Dr. Norbert Stefan and colleagues at the University of Tubingen, Germany, studied 314 people ages 18 to 69 -- average age 45. Using magnetic resonance tomography, the researchers measured participants' total body fat, visceral fat -- or abdominal fat around the internal organs -- and subcutaneous fat -- fat under the skin. Insulin resistance was measured.

The subjects were divided into four groups: normal weight, overweight, obese but still sensitive to insulin and obese with insulin resistance.

Those in the overweight and obese groups had more total body and visceral fat than those at a normal weight, but obese individuals with insulin resistance -- linked to heart risk -- had more fat within their skeletal muscles and their livers than obese individuals without insulin resistance.

Those with insulin resistance also had thicker blood vessels -- a sign of heart disease -- but the obese people without insulin problems were no different than those with a healthy weight. This group, called metabolically benign obese, may be protected from insulin resistance and atherosclerosis.

The study is published in the Archives of Internal Medicine.