Your body fights the ice

The beginning of winter can seem unendurably cold, but by spring our bodies are seemingly immune to the chill in the air. We get under your skin to find out why.

By Holly McKelvey

You pull on your coat over two sweaters that are bundled tight on top of shirts and thermal underwear, and wrap your scarf snug across your face. Gloves are on, and hat is drawn down around your ears; feet are swathed in socks over stockings, buried deep in boots that will keep out ice and snow and storms. You are ready for this.

You step outside, and the freezing air hits that vulnerable strip of skin still exposed between nose and forehead. You squint your eyes against the reflective brightness, and you concede: hey. this isn't that bad.

In fact, you admit to yourself, even as a little shiver runs through your arms against the biting morning air, I could get used to this.

And that's when it hits you. You already have gotten used to this. These layers of clothes wouldn't have cut it a few months ago; now they're more than enough. You have acclimated to winter. You and your body have learned to react to the cold, keep it at bay, and enforce a tough thermo-regulation process that is keeping you at a happy, healthy temperature.

The temperature that makes your body glow healthy is that oft-quoted 98.6 degrees Fahrenheit (37 degrees Celsius). Too high above this, and you can experience heatstroke; too low, and you risk hypothermia.

Your body makes unceasing adjustments moment-to-moment– regulating how much you sweat, shiver, and respire– to keep your temperature constant. It can keep you tuned to just the right degree, and during winter, it works harder than ever.

The most immediate responses of our bodies to the cold are very familiar. Shivering is caused by muscle contractions that generate heat through movement, and become stronger and more uncontrollable as we grow colder. Goose bumps rise up on our arms, standing our hairs upright to trap a thin layer of heat against the skin. With our paucity of body hair, this is one of our less effective defenses against the cold. Finally, blood vessels at the surface of our body constrict, reducing the amount of heat lost across the skin. This last one is the sneaky culprit behind icy-cold fingers, toes and noses.

But beyond these immediate responses, the body can also implement more significant changes to adjust to lower temperatures over the long term. So what exactly is happening inside our bodies during those five rough months of winter that makes 40 degrees Fahrenheit (4 degrees Celsius) in April feel so much more tolerable than the same temperature in November?

To start, there is metabolic rate. Your metabolism is the speed at which your cells convert food into energy; this process determines how warm your body is while at rest (in other words, how warm you feel standing outside on a winter day). A high metabolic rate produces energy quickly, and therefore generates a greater amount of heat the faster it goes.

When faced with the cold, your body nudges your metabolic rate upwards. Higher metabolism: more heat. Check.

When your metabolic rate goes up, you also need more food to fuel it. That's why we tend to eat more in autumn and winter. This increase in calories plays a second role, as well: it contributes to your fat reserves, which help you grow your cozy buffer against the cold.

The fat reserves are more than just a buffer, though. In fact, they are the ultimate source of your body's long-term acclimation to the cold. It comes down to the existence of brown fat, which, come winter, is your (grossly-named) best friend. More a muscle than a fat, brown fat generates heat when activated by exercises like shivering, and repeated exposure to cold. Moreover, it converts normal fat into more super-powered brown fat as it does this. This is what carries you through the winter.

So the meals you eat in the winter months facilitate your heightened metabolism and support that healthy heat buffer of fat under your skin. When that's not enough, and your body starts to shiver from the cold, the very act of shivering increases your larder of heat-generating brown fat. This is a series of processes that keep your internal temperature stably where it should be.

It's an amazingly well-calibrated system, and it helps us get through winter.

Of course, the process of acclimating to the cold is not an abrupt one, and certainly not an easy one. Anyone who has moved from California to New York, from the sun towards colder wintering grounds, can attest to the miserable difficulty of that first winter in the cold. The process can take weeks (at least); and as temperatures dip lower and lower, the adjustment period can be drawn out substantially.

There are limits to this self-regulation, as well. The core temperature of the body cannot fluctuate more than about 1 degree Fahrenheit (0.5 degrees Celsius) in either direction, and that does not change, even when the body has become habituated to the cold. The process of acclimation increases your body's ability to retain heat – but very prolonged exposure to the cold would ultimately still be deadly.

But there is something fantastic, and something awe-striking in our ability to acclimate. Namely, that our bodies are well-equipped to deal with such wildly variable changes in temperature. But also, because you can look forward to that day in spring when the temperature is still frigid by summer standards, and you can prance outside without even a sweater. No big deal. It's 40 degrees Fahrenheit (4 degrees Celsius) out, and it's t-shirt weather.

Your body has got things under control.

Holly McKelvey is a graduate student in Applied Ecology at the University of Poitiers, France, working on bio-indicators in stream ecology. She can be reached at holly.mc.kelvey@etu.univ-poitiers.fr