British researchers have found that those who control their weight through diet rather than exercise are more likely to have major deposits of internal fat, even if they are otherwise slim.
The researchers from Imperial College, London say whether a person is thin or fat is really determined by how they appear from the inside and is not based on external appearance.
Doctors believe that the internal fat surrounding such vital organs as the heart, liver or pancreas, which cannot be seen from the naked eye, could be as deadly to health as the more obvious external fat that is easily seen.
The study which was funded by the Medical Research Council involved 800 people who were scanned using MRI devices and Dr. Jimmy Bell, a professor of molecular imaging at Imperial College, says being thin does not automatically mean a person is not fat.
Dr. Bell's "fat maps" taken since 1994 which were created from the scans, show where excess fat was stored and it was found that people who were thin were not necessarily trim; as many as 60 percent of men and 45 percent of the women with normal BMI scores (20 to 25) screened who were externally thin had dangerous deposits of internal fat.
The researchers believe that the "concept of being fat needs to be redefined" as people who are thin outside, but fat inside are hovering on the verge of being obese.
Experts say that it is at present unclear what the exact dangers of internal fat are but many believe that it contributes to the risk of heart disease and diabetes.
They say internal fat disrupts the body's communication systems by sending mistaken chemical signals to the body to store fat inside organs such as the liver or pancreas.
On a more reassuring note however the researchers add that internal fat can be easily burned off through exercise or even by improving the diet and they recommend cutting down on the calories and beginning a physical exercise programme to ensure a healthy life free from heart diseases.