Gut check: Study sizes up types of belly fat

An apple-shaped body with a wide waistline will have anyone's doctor pushing an exercise regimen and healthier eating. Pear-shaped people, meanwhile, might get a break.

That's because, as researchers are learning, all fat tissue is not the same -- and a new test might help take aim at the worst of it.

"It's not so much how much fat one has, it's really how fat is distributed," says Gerald Shulman, professor of internal medicine and cellular and molecular physiology at Yale University School of Medicine. Visceral fat, which accumulates in the belly and clings to the abdominal organs, appears to be more harmful than subcutaneous, superficial fat that's carried around the thighs, hips and backside.

That's important because even a relatively lean person who doesn't have a visible spare tire might have dangerous deep-belly fat. Such people, in addition to those carrying obvious guts, are at risk of developing metabolic and cardiovascular problems, including diabetes, high blood pressure, and high cholesterol. So researchers are working to find a simple way to test for visceral fat.

Once thought of as just an unsightly, inert mass, fat has gained recognition as a dynamic force in the body. "Fat is now thought to be an endocrine organ," like the thyroid and pituitary glands, which secrete important hormones, says Judith Korner, director of the weight control center at Columbia University Medical Center in New York City.

Fat releases proteins called cytokines, which cause detrimental inflammation, yet it also releases adiponectin and leptin, both hormones that help regulate metabolism. On balance, visceral fat does more harm than good. "The molecules it secretes ... rapidly enhance the development of disease," says endocrinologist Barbara Kahn, a division chief at Beth Israel Deaconess Medical Center in Boston.

There might soon be a new way to detect visceral fat, which doctors currently can see only on an MRI or CT scan. Research that Kahn and her colleagues published in the July issue of Cell Metabolism indicates that a protein called retinol-binding protein 4, or RBP4, is a marker for the dangerous, well-hidden tissue. RBP4 is associated with insulin resistance, cardiovascular disease and diabetes, and Kahn found that it's much more concentrated in obese people who have abundant visceral fat than in those with mainly subcutaneous fat.

That said, a relatively lean person can have a high RBP4 blood level -- and a correspondingly high risk of health problems, according to Kahn. Lean people who have close relatives with diabetes or cardiovascular disease might stand to benefit most from getting a test that measures RBP4. The test Kahn used in her study is experimental, but she expects a clinical version to become available within the year.

Kahn's research "suggests RBP4 might become a useful marker for diagnosis," says Bob Farese Jr., professor of biochemistry and biophysics at the University of California-San Francisco. Measurements of the protein could also be used to assess whether a particular treatment is working, he adds.

Regardless of their physical appearance, people who have visceral fat have especially good reasons to get active: Cris Slentz, an exercise physiologist at Duke University Medical Center in Durham, N.C., has found that a brisk, 30-minute walk six times a week keeps visceral fat at bay, and more strenuous activity helps the body shed the dangerous tissue. In study subjects who didn't move, meanwhile, visceral fat increased by 8.5 percent over six months.

"You don't have to (re)move much weight to see results," Korner says. "With exercise, visceral fat is the first to go."