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For most of us, body fat has a bad reputation. From the dimply stuff that plagues women's thighs to the beer bellies that can pop out in middle-aged men, fat is typically something we agonize over, scorn, and try to exercise away.

But for scientists, fat is intriguing -- and becoming more so every day. "Fat is one of the most fascinating organs out there," says Aaron Cypess, MD, PhD, an instructor of medicine at Harvard Medical School and a research associate at the Joslin Diabetes Center in Boston. "We are only now beginning to understand fat."

"Fat has more functions in the body than we thought," agrees Rachel Whitmer, PhD, research scientist at the Kaiser Permanente Division of Research in Oakland, Calif., who has studied the links between fat and brain health.

To get the skinny on fat, WebMD asked four experts on fat -- who, not surprisingly, prefer not to be called fat experts -- to fill us in.

Fat is known to have two main purposes, says Susan Fried, PhD, director of the Boston Obesity and Nutrition Research Center at Boston University and a long-time researcher in the field.

  • Fat stores excess calories in a safe way so you can mobilize the fat stores when you're hungry.
  • Fat releases hormones that control metabolism.

But that's the broad brushstroke picture. Read on for details about various types of fat -- brown, white, subcutaneous, visceral, and belly fat.

Brown Fat

Brown fat has gotten a lot of buzz recently, with the discovery that it's not the mostly worthless fat scientists had thought.

In recent studies, scientists have found that lean people tend to have more brown fat than overweight or obese people -- and that when stimulated it can burn calories. Scientists are eyeing it as a potential obesity treatment if they can figure out a way to increase a person's brown fat or stimulate existing brown fat.

It's known that children have more brown fat than adults, and it's what helps them keep warm. Brown fat stores decline in adults but still help with warmth. "We've shown brown fat is more active in people in Boston in colder months," Cypess says, leading to the idea of sleeping in chillier rooms to burn a few more calories.

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