"BMI won't give you the exact indication of where fat is, but it's a useful clinical tool," said Dr. Toni Steer, a nutritionist at Britain's Medical Research Council.

Doctors are unsure about the exact dangers of internal fat, but some suspect it contributes to the risk of heart disease and diabetes. They theorize that internal fat disrupts the body's communication systems. The fat enveloping internal organs might be sending the body mistaken chemical signals to store fat inside organs like the liver or pancreas. This could ultimately lead to insulin resistance, type 2 diabetes, or heart disease.

Experts have long known that fat, active people can be healthier than their skinny, inactive counterparts. "Normal-weight persons who are sedentary and unfit are at much higher risk for mortality than obese persons who are active and fit," said Dr. Steven Blair, an obesity expert at the University of South Carolina.

For example, despite their ripples of fat, super-sized Sumo wrestlers probably have a better metabolic profile than some of their slim, sedentary spectators, Bell said. That's because the wrestlers' fat is primarily stored under the skin, not streaking throughout their vital organs and muscles.

The good news is that internal fat can be easily burned off through exercise or even by improving your diet. "Even if you don't see it on your bathroom scale, caloric restriction and physical exercise have an aggressive effect on visceral fat," said Dr. Bob Ross, an obesity expert at Queen's University in Canada.

Because many factors contribute to heart disease, Teichholz says it's difficult to determine the precise danger of internal fat — though it certainly doesn't help.

"Obesity is a risk factor, but it's lower down on the totem pole of risk factors," he said, explaining that whether or not people smoke, their family histories and blood pressure and cholesterol rates are more important determinants than both external and internal fat.

When it comes to being fit, experts say there is no short-cut. "If you just want to look thin, then maybe dieting is enough," Bell said. "But if you want to actually be healthy, then exercise has to be an important component of your lifestyle."