonetheless, these improvements were unlikely to have resulted solely from caloric deficit. More likely, the ice bath help her reduce inflammation, while signaling her body to recruit new brown fat. The thyroid hormone produced by her increased levels of brown fat likely normalized her thyroid function, which corrected a metabolic disorder that resulted from growing up in Florida, living in Arizona, and "hating the cold." In other words, without the cold exposure her body is programmed by evolution to expect, her thyroid dysregulated, and her body composition changed. Ice bath created the conditions necessary for her body to correct her metabolism, and weight loss likely followed.
Body fat scanners & brown fat
Measuring body fat is an inexact and laborious science, but now that body impedance meters are available at gyms for for home use, we can get more data on body composition than ever before. For example, I started my weight loss journey shortly before I turned 50 years old. During that time, I made regular measurements of my own body fat composition using an InBody scanner at my local gym. By the time I'd dropped from almost 250lb to around 195lb (at 5'-11 3/4") my body fat composition dipped as low as 20.3%.
That was before ice baths.
Here's a timeline of my measurements from 2018:
According to the InBody scanner, in May 2018 I'd gained 3lb of fat and lost more than 3lb of muscle, while my body fat percentage jumped from 20.3% to 22.0% -- in less than a month. In men, a body fat percentage of 25% is considered obese, which meant at the rate I was going, I was going to back in the "obese" category in about six weeks. Despite the fact that my weight hadn't budged, the InBody scanner was sounding an alarm.
Jason and I founded Morozko Forge in May 2018, and that is when my regular practice of ice baths began in earnest.
It makes sense that the ice baths would boost my brown fat, alter my body composition, and change the InBody scan measurements. As it turns out, electrical impedance measurements do not yield reliable body fat percentage results, because they do not measure fat directly. Instead, they measure total body water. Then, they make assumptions about the water content of fat-free body mass, and estimate body fat percentage based on other parameters including height, weight, and age (Ward 2019).
The statistical parameters employed by the body impedance analysis were calibrated for the general population, and you can surmise from the graph on brown fat and ageing that any sampling of the general population of 52 year olds will not included many subjects with detectable brown fat. In other words, the InBody scanner was detecting a change in my body composition resulting from my ice bath practice -- it just didn't know how to interpret that change.
Which is not an experience that is exclusive to middle-aged fat guys like me.
The other day I got an email from Kinsey Williams, co-Founder of Source Float & Wellness in Midland, TX. Kinsey is decades younger than me, in much better shape, and regular user of the Morozko ice bath at his float spa.
Although he and I have different bodies, just like me, Kinsey's InBody scanner was giving him different results, starting this summer. If the electrical impedance calculations are to be believed, Kinsey gained 10lbs of fat, and lost 2.5lb of muscle.
But Kinsey told me that didn't sound right to him. This is what he wrote:
The scanner numbers don’t sound right, actually. I’ve stayed very consistent with using the ARX & even use the LiveO2 oxygen contrast training in conjunction with sauna/forge, with a very healthy primal style food intake with lots of local protein & fruit. I don’t eat pasta & grains. I was thinking I’ve added on lots of brown fat, but my body doesn’t show that gain. - Kinsey Williams, co-Founder Source Floatwell
Despite Kinsey and I reporting similar experiences, most people will probably believe the machine over the man, but Kinsey sent a video of his workout, so we can all judge for ourselves. If there's 18.3lb of body fat on that man, I can't find it.
That tells me that when you start a regular practice of intense cold exposure, you have to reset your expectations of your body scanner. As your body adds brown fat, your results are going to show more body fat mass. And even though the InBody scanner might say that your increased fat levels are unhealthy, they're not, because electrical impedance measurements can't distinguish between brown and white fat, or between visceral and subcutaneous. As a result, the InBody will underestimate your muscle mass, and over estimate your fat as your acclimated to cold exposure.
Protocols for building brown fat
Although it has been well-established that deliberate cold exposure is the most effective way to activate and recruit brown fat, no one knows what minimum effective dose will result in gains. One of the most comprehensive studies of brown fat and cold exposure was recently conducted in Denmark. Researchers at University of Copenhagen compared two dozen "winter swimmers" -- i.e, people who combine sauna with a plunge in cold lakes or fjords -- with a similar control group without experience with cold water therapy.
They discovered that the winter swimmers had more brown fat, higher levels of thermogenic energy expenditure in response to cold, and better cold tolerance, compared to the control group (Søberg 2021). They also measured core body temperatures throughout a 24hr cycle, and discovered that winter swimmers do experience lower nighttime body temperatures that compensate for increased metabolic activity during the cold -- just like the rats in the weight loss study at University of Colorado. In other words, the winter swimmer's regular cold exposure practice had exactly the effect we would expect: successful cold acclimation through increased brown fat.
But how much cold dose were they getting?
On average, it was only eleven minutes a week, and it didn't seem to matter whether it came all at once, or spaced out over several sessions. Some winter swimmers did three plunges all in one day by alternating them with sauna sessions. Others did single plunges, several days a week. One swimmer didn't use sauna at all.
The researchers note that brown fat (compared to white) is highly innervated, which is to say that it contains a greater density of nerve fibers that likely increase responsiveness to the sympathetic nervous system. Although cold acclimation does reduce the shock of cold wat
er immersion, any exposure that creates a sympathetic nervous system response may be sufficient to activate brown fat. For example, the study explains that "winter swimming for 1 min or less increases norepinephrine plasma levels acutely," exactly as prior studies have shown. As we learned earlier, norepinephrine is what activate brown fat. Thus, even a microdose of ice bath might be enough to retain youthful levels of brown fat.
Although everyone is different, there are two signs to look for when cold plunging to increase brown fat. The first relates to temperature, and the second to time.
The gasp reflex. When you feel the involuntary, physiological response to suck your breath in fast as you first enter the ice bath, that's the gasp reflex. That's how you know you've activated your sympathetic nervous system and you will soon be producing norepinephrine to activate your brown fat. The temperature of the water governs the gasp reflex. If you're not feeling it, go colder.
Shivering. For the purpose of managing weight, some scientists advise ending your thermal contrast session with cold exposure, to force your body to rewarm exclusively through cold thermogenesis. This technique maximizes energy consumption during the day, and it might help you sleep at night, but as we've already seen it's not likely to help you lose weight. However, when you're cold plunging for brown fat, and not calorie burn, one way to be sure you've entered cold thermogenesis when you feel the urge to shiver. When your muscles begin shivering, that tells you that your brown fat is already busy, and can't keep your core body temperature up by itself. The urge to shiver is proportional to the duration of your ice bath, because it takes time to drop your core temperature far enough to max out the non-shivering thermogenesis in your brown fat. Cold-acclimated people will have to stay immersed for longer, because they already have more brown fat.
How often, how cold, how long to ice bath?
There is no single protocol that is ensured to provide your minimum effective dose for activating and recruiting brown fat. However, you can design your own protocol that works for you by keeping these rules of thumb in mind.
Frequency. We know that 11 min/week of winter swimming is effective, no matter how you divide it up. So plan your ice bath practice around getting cold at least once a week. For example if you use the ice bath at a local med spa or wellness center, and it's not convenient for you to go there several times a week, then at least go once. During that one weekly session, extend your plunge to try for 11 minutes.
Temperature. Although you can obtain some metabolic benefits like increased insulin sensitivity at temperatures as warm at 60F, you aren't likely to build much brown fat until you set your cold plunge to a temperature that frightens you. Go cold enough to feel the gasp reflex.
Duration. Even regular, cold-acclimated ice bath practitioners sometimes shiver, but not always when they're in the ice bath. A phenomenon called afterdrop will cause your core body temperature to keep falling for awhile, even after you emerge from the ice bath. In afterdrop, blood circulation returns to your cold limbs, where it loses heat and drops in temperature before returning to the heart. It is this cold, returning blood that causes a shivering response. When establishing a ice bath duration that is right for you, consider what duration will cause you to feel the urge to shiver, even if the shivering doesn't start until several minutes after you're dry.
Finally, the definitive test of brown fat is expensive and requires specialized equipment. It's not something you can monitor at home. However, if you're using an electrical impedance body fat scanner like the InBody, you'll know you've started adding brown fat when you see the body fat percentage jump on your scan results even though you haven't gained any weight.
Fooling your electrical impedance body fat scanner into thinking you've lots muscle and gained fat may be the surest measurable indicator that you've increased your brown fat, and started ageing in reverse.
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