Now that the concept of cold training for cold adaptation and fat loss has received scientific support, I've been thinking more about how to apply it. A number of people have been practicing cold training for a long time, using various methods, most of which haven't been scientifically validated. That doesn't mean the methods don't work (some of them probably do), but I don't know how far we can generalize individual results prior to seeing controlled studies.
The studies that were published two weeks ago used prolonged, mild cold exposure (60-63 F air) to achieve cold adaptation and fat loss (1, 2). We still don't know whether or not we would see the same outcome from short, intense cold exposure such as a cold shower or brief cold water plunge. Also, the fat loss that occurred was modest (5%), and the subjects started off lean rather than overweight. Normally, overweight people lose more fat than lean people given the same fat loss intervention, but this possibility remains untested. So the current research leaves a lot of stones unturned, some of which are directly relevant to popular cold training concepts.
In my last post on brown fat, I mentioned that we already know a lot about how brown fat activity is regulated, and I touched briefly on a few key points. As is often the case, understanding the underlying biology provides clues that may help us train more effectively. Let's see what the biology has to say.
Biology of Temperature Regulation
As warm-blooded mammals, maintaining a stable core body temperature is essential for our survival. Since the rate of chemical reactions depends heavily on temperature, and our reactions are optimized for 97-99 degrees F, we must maintain a stable core temperature to support core organ function. For this reason, we have a sophisticated temperature control system that acts to maintain thermal stability, also called homeostasis (3).
As with most large-scale homeostatic systems in the body, body temperature is controlled by the brain, and particularly a brain region called the hypothalamus (3). The brain senses changes in core temperature via "thermometers" in the brain and the abdominal cavity. In addition, the brain uses "thermometers" in the skin to detect situations that threaten future core temperature stability (3). In other words, if the skin is cooling rapidly, this tells the brain that the core is at risk of cooling soon. This allows us respond to thermal challenges before core temperature is compromised.
Humans maintain body temperature by a hierarchy of sequentially deployed responses that are coordinated by the brain. Behavioral responses are one of the first lines of defense. Humans can use a wide variety of behaviors to maintain thermal stability, such as putting on a sweater, moving under the shade of a tree, turning on the heat, and changing posture (e.g. crossed arms and legs vs. sprawled out limbs). These behaviors "feel good" because they are made rewarding in situations where they help maintain thermal stability. This is analogous to how food is more motivating and tastes better when you're hungry.
Changes in the diameter of small blood vessels (capillaries) in the skin are another early line of defense against cold or hot temperatures. In cold conditions, capillaries contract, limiting blood flow to the skin and extremities and therefore limiting heat transfer from the core to the environment. In this scenario, the skin and extremities cool down, acting as a thermal insulator for the core. In hot conditions, capillaries dilate to allow the dissipation of excess heat generated inside the body. Another line of defense is sweating, which takes advantage of the large amount of heat that is absorbed by the evaporation of water.
Other lines of defense against cold are deployed later, presumably because the're energetically costly. These include non-shivering thermogenesis (via brown fat) and shivering. Brown fat is activated first, and if that isn't sufficient, shivering kicks in to make up the shortfall. The brain is reluctant to deploy these strategies because they're extremely costly: you're burning a lot of calories for heat that could be used to do other useful things instead, like performing muscular work, reproducing, and building or maintaining tissues. These lost calories may also have to be replaced by seeking more food. Not an ideal situation in a natural environment where food may be limited and obtaining it presents risks.
The brain lets you know that it prefers not to burn calories for heat by making you feel uncomfortable. This is negative reinforcement and negative affect, the opposite of what happens when you do something your brain 'likes'. Shivering is uncomfortable, and brown fat activation probably also correlates with mild to moderate discomfort. Your brain would rather you put on a sweater than waste calories through non-shivering thermogenesis or shivering.
Understanding the underlying biology can often help us understand how best to apply techniques that are based on biological processes. Here are a few points of informed speculation that I'll offer to people who are interested in cold training for fat loss or performance.
1. You don't have to lower your core temperature to activate brown fat. The brain activates brown fat in response to perceived threats to future core temperature, including a rapid drop in skin temperature. This is why cold showers probably activate brown fat strongly, even when the exposure time isn't sufficient to lower core temperature significantly.
2. Simply revving up your brown fat for brief periods may not make much of a difference in calorie balance or fat loss. Like muscle tissue, what burns calories is using brown fat, not just having the capacity to use it. Resting metabolic rate for muscle and brown fat tissue are both low, despite both tissues having the capacity to use a huge amount of fuel when activated. You can have a highly trained non-shivering thermogenesis system, but if you don't use it for more than a few minutes a week, it may not make much difference in your body weight. Still, having a highly trained system does make you more resistant to cold stress, which may be advantageous, and may also have metabolic benefits. Also, it remains possible that brown fat activation has neuroendocrine effects on body fatness and metabolism that go beyond calorie use.
3. The sensors that detect temperature for the purposes of activating brown fat are not, to my knowledge, located in brown fat itself (3). Some people have used ice packs for cold training. There's probably no advantage to putting ice on your upper back, where brown fat is located, relative to any other patch of skin on your torso. In fact, I might actually avoid putting ice on my upper back because the more you cool your brown fat, the slower uncoupling will proceed. Ice may also be a weaker stimulus than cold water due to the smaller surface area of skin it targets. Water doesn't actually have to be that cold to activate non-shivering thermogenesis, due to its high conductance.
4. If you don't feel uncomfortable at all, it probably isn't doing anything. When the brain perceives a thermal threat, it initiates a coordinated physiological and behavioral program designed to oppose core temperature change. Part of that program includes activating brown fat, and part of it involves making you feel uncomfortable so you'll hurry up and put on a sweater or stop standing in a cold shower and wasting energy.
To illustrate that principle with an example, if you're overheated and you get into a cold shower, it feels good. Cold water doesn't activate your brown fat in that scenario, because it's helping you achieve thermal homeostasis, not posing a threat to it. Likewise, some people have found ways to feel more comfortable during cold training, such as keeping extremities warm. Since feeling uncomfortable is part of the program that the brain turns on when it perceives a cold threat, I wonder if these strategies make cold training less effective by reducing the degree of brown fat activation. You may not need to feel very uncomfortable to activate brown fat, but if you don't feel uncomfortable at all then I question whether you're having the desired effect.
I look forward to research on these questions so we can have more definitive answers. Any additional thoughts on cold training are welcome in the comments.