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Out in the cold

JAN 2010

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No doubt cold weather can be hard on your health, but there may be a silver lining or two.

Winter and its chilly temperatures are a mixed blessing when it comes to human health. We might not appreciate it at the time, but cold temperatures perform a great public health service by killing off disease-mongering insects and microorganisms, and one of the big worries about climate change is that winter will lose its pestilence-fighting punch. Although it's a bit theoretical, cold weather may also help us slim down by stimulating metabolically active brown fat. And in Scandinavia and Russia, many people actively seek out the cold: wintertime swimming in frigid water is believed to do health wonders, and there's some science (not much, but some) suggesting that it might be so.

But there's also a dark side to consider. Numerous studies have shown that death rates peak this time of year. Blood pressure increases during the winter, and, by some reckonings, 70% of the wintertime increase in the death rate can be traced back to heart attacks, strokes, and other cardiovascular causes of death. And, of course, flu season is a winter event, and flu viruses spread more readily once the air is dry and chilly.

Winter darkness, in a literal sense, may make matters worse. Sun-exposed skin makes vitamin D, a vitamin that seems to have all kinds of health benefits. During the winter, when days are short and the sun is at a low angle, levels of the vitamin in the body tend to dip. Cold temperatures and low vitamin D levels: that may be a bad combination.

The shunt and shiver

If it's not too cold, our bodies adapt to cold temperatures pretty well. When we encounter cold air or water, the lacy network of blood vessels in the skin constricts, and blood is hastily shunted to the interior. That response adds to the insulating power of the skin because there's less heat lost from blood circulating near the surface. It also protects vital organs against the falling temperature. But we pay a price for the rerouting: diminished blood flow makes fingers, toes, and other peripheral parts of the body (the nose, the ears) vulnerable to frostbite, which occurs when the fluids in and around tissue freeze. Under the right conditions, blood vessels in the skin will open and close in an oscillating pattern, so skin temperatures rise temporarily, especially in the fingertips.

Shivering is another familiar defense mechanism against falling body temperatures. The rapid, rhythmic muscle contractions throw off heat that helps the rest of the body stay warm. The body may recruit more and more muscles as the temperature drops, so shivering can get intense and very uncomfortable. Voluntary movement — stomping your feet, swinging your arms — is another way to generate heat, and depending on the circumstance, may cancel out the need to shiver. It's not a total gain, though, because exercise also increases blood flow to the skin, so some body heat escapes.

Body type explains some of the varying reactions to cold weather. Taller people tend to get cold faster than shorter people because a larger surface area means more heat loss. And fat's reputation as an insulating material is well deserved, although for warmth during the winter, you want it to be the subcutaneous fat layered under the skin, not the visceral fat that collects in the abdomen.

Cold reaction

illustration of circulatory system reacting to cold

In cold temperatures, blood is shunted from the periphery to the interior.

Warming up to cold

Most of us spend the winter trying to stay warm and avoid getting cold, but a little bit of exposure may not be such a bad thing. It's been suggested, for example, that moderately cold temperatures could be good for the vasculature because it trains blood vessels in the skin to be responsive. (An added benefit: rosy cheeks.)

Brown fat is the heat-producing, calorie-burning fat that babies need to regulate their body temperatures. Most of it disappears with age, but PET scans have shown that adults retain some brown fat. Years ago, Finnish researchers reported that outdoor workers had more brown fat than indoor workers. Dutch researchers reported findings in The New England Journal of Medicine in 2009 that showed that moderately cool temperatures of 61? F activated brown fat in 23 of 24 study volunteers. No one is suggesting that cold weather be used for dieting purposes (not yet anyway). But when we get chilled this winter, we may take some consolation that at least we're firing up those brown fat cells.

Using cold temperatures for medical purposes is taken quite seriously in other countries. "Whole-body cryotherapy" was developed in Japan to treat pain and inflammation from rheumatic and other conditions. Patients spend one to three minutes in a room cooled to -166? F. And people in Finland, Russia, and elsewhere are passionate about winter swimming having health benefits. Several years ago, Finnish researchers reported the results of a study of 10 women who for three months took cold-water plunges (20 seconds in water just above freezing) and submitted to whole-body cryotherapy sessions. Blood tests were unremarkable except for a two- to threefold jump in norepinephrine levels minutes after cold exposure. Norepinephrine is a chemical in the nervous system that wears many hats, including, possibly, a role in pain suppression. (Spikes in norepinephrine levels may also explain the rise in blood pressure that occurs in cold weather).

Cold ain't so hot

But before heading north in search of a more healthful clime, or ringing in the New Year with a polar bear swim, you may want to think about the toll that cold and winter takes.

Research documenting wintertime increases in blood pressure goes back decades. French researchers rounded out the record in 2009 with findings that showed the cold-weather increase in systolic blood pressure (the top number) was especially pronounced in those ages 80 and older. Cold may trigger the fight-or-flight response: blood pressure goes up because the heart pumps faster and blood vessels tense up.

Cold weather and respiratory disease, including flu, also go hand in hand. Research has shown that cold spells are reliably followed by upticks in the number of deaths from respiratory disease. Some of this may have to do with a few infectious organisms, like flu viruses, thriving in colder temperatures, but there's also evidence that exposure to cold temperatures suppresses the immune system, so the opportunities for infection increase. A study published in The New England Journal of Medicine in the late 1970s famously debunked the belief that the common cold is linked to cold exposure, but British cold researchers have maintained that there is a cold–to–common cold connection. Their hypothesis: cold air rushing into the nasal passages makes infections more probable by diminishing the local immune response there.