What part of the body is affected by fats?

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 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.

Brown fat is now thought to be more like muscle than like white fat. When activated, brown fat burns white fat.

Although leaner adults have more brown fat than heavier people, even their brown fat cells are greatly outnumbered by white fat cells. "A 150-pound person might have 20 or 30 pounds of fat," Cypess says. "They are only going to have 2 or 3 ounces of brown fat."

But that 2 ounces, he says, if maximally stimulated, could burn off 300 to 500 calories a day -- enough to lose up to a pound in a week.

"You might give people a drug that increases brown fat," he says. "We're working on one."

But even if the drug to stimulate brown fat pans out, Cypess warns, it won't be a cure-all for weight issues. It may, however, help a person achieve more weight loss combined with a sound diet and exercise regimen.

White fat is much more plentiful than brown, experts agree. The job of white fat is to store energy and produce hormones that are then secreted into the bloodstream.

Small fat cells produce a "good guy" hormone called adiponectin, which makes the liver and muscles sensitive to the hormone insulin, in the process making us less susceptible to diabetes and heart disease.

When people become fat, the production of adiponectin slows down or shuts down, setting them up for disease, according to Fried and others.

Subcutaneous fat is found directly under the skin. It's the fat that's measured using skin-fold calipers to estimate your total body fat.

In terms of overall health, subcutaneous fat in the thighs and buttocks, for instance, may not be as bad and may have some potential benefits, says Cypess. "It may not cause as many problems" as other types of fat, specifically the deeper, visceral fat, he says.

But subcutaneous fat cells on the belly may be another story, says Fried. There's emerging evidence that the danger of big bellies lies not only in the deep visceral fat but also the subcutaneous fat.

Visceral or "deep" fat wraps around the inner organs and spells trouble for your health. How do you know if you have it? "If you have a large waist or belly, of course you have visceral fat," Whitmer says. Visceral fat drives up your risk for diabetes, heart disease, stroke, and even dementia.

Visceral fat is thought to play a larger role in insulin resistance -- which boosts risk of diabetes -- than other fat, Whitmer tells WebMD. It's not clear why, but it could explain or partially explain why visceral fat is a health risk.

Whitmer investigated the link between visceral fat and dementia. In a study, she evaluated the records of more than 6,500 members of Kaiser Permanente of Northern California, a large health maintenance organization, for an average of 36 years, from the time they were in their 40s until they were in their 70s.

The records included details on height, weight, and belly diameter -- a reflection of the amount of visceral fat. Those with the biggest bellies had a higher risk of dementia than those with smaller bellies. The link was true even for people with excess belly fat but overall of normal weight.

She doesn't know why belly fat and dementia are linked, but speculates that substances such as leptin, a hormone released by the belly fat, may have some adverse effect on the brain. Leptin plays a role in appetite regulation but also in learning and memory.

Belly fat has gotten a mostly deserved reputation as an unhealthy fat. "Understand that belly fat is both visceral and subcutaneous," says Kristen Gill Hairston, MD, MPH, an assistant professor of medicine at Wake Forest University School of Medicine, Winston-Salem, N.C. "We don't have a perfect way yet to determine which [of belly fat] is subcutaneous or visceral, except by CT scan, but that's not cost-effective."

But if you've got an oversize belly, figuring out how much is visceral and how much is subcutaneous isn't as important as recognizing a big belly is unhealthy, she says. How big is too big? Women with a waist circumference more than 35 inches and men with a waist circumference more than 40 inches are at increased disease risk.

Abdominal fat is viewed as a bigger health risk than hip or thigh fat, Whitmer and other experts say. And that could mean having a worse effect on insulin resistance, boosting the risk of diabetes, and a worse effect on blood lipids, boosting heart and stroke risks.

While men tend to accumulate fat in the belly, it's no secret women, especially if "pear-shaped," accumulate it in their thighs and buttocks.

Evidence suggests that pear-shaped women are protected from metabolic disease compared to big-bellied people, says Fried.

"Thigh fat and butt fat might be good," she says, referring to that area's stores of subcutaneous fat. But the benefit of women being pear shaped may stop at menopause, when women tend to deposit more fat in the abdomen.

So when you lose weight, what kind or kinds of fat do you shed? "You're losing white fat," Fried tells WebMD. "People tend to lose evenly all over."

The results change a bit, however, if you add workouts to your calorie reduction, she says. "If you exercise plus diet you will tend to lose slightly more visceral fat from your belly."

"We're at an exciting point in science," says Whitmer, echoing the input from other scientists in the field.

Whitmer and others expect more discoveries about fat of all types to be made in the near future.