It’s been pretty hot here in the UK recently (at least by our standards). You may have found yourself yearning for a cold bath to beat the heat, but submerging yourself in cold water could do more than offer temporary relief from warm weather. Research suggests that deliberate exposure to cold temperatures could have a variety of health benefits, and may even slow down the ageing process. Here, we’ll give an overview of these benefits, and how you can get the most out of cold exposure.
To understand what the benefits of cold exposure are, we first have to understand what happens when you take a cold plunge, shower, or any practice that significantly lowers the temperature of your skin. As soon as you are exposed to cold temperatures, cold receptors in the skin send signals to an area of the brain called the hypothalamus, which is responsible for maintaining body temperature by coordinating the release of different hormones. The hypothalamus then activates a stress response by triggering the release of cortisol (aka the ‘stress hormone’) and noradrenaline (which is similar to adrenaline in function).
We usually associate stress with negative health outcomes, but the stress we are concerned with here is not psychological but physiological – the body is being cooled and must generate its own heat in order to maintain its core temperature. The main tissue responsible for generating this heat is a special type of fat known as brown fat (or thermogenic adipose tissue if you want to get fancy).
Brown fat cells are packed with mitochondria, the ‘cellular organs’ responsible for converting nutrients from food into the cellular fuel ATP. Mitochondria also have a ‘heating mode’ in which they can consume fat to produce a lot of heat instead of powering the cell. The stress response triggered by cold exposure activates the brown fat, which starts burning fat in order to warm the body. Cold receptors in the skin also signal directly to the brown fat and activate it. This means that brown fat becomes active almost immediately when you are exposed to cold temperatures.
If you remain cold for some time, you will also begin to shiver. Muscles make small, rapid contractions in order to generate yet more heat. Both brown fat and shivering consume calories, but the real benefits of cold exposure are the long term effects.
In exercise (another beneficial form of physiological stress), repeated physical exertion will increase your ability to exercise harder and for longer. Likewise, you can become adapted to cold exposure. Each time your cells are forced to respond to cold temperatures, they are ‘trained’ to respond faster and more effectively next time. This is mainly due to an increase in the number of mitochondria in each brown fat cell, as well as the growth of brown fat tissue. Peripheral blood vessels also get better at constricting, allowing them to cut off blood flow to the extremities in order to conserve heat.
These changes seem to bring about beneficial changes in blood pressure and blood sugar while protecting against obesity. Studies in winter swimmers and cultures in which winter cold submersion is common show that these practices increase sensitivity to insulin, the hormone that controls blood sugar.
Insulin resistance (in which cells don’t respond correctly to insulin) is closely linked with inflammation and most age-related diseases, so any practice that improves insulin sensitivity is likely to be very beneficial. The short-term response to cold also burns fat stored within white fat tissue (with this fat being a contributor to inflammation) and activates muscle tissue, which is important for controlling blood sugar.
Getting cold should feel unpleasant to an extent – the unpleasantness is due to the activation of the stress response, which is the goal. The more adapted you are to cold, the less unpleasant it will feel. Most people find that going from hot to cold feels good, which is one reason why many people use a sauna to warm up before entering cold water.
What type of cold?
Research suggests that the greater the area of skin exposed to cold temperature, the stronger the activation of the stress response. However, even submerging a hand in cold water seems to be enough to activate brown fat, as is sleeping in a cold (19°C) room with only your head poking out from under the covers. So yes, going out in the cold wearing only a T-shirt should still be beneficial, just not as beneficial as a cold plunge. There’s not much data on the benefits of cold showers, but they are probably somewhere in between cold plunges and T-shirts, depending on whether you stand directly under the shower.
Studies comparing different cold immersion temperatures are lacking, but existing studies have typically used temperatures between 2 and 15°C and found beneficial effects. Colder temperatures are likely to be more beneficial.
There’s no definitive answer, but studies suggest that just a few minutes of cold exposure is long enough to yield benefits. While the initial response to cold exposure is beneficial, a prolonged stress response may not be a good idea, especially if you aren’t cold adapted. 20 minutes is probably too long.
Getting out of the water
When you leave cold water, the skin begins to warm up, which causes peripheral blood vessels to dilate. This means that warm blood from the core can flow to the skin, resulting in a drop in core body temperature. It can take quite a long time for temperature throughout the body to be restored to normal after cold immersion, which is why some people suggest you should let the body warm up naturally (not take a warm shower or use a sauna, for example) so that you can extend the benefits of cold immersion. There’s no scientific consensus on this, however.
There’s plenty of evidence to show that cold immersion is beneficial, but not much evidence about the best way to do it. It’s difficult to compare different methods, because there are so many variables: the temperature of the water, the duration of the exposure, the surface area of skin covered (which of course depends on the size of the person) and more. However, if you keep the above information in mind, you should be able to find the method that is right for you while obtaining close to the maximum benefit possible.
Effect of Acute Cold Exposure on Energy Metabolism and Activity of Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis https://doi.org/10.3389%2Ffphys.2022.917084
Cold and Exercise: Therapeutic Tools to Activate Brown Adipose Tissue and Combat Obesity https://doi.org/10.3390%2Fbiology8010009
Title image by Tobias Oetiker, Upslash
Brown Adipose Tissue and Its Role in Insulin and Glucose Homeostasis https://doi.org/10.3390%2Fijms22041530
Dr. Susanna Søberg: How to Use Cold & Heat Exposure to Improve Your Health | Huberman Lab Podcast https://www.youtube.com/watch?v=x3MgDtZovks