Everyone has at least a little bit of brown fat. Unlike regular old white fat, which stores calories, mitochondria-packed brown-fat cells burn energy and produce heat.
It was once thought that, in humans, only babies had brown fat. But in 2009, researchers found small amounts of brown fat in adults. What's more, they found that people with lower body mass indexes (BMIs) tended to have more brown fat. This finding suggests "a potential role of brown [fat] in adult human metabolism," the researchers wrote in their findings.
Because of brown fat's ability to burn calories, scientists are looking for ways to exploit its power to help fight obesity. Here are five fascinating facts about this hot topic.
1. Brown fat is activated by cold.
Spending time in the cold makes your brown fat more active, and could even cause you to grow new brown-fat cells, according to a 2014 study conducted by National Institutes of Health researchers and published in the journal Diabetes. "It helps us to defend our body temperature in a comfortable manner," said Barbara Cannon, a professor of physiology at the Wenner-Grenn Institute in Stockholm and president of the Royal Swedish Academy of Sciences. "Mammals and birds [maintain] a more or less constant body temperature."
Brown fat helps babies — who don't yet have the ability to shiver — to stay warm. In adults exposed to cold temperatures, brown fat may serve as an "internal heating jacket" to keep blood warm as it flows back to the heart and brain from our chilly extremities, Harold Sacks, of the VA Greater Los Angeles Healthcare System, and Michael Symonds, of the University of Nottingham in England, suggested in a 2013 paper.
2. It's found in weird spots.
Brown fat is found in unpredictable locations in the body. "We know where brown fat can be found, but it's not always there in every single person. There's a region in the neck and the shoulders, and that is where you typically find it, but not everybody has it there," said Aaron Cypess, head of the Diabetes, Endocrinology and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases and part of the research team that found brown fat in adults in 2009. [Ready for Med School? Test Your Body Smarts]
In a recent study, Cypess and his colleagues also found brown fat in the chests and down the spines of a group of healthy young men. "It's in a lot of interesting places in the abdomen," Cypess said.
3. You have at least some brown fat.
Everyone has it. "Probably everybody has a few cells of brown fat, even if you can't see it with a PET/CT scan," Cypess told Live Science. "We do believe that you can grow them. We can probably grow it in anybody."
4. It's hard to find.
Brown fat can be hard to find, and to study. That's because brown and white fat cells are often mixed in together in fat tissue, Cypess said. Finding the brown fat cells requires performing CT scans to show where the fat is, combined with PET scans (which requires injecting people with radioactive glucose) to identify the most metabolically active cells. Cypess and others are working to find less invasive, and less expensive, ways to spot brown fat and measure its activity.
5. Someday, you might be able to take a pill to activate your brown fat.
A drug that treats people with overactive bladder can boost brown fat activity, according to findings that Cypess and his colleagues reported in January 2015 in the journal Cell. The medication, called mirabegron, stimulates receptors called beta 3 receptors, which cause smooth muscle — in, for example, your bladder — to relax. These receptors also are found on both brown and white fat cells, the researchers said.
Drugs that fire up brown fat could be useful for treating people with fatty liver disease, a buildup of fat cells in the liver that may affect up to 25 percent of people in the United States, Cypess said.
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