Why are so many people in first-world countries so overweight? Why is metabolic syndrome so prevalent? The familiar contenders are diet and exercise – more specifically, the wrong kind of each. Both Conventional Wisdom types and nutrition nerds (myself included) agree that we’re doing something wrong in the kitchen and the gym, and that fixing that stuff could solve most of our weight (and even health) problems. Of course, that’s about all we agree on. Specific definitions of “fixing” and “that stuff” remain subjects of vociferous debate. That said, I like when we can agree on something, even if that something is just speculation about another possible factor in the obesity problem. In today’s Monday Musings we’ll take a look at one such factor.

A recent study out of the journal Obesity Reviews notes that it’s not just diet and activity levels that have changed in correlation with rising obesity numbers, but ambient temperature. To be more specific, people are heating their homes at all hours of the day, even as they sleep, and spending less time outdoors exposed to the elements. Central heating is more common, while space heaters, fireplaces, and electric heaters are less common, meaning the entire house gets and stays warm. People in developed countries exist in relative thermoneutrality: a nice 68-72 degrees F. The authors guess that with less exposure to thermal stress, we’re burning fewer calories. Our bodies have an easier time regulating our internal temperatures, and expend less energy doing so.

On the surface, their ideas might remind you of the outdated, overly-simplistic calories in, calories out model, where people are fat because they eat a few extra candies between lunch and dinner that add a few dozen calories to their daily allotment. It’s more complex than that, though. Exposure to cold is a type of stressor; to be specific, it’s a thermal stressor. Our bodies respond to stressors by adapting and (hopefully) improving, as you well know, and hypothermal stressors, like taking a cold bath, going for a swim in the middle of winter, or even letting the heat go off at night, induce the creation of brown fat. Brown fat is different from the reviled and feared “white fat” in that it keeps us warm by burning white fat. Newborns have lots of brown fat because they can’t shiver, can’t crawl (away from cold and toward warmth), and have underdeveloped central nervous systems that can’t be counted on to react quickly enough to changes in ambient temperature. It’s how they stay warm. Adults have far less brown fat, but they can develop more through exposure to cold. Furthermore, brown fat levels in adult humans are highest during winter and linked with less visceral fat and a lower BMI. If you’re subjectively cold, chances are you’re spurring the creation of new brown fat or increasing the activity of already-present brown fat.

This all seems quite reasonable, doesn’t it? I do love it when I can agree with obesity researchers.

On a different note, a quick word about that Yahoo! Shine article floating around. You know, the one telling you the eight ways carbs will make you lose weight. It’s silly and not worth a lot of typing, so I’ll make it short. The thing that jumps out at me is the author’s obsession with “Resistant Starch.” First of all, I’m not sure why it deserves repeated capitalization (maybe it’s some sort of deity?), and second, resistant starch is just another type of prebiotic whose fermentation by microbiota releases beneficial short chain fatty acids. You can get the same kind of reaction by eating other sources of soluble fiber, many of them decidedly low-carb. Think leafy greens, broccoli, berries, apples, jicama, onions, garlic, leeks, Jerusalem artichokes. And yes, if your activity levels and metabolic health permit, Primal starches are good sources of resistant starch and soluble fiber alike, but it’s not the carbs doing it. It’s the “carbs” that you literally cannot digest without your little microscopic friends’ assistance.

Can keeping central heating on really be at least somewhat responsible for the obesity epidemic? Have you ever noticed a correlation between ambient temperature and your own weight? Let me know in the comment section!

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