Updated: Nov 18, 2022
Minimum effective cold dose depends on acclimation
Although we emphasize that cold exposure should always be deliberate, to avoid the dangers of accidental, there is nothing in the scientific literature for measuring what we would call "dose" for deliberate cold exposure.
When I was a graduate student in environmental engineering, I took a course in toxicology that introduced one of most bedrock principles in environmental risk called the dose-response curve. In most of our engineering classes, we learned that more is usually better, to provide for a factor of safety. In business classes, when it comes to profits, more is always better.
But in biology, that is not the case. A typical dose-response curve is non-linear.
Too little is harmful. Just enough is healthy, and then too much is worse.
The figure shows a typical dose-response curve. At the left edge, where the dose (e.g., magnesium) is too low, you suffer from nutrient deficiency. As dose increases, the deficiency subsides and your health improves, but at the right edge even essential nutrients become poisons when the dose is too high.
The same is true for deliberate cold exposure. Not enough cold, and your metabolism will suffer from ill-effects, including thyroid dysregulation and insulin resistance. Too much cold can lead to hypothermia.
Most people don't need a graduate class in toxicology to understand this, because it makes intuitive and experiential sense. That's probably why we often get the question, "How much ice bath is just the right amount?"
Because none of the hundreds of scientific studies on deliberate cold exposure, cryotherapy, winter swimming, or ice baths establish a standardized comparative measure for cold exposure dose, I decided to invent one for whole-body cold water immersion.
There are two important variables: 1) water temperature, and 2) time. It stands to reason that the colder the water and the greater the time, the higher the dose.
But how should these variables be combined?
As a first approximation, heat transfers is proportional to temperature differences. When the water in he ice bath is colder than the temperature of your skin, then heat will transfer from your skin to the water.
But reality is more complicated, because every body is different in their physiological response to the cold. As you gain experience in the ice bath, your smooth muscle tissues will adapt to improve your vasoconstriction to cut off blood circulation to your extremities, thereby reducing heat transfer. And the mitochondria in your brown fat will burn glucose and fatty acids to generate heat via non-shivering thermogenesis, thereby keeping your core warm.
To get a measure of the effects of temperature on dose, we must first establish your thermal comfort line. This is the water temperature that feels neutral to you. Average skin temperature is cooler than core temperature -- about 80F -- so the average beginner won't feel cold in water that measures about 80F.
Let's call the difference between your thermal comfort line and water temperature your delta, which is the portion of the dose that is due to temperature effects.
To calculate your total dose, multiply your temperature delta by your time of whole body exposure. That means that someone with a thermal comfort line at 80F would have to spend twice as much time at 60F to get the same cold dose they would get at 40F.
More experienced cold practitioners are going to have a lower thermal comfort line. From me experience with winter swimming, I find that anything above 60F barely registers for me as cold anymore. That is, my sense is that I could be in 60F water almost indefinitely, so for the sake of argument, I might take that as my thermal comfort line.
I keep my Forge at about 35F, so my typical temperature delta is 60F minus 35F, or +25F. A beginner with the thermal comfort line of 80F would experience a delta of 80F minus 35F, or +45F. That means it takes me 3.6 min to get the same dose that a beginner will get in 2 min.
Right now, this measure is hypothetical. To validate it, we'd have to gather several subjects, establish their thermal comfort line, and measure their skin temperature, core temperature, and other physiological makers to compare responses and see if they correspond to our estimate of dose. After that, we could establish a minimum effective dose for metabolic benefits.
We haven't done that (yet), but researchers at the University of Copenhagen in Denmark recently established that as little as 3-4 minutes of winter swimming each week was sufficient to recruit and maintain active brown fat -- no matter whether it came all at once, or in several brief sessions (Søberg et al. 2021).
The fjords of Denmark are very cold in the winter, typically less than 40F, so the delta-time measurement approach suggests that more time will be required when your cold plunge practice is a higher temperatures.
To test for yourself whether this measure offers you any guidance, establish your own thermal comfort line and then test the dose measure against your subjective experience of cold, and objective measures of response, including: required time for rewarming, or time before the onset of shivering. If the measure is useful, you might find that you get better at predicting how you will respond to different temperatures of water, and be able to tell the friends you introduce to your Forge what they could expect, too.
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