On any given group run in sub-freezing temperatures, it’s amazing to see the variety of hand protection on display. Some people have thin gardening gloves; others (and I count myself among them) have what look like boxing gloves lined with fleece and stuffed with down.
It’s not a question of toughness: as a new study in Experimental Physiology illustrates, people’s fingers and toes vary dramatically in their response to cold. And scientists still aren’t really sure what makes the difference, how to change it, or even whether you get better or worse with experience.
Here’s a telling figure from the study, which was led by Clare Eglin of the University of Portsmouth’s Extreme Environments Research Group. It shows skin temperature of the toes before (-2 on the figure below) and after (0 to 10 min) a two-minute dunk in cool water at 59 degrees Fahrenheit, for a group of cold-sensitive subjects (black circles) and a group of normal control subjects (white circles):
What jumps out at me in this graph is the big difference in toe temperature even before the cold water dip: around 35 degrees Celsius versus 30 degrees Celsius, which corresponds to 95 degrees Fahrenheit and 86 degrees Fahrenheit. Some people have cold feet pretty much all the time!
To be fair, this difference is a bit of a self-fulfilling prophecy, because the two groups were selected based on their toe temperatures before immersion and after five minutes of rewarming. Those whose toes were below 90 degrees Fahrenheit in both cases were classified as cold-sensitive. Out of an initial testing pool of 27 volunteers, nine were identified as cold-sensitive (five men and four women), and another nine were chosen as the control group based on their similarities to the cold-sensitive group in age, sex, body shape, and exercise habits.
The key question is whether there are any differences between the two groups that might explain why some of them have such cold feet. One aspect of the study was a series of questions about past recreational cold exposure, focusing on duration, frequency, and severity during the last two years. Based on the responses, the 27 participants were ranked from greatest to least cold exposure. Topping the rankings was an open-water swimmer who, among other feats, had completed an “ice mile” (meaning 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-around outdoor athletes like runners and cyclists; and finally those who did basically no cold-weather outdoor activities.
Pause for a moment to consider what you’d expect to see. Are the surfers and open-water swimmers the ones with unusually warm feet, or unusually cold feet?
Personally, I guessed wrong. Here’s a graph showing toe temperature five minutes after the cold dip, sorted by cold exposure ranking (number one is the ice-mile swimmer, number 27 spends the winter sipping cocoa on the couch). The black dots, once again, are the frigid-toed cold-sensitive group; the white dots are the matched control group; and the grey dots are the other subjects who weren’t slotted into either nine-person group.
The correlation isn’t perfect, but those with the most cold exposure (i.e. the top-ranked, on the left) tend to have the coldest toes, and those with the least cold exposure have the warmest toes. This argues against the idea that the people who gravitate to activities like cold-water swimming are the ones whose toes stay warm.
Instead, it’s more consistent with the idea that repeated cold exposure might actually impair your toes’ ability to handle the cold. The focus of Eglin’s research is something called “non-freezing cold injury” (NFCI) which results from prolonged exposure to cold and wet conditions but doesn’t actually freeze the tissue and develop full-blown frostbite. The classic example is trench foot, which can have serious permanent consequences like gangrene. But Eglin’s results suggest the possibility of less severe versions of NFCI that might accumulate over time and leave lasting consequences.
It’s well known that repeated exposure to heat triggers a series of physiological changes like enhanced sweating and increased blood plasma volume that make us better at dealing with hot conditions. There’s a long-running debate about whether the reverse—cold acclimatization—also occurs. For example, studies in the 1960s showed that fishermen tended to have warmer fingers than non-fishermen, but that again runs into the possibility that only people with good circulation can hack it in the profession.
Experiments that attempt to induce acclimatization by exposing people to cold repeatedly have produced mixed and mostly negative results. One 2012 study had volunteers dunk their hands and feet in frigid 46-degree water for half an hour daily for 15 days. By the end, their perception of cold had lessened—no surprise to anyone who has noticed how the same temperature that felt miserably cold for a run in November can feel delightfully warm in March. But blood circulation and skin temperature during the cold exposure actually worsened in the fingers. That’s a dangerous combination, because it means your fingers are still getting cold but you’re less likely to realize the danger.
Eglin’s new study also explored the possibility that repeated cold exposure could in some cases be harmful rather than just useless. The hypothesis was that the mild version of non-freezing cold injury might damage the ability of your blood vessels to dilate and bring warm blood to your extremities, and compromise your ability to detect subtle changes in temperature. But the experiments didn’t bear this out. The group with cold toes and high levels of recreational cold exposure had roughly the same ability to detect temperature changes as the control group, and their blood vessels dilated to a similar degree.
It’s clear, in other words, that our understanding of the long-term effects of mild cold exposure is still pretty murky. We don’t know exactly what happens or why. But I think we can draw two reasonable conclusions. First, despite decades of speculation among thermal physiologists, it’s not worth the effort (and is possibly counterproductive) to deliberately expose yourself to cold in the hopes of triggering adaptations that make you more cold-resistant. And second, people vary dramatically in how their extremities respond to cold. My only regret, after decades of running through the Canadian winter, is that it took me so long to realize that I really do need those massive boxing gloves.
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