Outdoor

How your physique adapts (and doesn’t adapt) to chilly

With every group run in freezing temperatures, it is amazing how diverse the hand protection is. Some people have thin gardening gloves; others (and I count myself among them) have something like boxing gloves that are fleece-lined and filled with down.

It’s not a question of toughness: as a new study in Experimental Physiology shows, people’s fingers and toes vary dramatically in their response to cold. And scientists still aren’t sure what makes the difference, how to change it, or whether the experience makes you better or worse.

Here’s a telling number from the study led by Clare Eglin of the Extreme Environments research group at the University of Portsmouth. It shows the skin temperature of the toes before (-2 in the figure below) and after (0 to 10 minutes) a two-minute immersion in cold water at 59 degrees Fahrenheit for a group of cold-sensitive subjects (black circles) and a group of normal control subjects (white circles ):

(Figure: Experimental Physiology)

What strikes me in this graph is the big difference in toe temperature before I even immerse myself in cold water: about 35 degrees Celsius versus 30 degrees Celsius, which corresponds to 95 degrees Fahrenheit and 86 degrees Fahrenheit. Some people almost always have cold feet!

To be fair, this difference is a self-fulfilling prophecy, as the two groups were selected based on their toe temperatures before immersion and after five minutes of rewarming. Those with toes below 90 degrees Fahrenheit in both cases were classified as sensitive to cold. Out of an initial test pool of 27 volunteers, nine were identified as being cold sensitive (five men and four women), and another nine were selected as the control group based on their similarity to the cold sensitive group in terms of age, gender and body shape and exercise habits.

The key question is whether there are differences between the two groups that could explain why some of them have such cold feet. One aspect of the study was a series of questions about past cold exposure that focused on duration, frequency, and severity over the past two years. Based on the answers, the 27 participants were classified from greatest to least exposure to cold. At the top of the ranking was an open water swimmer who, among other things, had completed an “ice mile” (ie water temperatures of 41 degrees Fahrenheit or less) without a wetsuit. Next came those who took part in cold water activities like kite surfing or swimming; then year-round outdoor athletes such as runners and cyclists; and finally those who basically didn’t do any outdoor cold weather activities.

Take a short break to consider what to expect. Are the surfers and open water swimmers the ones with unusually warm or unusually cold feet?

Personally, I guessed wrong. Here’s a graph showing toe temperature five minutes after being soaked in the cold, sorted by exposure to the cold (number one is the ice mile swimmer, number 27 spends the winter sipping cocoa on the couch). The black points are again the cold-sensitive group with the cold toes; the white dots are the matching control group; and the gray dots are the other topics that were not assigned to any of the nine groups of people.

science(Figure: Experimental Physiology)

The correlation isn’t perfect, but those with the highest exposure to the cold (that is, the highest ranked on the left) tend to have the coldest toes and those with the least exposure to the cold tend to have the warmest toes. This speaks against the idea that the people who get interested in activities like cold water swimming are the ones whose toes stay warm.

Instead, this is more in line with the idea that repeated exposure to the cold could actually affect your toes’ ability to handle the cold. The focus of Eglin’s research is on what is known as “non-freezing cold damage” (NFCI), which results from prolonged exposure to cold and wet, but does not actually freeze tissue and develop full-blown frostbite. The classic example is the trench foot, which can have serious permanent consequences such as burns. However, Eglin’s results suggest the possibility of less severe versions of NFCI that could build up over time and leave lasting consequences.

It is known that repeated exposure to heat induces a number of physiological changes, such as: B. increased sweating and increased blood plasma volume, which make it easier for us to deal with hot conditions. There has been a long debate about whether the opposite – cold acclimatization – will also occur. For example, studies in the 1960s showed that fishermen tend to have warmer fingers than non-fishermen, but this in turn leads to the possibility that only people with good circulation can hack this at work.

Experiments attempting to induce acclimatization by repeatedly exposing people to the cold have produced mixed and mostly negative results. In a 2012 study, volunteers immersed their hands and feet in cold 46-degree water for half an hour a day for 15 days. By the end, their perception of the cold had diminished – no surprise to anyone who noticed how the same temperature that felt miserably cold for a run in November can feel deliciously warm in March. But blood circulation and skin temperature during the exposure to the cold actually deteriorated in the fingers. This is a dangerous combination because it means your fingers will still get cold but you are less likely to realize the danger.

Eglin’s new study also examined the possibility that repeated exposure to the cold could in some cases be harmful, not just useless. The hypothesis was that the mild version of non-freezing cold damage could affect your blood vessels’ ability to dilate and bring warm blood to your extremities and affect your ability to sense subtle changes in temperature. But the experiments did not confirm this. The group with cold toes who had high recreational exposure to cold had roughly the same ability to sense temperature changes as the control group, and their blood vessels expanded to a similar extent.

In other words, it is clear that our understanding of the long-term effects of mild exposure to the cold is still rather murky. We don’t know exactly what is happening or why. But I think we can draw two reasonable conclusions. First, despite decades of speculation among thermophysiologists, it’s not worth the effort (and potentially counterproductive) to purposely exposure to cold in hopes of triggering adjustments that will make you more resistant to cold. Second, people differ dramatically in how their extremities react to cold. My only regret after decades in the Canadian winter is that it took me so long to realize that I really need these massive boxing gloves.

If you’d like to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Strangely Elastic Limits of Human Achievement.

Main photo: Studio Firma / Stocksy

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