Right this moment, in early fall, the average adult will be lighter than at any other time of the year. His or her weight will climb steadily until Thanksgiving, level off, and then peak around New Year's Day. It then falls for the next 10 months. But for most of us, it remains a pound or two higher than it was the previous October. The cumulative load is considerable: On average, a middle-class American woman gains 28 pounds from early adulthood to middle age, while the average guy packs on 21.
I should note that this data comes from people who know their weight is being tracked. In the first study, volunteers weighed themselves on wireless scales for 12 months. The second is a combination of two long-running studies from the Harvard School of Public Health. Not only do the participants provide regular updates to the researchers, they're all doctors and nurses.
If health professionals and people who understand their digital scale is spying on them can't avoid annual weight gain, what hope is there for the rest of us? Lots, actually.
For the past five years, my weight has gone the opposite direction. I lose a few pounds every winter, starting in early December, and then slowly regain some of it through the spring and summer. I'm typically at my heaviest in the fall, when most people are at their lightest. My working theory: It's because I keep the thermostat at 66 degrees through the winter.
I started the experiment in early 2012, when I came across a study titled "Putative Contributors to the Secular Increase in Obesity: Exploring the Roads Less Traveled." (Admit it: You're jealous of my reading material.) It included this tidbit: In 1923, the average US home was heated to just 64 degrees in winter. By 1986, it had increased to 76.
Living in a climate-controlled world that's never uncomfortably cold means our bodies don't burn all the energy we'd normally expend to keep ourselves warm in winter. I can't prove it, but I think it's logical to assume that contributes to these slow, incremental weight gains over the years. If we allowed ourselves to be cold again, we should burn more calories and lose weight, or at worst neutralize all the food we inhale over the holidays.
The link between cold exposure and weight loss isn't exactly news. Tim Ferriss included ice baths in The 4-Hour Body. Wired wrote about it in 2013. The Atlantic covered it in 2015. Penn Gillette said it helped him lose 100 pounds. But here's the thing:
If I'd read those stories before I did my own experiment, I would've thought you needed to join the Polar Bear Club to lower your core temperature, or walk around wearing icepacks, and I wouldn't have done it. I'd rather have a few extra pounds than be miserable. But by simply turning the thermostat down, I was just mildly uncomfortable, and the pounds fell off.
Weight-loss experts are understandably skeptical. "If we added it in to a bunch of other factors, it becomes plausible," says Spencer Nadolsky, a board-certified obesity specialist. "But I doubt if it's relevant by itself."
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That's what I thought at first. Because it was so effortless, I initially attributed the results to my diet. At the time I was working on a story about the Paleo diet, and following the principles in a semi-earnest way. I had such low expectations for the project that I didn't think to weigh myself before I started, but I lost at least 10 pounds in a few months. When I switched from winter to summer clothes, shorts that fit fine the year before were now falling off my hips. The next winter, when my diet wasn't even quasi-Paleo, I lost the weight I'd regained over the summer and fall. The same thing happened the following winter, and the one after that. It was only a couple of pounds each time, but the timing seemed too consistent to be coincidental. To understand how it works, it helps to think of the human body as a sentient stack of firewood. We're three parts water and two parts fuel, with some bones to hold it all together. It takes a lot of fuel to keep us alive, much of which goes toward thermoregulation—keeping the body's temperature at or near 98.6 degrees. "All of our metabolic processes are heating our body at some level," Nadolsky says. Cold weather is a potentially existential threat to thermoregulation, and we have multiple ways to survive it. The muscles just below your skin contract to give you goose bumps, the object of which is to raise your hairs higher so they can provide more insulation. We also shiver, using our skeletal muscles to generate heat. Both mechanisms are the equivalent of throwing more logs on the fire to keep the room warm. Our bodies have plenty of logs in reserve, of course; even a relatively lean athlete has enough body fat to keep going for days without food. When fat runs low, the body can use some of the protein you have stored in your muscles and organs, which is obviously not ideal; it's tantamount to burning your floorboards to keep your house warm. It's much better to get fuel from the outside, which is why you get hungrier when you're cold. But even that has a benefit: Eating more fuel means burning more fuel, increasing your energy flux. Higher energy flux, in turn, helps you stay leaner and healthier. One more interesting thing happens as your body adapts to cold exposure: As the nutrition researcher Kamal Patel explains in this article, you increase your supply of brown fat, a metabolically active tissue that specializes in generating heat. This is in contrast to the rest of your body's fat, which exists to store fuel. Cold weather activates brown fat, and surviving cold weather triggers your body to create more of it. You don't actually have to shiver to get the benefits of lower temps: Thanks to a phenomenon called non-shivering thermogenesis, you can get the fuel-burning effects of shivering without being miserable. Now we come to the fine print: My experiment was only possible because I work from home, where I control the thermostat. When I go to someone else's office, or stay in a hotel, I'm struck by how warm they are in winter, and how insanely cold they are in summer. (On a recent business trip to Southern California, the hotel's hallways, restaurant, and lobby were so cold I had to wear long sleeves, even though it was at least 90 degrees outside.) Even for those who can control their own environment, different exposures at different temperatures will have different effects for different people—same as every other aspect of human metabolism. Some research shows that leaner people have a stronger thermoregulatory response than those who're overweight or obese. In fact, as Patel points out, research in this area is so new that we have no idea what's typical. The only long-term experiments are anecdotal, like mine, and the short-term studies use a range of different temperatures. So while the idea makes perfect sense in theory, no one knows how it'll work for any individual, or for how long it'll work. But everyone agrees on one thing: Gaining weight throughout adulthood, even if it's just a couple pounds a year, has genuine consequences. The more you gain, and the longer those extra pounds stick around, the higher your risk for diabetes, heart disease, and all the nasty stuff you wouldn't wish on your worst enemy. If turning your thermostat down to somewhere between "toasty" and "are those your teeth chattering?" simply keeps you from gaining weight year in and year out, that's a win. Or, as Nadolsky says, "It's not harmful to be slightly uncomfortable, and you might save some money on energy. Just don't expect a miracle."
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