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.
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.
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.
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.