And that's not all. A few years ago, scientists discovered brown adipose tissue, or brown fat, in adults; it was previously believed that only babies had brown fat and that it was lost as you age. What's the big deal? Brown fat is a highly metabolically active "good fat" that burns calories, maintains your body temperature, and may play a role in conditions like diabetes and obesity. Recently, researchers at the University of California–San Francisco, publishing in the journal Nature, found that mice with a gene that triggers production of brown fat were thinner, had lower blood sugar levels, had less insulin resistance, and had less fat in their livers than mice who'd been genetically modified to not produce that gene. The researchers concluded that developing mechanisms that activate brown-fat production could play a significant role in treating obesity.
All adults contain some level of brown fat, but until it's activated, brown fat doesn't really do much. And you can lose it as you age, which means more middle-age weight gain; a 2011 study in the journal Obesity found that the body fat levels increased with age in people with very low levels of brown fat, while body fat levels remained steady in people with high levels of brown fat.
So even if you're not overweight now, you can protect yourself against obesity in the future by activating your brown fat. Here are three ways to do that:
1. Crank down your thermostat. Or open a window when you're driving—whatever will cool your living space down a few degrees.
Why it works: A number of studies have shown that exposure to cold temperatures is the most effective way to stimulate activity of brown fat. When you're cold, even just mildly cold, your tissues activate brown fat as a way to generate heat, burning calories in the process. The problem is, we're not as cold as a whole as we used to be. A 2011 review of the science on brown fat and body temperature, published in Obesity Reviews, theorized that our increasingly comfortable homes, cars, and offices are reducing our body's reliance on brown fat as a way to generate heat. The authors noted that home living temperatures have increased roughly 2ºF on average since 1985, and the studies they reviewed showed a distinct correlation between average living temperatures and the number of calories burned per day. The warmer your living environment, the fewer calories you burn.
How to do it: Spend at least two hours a day at a less-than-comfortable temperature. For an October 2013 study whose results appeared in The Journal of Clinical Investigation, scientists exposed one group of men to two-hour periods of 66ºF temperatures daily for six weeks, while men in another group went about their daily lives in normal living temperatures. By the end of the study, the group exposed to cold showed higher brown-fat activity in PET scans, and the men were burning 200 extra calories per day compared to those in the group not exposed to cold. The cold group also had 5 percent less total body fat mass, while the other group's fat levels remained unchanged.
More from Rodale News: 5 Surprising Reasons You're Not Losing Weight
2. Eat more hot peppers.
Why it works: Chile peppers contain capsinoids, compounds that have been linked to brown fat activation (capsinoids don't generate spicy heat, as the similar-sounding compound capsaisin, also found in peppers, does). Why capsinoids work isn't quite as fully understood as why cold temperatures activate brown fat, but research suggests that capsinoids stimulate receptors in your gut that signal your nervous system to produce more brown fat.
How to do it: In the same Journal of Clinical Investigation study, 10 men were given either a placebo or a capsule containing 9 milligrams of capsinoids from a variety of red chile pepper known as CH-19 Sweet, once a day for six weeks. As was seen in those with greater exposure to cold, men in this study taking chile pepper capsules exhibited higher levels of brown-fat activity and were burning about 150 more calories per day than those in the placebo group. CH-19 Sweet peppers are grown exclusively in Thailand and are bred to contain high levels of capsinoids, but you can get low levels of the compounds in most varieties of hot chile peppers.
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3. Grab an apple.
Why it works: Apple skins contain ursolic acid, a compound that, according to a 2012 study in PLoS One, has been found to increase production of both skeletal muscle and brown fat in mice. Those mice, which were living off a very high-fat diet, exhibited increased calorie burn as well, which meant they weighed less and had more stable blood sugar levels. They were also less likely to suffer from fatty liver disease.
How to do it: The amount of ursolic acid fed to the mice in the study was equivalent to the amount you'd get from one to two apples per day. You can also find ursolic acid in basil, cranberries, peppermint, rosemary, lavender, oregano, thyme, and prunes. Add some strength training to your workout routine: Skeletal muscle, which also burns calories, can help your body produce more brown fat.
BONUS: Pop a Viagra. So, this "crazy trick" may not be for you, but a study out of Germany's Institute of Pharmacology and Toxicology at the University of Bonn Biomedical Center found, interestingly enough, that the main ingredient in Viagra and other erectile dysfunction drugs, sildenafil, directs your body to store more brown fat than white fat. The study, conducted on mice, found that sildenafil prevents the breakdown of a molecule simply called GMP, the absence of which leads your body to store more unhealthy fat. A possible new use for that little blue pill? Maybe.