LONDON — If it really is what's on the inside that counts, then a lot of thin people might be in trouble.

Some doctors now think that the internal fat surrounding vital organs like the heart, liver or pancreas — invisible to the naked eye — could be as dangerous as the more obvious external fat that bulges 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. Since 1994, Bell and his team have scanned nearly 800 people with MRI machines to create "fat maps" showing where people store fat.

According to the data, people who maintain their weight through diet rather than exercise are likely to have major deposits of internal fat, even if they are otherwise slim. "The whole concept of being fat needs to be redefined," said Bell, whose research is funded by Britain's Medical Research Council.

Without a clear warning signal — like a rounder middle — doctors worry that thin people may be lulled into falsely assuming that because they're not overweight, they're healthy.

"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 University Medical Center in New Jersey, who was not involved in Bell's research.

Even people with normal Body Mass Index scores — a standard obesity measure that divides your weight by the square of your height — can have surprising levels of fat deposits inside.

Of the women scanned by Bell and his colleagues, as many as 45 percent of those with normal BMI scores (20 to 25) actually had excessive levels of internal fat. Among men, the percentage was nearly 60 percent.

Relating the news to what Bell calls "TOFIs" — people who are "thin outside, fat inside" — is rarely uneventful. "The thinner people are, the bigger the surprise," he said, adding the researchers even found TOFIs among people who are professional models.

According to Bell, people who are fat on the inside are essentially on the threshold of being obese. They eat too many fatty, sugary foods — and exercise too little to work it off — but they are not eating enough to actually be fat. Scientists believe we naturally accumulate fat around the belly first, but at some point, the body may start storing it elsewhere.

Still, most experts believe that being of normal weight is an indicator of good health, and that BMI is a reliable measurement.

"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.