Adaptation to cold: habituation & acclimatization

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You needn’t be a special snowflake to swim in cold water, lots of different people do it. Some people might be more naturally skilled at it than others, but like any sport, it’s just as much learned and practiced.

When I first decide to keep swimming into the fall and winter, my goal was to continue enjoying the experience but also to push my level of adaption so that I could swim in much colder temperatures. Understanding the role of adaptation has come to play a huge part in the process…

Means of Adaptation to Cold

Three patterns of physiological cold adaptation are observed in humans: cold habituation, metabolic acclimatization, and insulative acclimatization. I’m going to throw in behavioural adaptation as well, as it’s important too.

1. Cold habituation

Cold habituation is often described as the process of getting used to entering cold water, a sort of desensitization of thermal sensation to cold as a stressor on the body. In simpler terms, repeatedly getting into cold water gets easier over time, a swimmer’s skin and body adapts to the sensations of cold.

Entering cold water felt painful at first, like sharp pins and needles, but with habituation I’ve found it become more like a firing up of the senses, every single cold thermoreceptor under my skin ignited. It sparks the start of an intense sensory experience. The feeling of cold never really goes away, I always feel it, but I simply got more used to it, it started to matter less, and I seem to have learned to compartmentalize any discomfort.

Getting using to entering cold has some interesting side effects. As the body gets more used to entering cold water, this type of adaptation to cold “blunts”, or stops, both the shivering response and peripheral vasoconstriction (both discussed in detail here). There are two sides to this coin:

Positive side: blunted vasoconstriction allows the blood flow to remain moving regularly through a swimmer’s skin and extremities (in particular, hands and feet), without yet redirecting any significant amount of bloodflow to the core. As a result, skin and extremeties stay warmer longer, and the swimmer will likely maintain manual dexterity for a bit too. And, with shivering blunted as well, the swimmer likely has a higher level of physical comfort when exposed to cold or first entering cold water. With habituation, I’ve gotten to the point where I can stand around in the cold with much less discomfort than before or much notice given to cold.

Negative side: both shivering and vasoconstriction are critical in maintaining/creating warmth around the core. So while repeated exposure to cold might make getting into cold water feel easier, this blunting (stopping) of two keys heat mechanisms renders the swimmer’s core more susceptible to cold, and core temperature may start to drop at this point – before we’ve even gotten fully submerged in the water.

This is the reason most cold swimmers practice entering the water and submerging their bodies within a reasonable time, 1-2 minutes max. Competitive races are much faster, 10-20 seconds. Standing in the shallow end for 10-15 minutes trying to ease your way in will only result in a deeper drop in core temperature before the swim starts, which a swimmer will not likely be able to raise back up during the swim.

2. Insulative acclimatization

One of the very first responses the body has when entering cold water is typically peripheral vasoconstriction, at least to some degree, which simply put is when the body adjusts its pattern of blood circulation to redirect some of the blood flow away from extremities, and gather it around the body’s core to help create and insulate warmth.

The more significant the exposure to cold, or drop on core temperature, the more pronounced the vasoconstriction will be. The very first time I swam in cold water, back then “cold” was 12C, almost immediately I felt a surge of heat surrounding my core – the effects of vasoconstriction were quite pronounced. I was perfectly fine, although at the time I didn’t quite understand what was happening.

However, with repeated entry into cold water, the rate and severity at which peripheral vasoconstriction occurs has been greatly reduced. I can’t know for sure why without proper testing in a lab setting, but my theory is this:

I’m habituated to cold water, so vasoconstriction is blunted when I first get in. My hands are fine, my feet are fine, I don’t even shiver;
As I continue to swim, at some point my core temp drops enough to activate vasoconstriction, and some blood flow is moved away from my extremities towards my core. The colder I get, the more blood moves to my core. It takes me longer and longer to reach this point, which luckily means my hands don’t get as cold as they used to. I also believe this pattern to be true because I experience very minimal afterdrop coming out of most swims. Or, I’m playing it safer than I realize, hard to tell;
I can stay in cold water for quite some time, other mechanisms must be playing a significant role in keeping me warm enough to continue swimming…

3. Metabolic acclimatization

Enter metabolic acclimatization, also known as “thermogenesis”.

Heat loss is 25-30 times faster in water than air, due to conductive heat loss, which simply means the body continues to conduct heat to whatever the skin is in direct contact with – in this case, cold water (and likely cold air, if you’re still dallying in the shallow end). The colder the water, the quicker the body expels its heat. The body seeks to replace this heat by boosting metabolism or burning more calories to stay warm, which is the process of “thermogenesis”, the production of heat by the body.

Thermogenesis happens in two ways: shivering, and non-shivering.

Generally speaking, shivering is not a bad thing, even if it’s uncomfortable. It’s the body’s attempt to create heat; muscles contract quickly and as a result generate some warmth. Shivering comes on during exposure to cold, or even as a result of strenuous physical activity which results in a drop in core temperature. Even once a swim is over, shivering during after-drop means your body is working to heat back up. It’s also not uncommon for me to shiver or experience after-drop a little after strenuous training swims where I’ve given it my all, even in the warmth of summertime.

However, shivering while still swimming in water is something all together different, and is very dangerous – it’s a warning sign to get out immediately. Considering that heat is lost 25-30 times faster in water than air, shivering will only result in the opposite of it’s intention, and any heat produced by shivers will conduct away from the body much faster than if the swimmer were only exposed to air. Shivering in water is a last ditch effort to heat up, but will actually cause core temperature to continue dropping, and quickly. Shivering in the water means you’re done, swim’s over, there’s no recuperating lost heat, you need to get out and rewarm immediately.

The other type of thermogenesis is non-shivering, which is an area many cold water swimmers seek to improve and adapt.

The human body contains several types of fat, or adipose tissue, two of which play key roles in cold water swimming; white adipose tissue, and brown adipose tissue. White adipose tissue is distributed across the body, often in higher volume around the hips and stomach, it’s the fat many of us are most familiar with (I’m trying to lose a little as we speak). It acts as an insulator for the body to some extent, so that swimmers with a higher body fat mass may possibly be more able to stay in the water for longer periods.

But it’s not a general rule of thumb. White adipose tissue won’t do anything to generate heat. Brown adipose tissue (“brown fat”) on the other hand, will. Brown fat burns through stored energy at a high rate, in order to perform the very important job of generating heat – but without shivering, thus where the term “non-shivering thermogenesis” comes from. Both fats provide some insulation, but unlike white fat, blood flows through brown fat, essentially warming it up and transferring that heat to the body. And guess where brown fat gets its energy source from…white fat. Yep, brown fat is a fat-eating fat. How awesome is that.

Babies are born with a fair amount of brown fat, since they cant shiver much yet, so it acts as a way to protect their small bodies. Until recently, it was thought that brown fat disappeared entirely into adulthood, but it’s been shown to not be the case, it’s only completely absent in some adults – most often, those who are overweight. To some extent, brown fat has been linked to leanness, as children with more brown fat have been shown to grow up to be leaner adults.

Brown fat sits more heavily around shoulders and the spinal cord in adults, and is more prevalent in women.

It’s interesting, as we tend to assume people with higher overall body fat mass deal best with cold water swimming, and yet one of the keys to generating heat when exposed to cold has been linked to leanness. I happen to be short and a bit chubby, and love swimming in cold water, so it made sense to me at the time. As I’ve gotten to know the cold water community better, I realize now that I have swimmie friends of all shapes and sizes who swim at varying degrees of cold, even skinny minnies who’ve done ice miles. Body shape and size = not always a determinant of how well a swimmer will do in cold.

But here’s the really interesting thing: research has also shown that cold exposure to areas where brown fat is active can further stimulate its growth. In other words, an adult can increase the amount of brown fat they have, and consequently, increase the amount of heat they can generate when exposed to cold. And guess how. Yep. By getting into cold water often, exposure to cold helps generate more brown fat.

4. Behavioural adaptation

The behavioural changes have been very interesting to me. Bear in mind where I started from, I never could have imagined I’d be swimming in cold water:

a: I’m a big chicken
b: I complain constantly about being cold
c: I hate being uncomfortable

I often think back to my very first cold swim; I stayed in 12C water for about 60mins, I was the last one out, I wasn’t terribly cold afterwards, and I couldn’t wipe the grin off my face for hours. I was excited more than anything, that heightened sensory experience felt amazing, but I still had work to do in order to swim safely into colder waters. But there’s hope, even for big, cold, complainy chickens.

I seem to have developed a resolve and determination that wasn’t there before. Once I show up on shore for a cold swim, I’m set on doing it, no matter how hard it might be for me to get in the water. I always have some fear, some anxiety about it beforehand, I always tell myself this time will be the last one, it’s not for me, I hate cold. And each and every time, I overcome my negative self-talk. I get undressed, I get in, I end up loving it, I never want to get back out. It’s starting to translate to other areas of my life as well, which is really nice…

How often, how much?

Knowing a bit more about how the body can adapt to cold, the question for me remains as to how much training, what kinds of training, and how often would it take to keep adapting further?

There isn’t much research on the subject to really make a determination specific to cold water swimming, but there’s consensus on the following:

brief, intermittent exposure to cold produces seemingly negligible results
pronounced physiological adjustments come about only when repeated exposure to cold causes body heat loss, the kind of loss that can’t be recovered with metabolic heat production

So…apparently I’ve got to get cold to get better at being cold, and more often.

That’s fine, I figured as much. I’ll continue to play outdoors in the snow with puppy, I find it helps with habituation, and if I push it a bit longer, I can probably create some level of adaptation in the process. But I understand now why there’s skepticism about the effectiveness of cold showers or sitting outside in the snow as training methods. I mean, if it’s all you’ve got, use it, but it’s not likely for a swimmer to get cold enough to acclimatize, to effectuate any form of true physiological adaptation. Soaking in a tank or tub full of cold water in the backyard would be better.

Ice baths in the backyard. Cozy.

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Practice is the hardest part of learning

I recently posted in a favourite swim forum about how eager I was to get back into cold water, for fear I’d lose some of my acclimatization. I was politely corrected by another swimmer, who insistent that someone fully “hardened” (= fully adapted) to cold never loses their acclimatization. They’d done all the research, they knew this for fact.

The comment didn’t sit well with me. I’ve always seen swimming as a learned and practiced activity; we learn the skills necessary to swim safely and swim well, and practice the sport to improve on our abilities. It’s not likely for us to unlearn these things, but more likely we could become out of practice at performing it. I don’t see why this doesn’t also apply to adaptation. I was redirected to the swimmer’s personal swim blog, where I read an article they’d written with interest, but frankly, there was no evidence provided, just supposition and personal experience. Which is fine and well, but one person’s experience doesn’t apply to us all.

I turned to trusted resources, and learned there is research to show that when cold stimulus is removed, adaptation will gradually started to disappear. Some suggest a swimmer will lose adaptation 4-5 times faster than they gain it. And yet for other swimmers, the passing of time apparently seems to have little effect on their ability to get back into cold water. Am I one of those swimmers? Maybe. But I like learning, and value practising. Practice makes perfect, or something like that.

Friends: question everything you hear, which includes whatever I write here. I’m just one swimmer, no one can know it all, and some things you just have to learn for yourself…

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