If you think skinny means healthy, you're wrong.

Many experts now believe that the internal fat around vital organs like the heart, liver or pancreas, which cannot be seen, is more dangerous than the obvious external fat that stores underneath the skin.

"Being thin doesn't automatically mean you're not fat," said Dr. Jimmy Bell, a professor of molecular imaging at Imperial College, London.

A team has scanned since 1994 about 800 people with MRI machines to build "fat maps" revealing where people deposit the fat.

People who maintain their profile just through diet rather than exercise were found to possess higher deposits of internal fat, even if slim.

"The whole concept of being fat needs to be redefined," said Bell.

Lacking a warning signal like swollen belly, thin people may be deceived by the idea that because they're not overweight, their health is strong.

"Just because someone is lean doesn't make them immune to diabetes or other risk factors for heart disease," said Dr. Louis Teichholz, chief of cardiology at Hackensack Hospital in New Jersey.

Bell's team discovered that even people with normal Body Mass Index (BMI) have surprisingly high levels of fat tissue: up to 45 % of the women with normal BMI scores had excessive levels of internal fat, while for men the number rose to about 60 %.

"People who are fat on the inside are essentially on the threshold of being obese." said Bell.

These people have a too abundant diet in fat and sugars without exercising: they just do not eat enough to be fat. Many experts still regard normal values of BMI as an indicator of good health. But many doctors believe the internal fat is increasing the risk of heart disease and diabetes.

The fat could make the body to mistakenly store more fat inside organs like the liver or pancreas, which ultimately leads to insulin resistance, type 2 diabetes, or heart disease. Indeed, experts have long known that fat but active people can be healthier than the skinny ones.

"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.

In the case of sports-practicing persons, the fat is deposed primarily under the skin, not mingling with their vital organs.

Still, there are hopes.

"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.

"If you just want to look thin, then maybe dieting is enough. But if you want to actually be healthy, then exercise has to be an important component of your lifestyle." Bell said.