If your belt is getting too tight, take it as a warning sign. Having a large waistline can put you at risk for chronic disease, even though your weight is otherwise in proportion to your height.

In fact, you can be lean but metabolically obese if you carry your weight around the midriff, says Osama Hamdy, M.D., of the Joslin Diabetes Center in Boston.

"A large waistline is one of the diagnostic criteria for metabolic syndrome (a cluster of conditions that greatly increases your chances of developing type 2 diabetes and heart disease)," says Hamdy.

If you're an adult male with a waistline of 40 inches or more, or an adult female with a waistline of 35 inches, your waist measurements should concern you, according to Hamdy. Even if you're otherwise slight but your waistline is 35 inches if you're a man, or 30 inches if you're a woman, your midsection could be a health factor.

Fat stored at your waist is different from the fat elsewhere on your body. It's not subcutaneous fat, but visceral fat and fills the abdominal cavity draping internal organs, according to Anne Peters Harmel, M.D., director of the clinical diabetes programs at the University of Southern California.

Earlier in human development, visceral fat was essential for survival. "There are a lot of theories about this,'' says Harmel, "but we think the fat is stored at the waist to help people survive famine. It's placed near the internal organs, where the body needs it."

When starvation isn't a concern, the internal fat is allowed to build up over time. Unfortunately, visceral fat isn't inert. "It makes different hormones that affect hunger, satiety, inflammation and other activity," says Harmel. "Metabolically active fat around the pancreas and liver send signals that trigger diabetes. In many cases, visceral fat is considered the most dangerous fat."

Visceral fat build-up is dangerous because it causes the body to secrete hormones that alter blood coagulation and increase the rate of fatty acids that go to the liver, she adds.

That in turn increases your risk of developing diabetes, blood-fat disorders and high blood pressure, possibly leading to heart disease.

"Every one of my patients who has diabetes points to the center as the area where they gain weight," says Harmel.

Tempting as it may be, you can't get rid of visceral fat through liposuction. Nor can you sit in a steam bath and sweat it off. You can, however, lose visceral fat the old-fashioned way, with a calorie-restricted diet and exercise. As you lose weight, you'll see a trimmer waistline. And unlike the fat you never seem to budge from other parts of the body, visceral fat is relatively easy to get rid of. Those first pounds you drop when you're on a diet are mostly visceral fat, says Harmel. Along with weighing yourself to measure your progress, try on your belt. The looser it is, the better you're doing.

Bev Bennett is co-author of "The Dictionary of Healthful Food Terms" (Barron's, 1997).