Fat That Makes You Thin

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This holiday weekend I slept in, enjoyed the sunshine, and started working on my tan. It seems like the summer weather is finally here to stay, and painful memories of the Polar Vortex are fading away. Although this past winter was a harsh one, there may be a few health benefits that we are leaving behind along with the chilly weather. Aside from fewer bugs, reduced inflammation, and motivation to go a bit faster on your morning run, exposure to cold temperatures has been shown to increase the activity of a certain type of body tissue called brown fat.

An increase in body fat may not sound like a good thing, but wait until you hear the punch line…brown fat actually burns calories. Just one ounce of brown fat can burn over 100 extra calories per day, which would add up to shedding up to 15 pounds over one year. However, the calorie-burning power of brown fat is only activated at cool temps; unless the ambient temperature is 60° F or below, you don’t get much of a burn.

Brown fat only fires up in the cold because its main purpose is to create heat (thermogenesis) and protect against drops in body temperature. Unlike normal white fat cells, brown fat cells contain tons of iron-rich mitochondria, which give the cells their brownish color. Mitochondria, you may recall, are the “powerhouses” of our cells. They harvest the caloric energy that comes from breaking down the protein, fats and carbs that we use to fuel our body. In most cells, mitochondria store the energy in molecular form, but in brown fat cells they dissipate all that energy as heat. Unlike their white fat cousins, which only store calories, brown fat cells literally burn up calories like fuel in a furnace.

It was long thought that humans only had brown fat during infancy; babies are more susceptible to the cold and can’t exactly whip up a cup of hot cocoa when the going gets chilly. Instead, babies have deposits of brown fat along the spine that can generate heat if the core body temperature is threatened. When the body’s musculature develops further, we are able to use a new method of thermogenesis when we get cold: shivering. Normally, you contract specific muscles to move your body around, and a small amount of heat is generated as a byproduct. When you get cold enough to shiver, your muscles make lots of small involuntarily contractions. Opposing muscles often contract against each other again and again, so there isn’t much movement – just small twitches – but there is lots of extra heat.

Adult humans are great at shivering (trust me, after this winter I’m an expert), so we tend to lose most of the brown fat we had as babies. However, recent research revealed that we don’t ditch it completely; small deposits can still be found along the upper spine and shoulders. And although adults have only a few ounces at most, a 2009 study showed that higher levels of brown fat activity do correspond with markers of bodily health, such as lower BMI, lower percentage of total body fat, and lower blood glucose levels. What’s more, exercise may actually stimulate white fat cells to act more like brown ones. This “beige fat” is being studied along with brown fat as a potential tool to combat obesity.

Although researchers hope to develop drugs that can activate brown fat artificially, cold exposure is still the only surefire way to rev your brown fat metabolism. (But careful, people also tend to consume more calories when they’re cold!) So maybe next winter I’ll try turning down the thermostat to spend a few hours being chilly – and try desperately not to think about that warm, creamy cup of cocoa!

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