Muscle: clues to the diabetic difference.

Although obesity boosts a person's risk of developing Type 11 diabetes, endocrinologists have few clues to explain why. a study of middle-aged men now indicates that heavy people, compared with normal-weight ones, lay down a less dense form of skeletal muscle as they gain weight.

Because an obesity-related resistance to insulin in skeletal muscle is one hallmark of adult-onset diabetes, understanding what's behind this density difference might ultimately enable scientists to figure out how excess weight fosters this disabling disease.

For their muscle study, researchers at the University of Pittsburgh recruited 10 healthy men and 10 with Type II diabetes. They matched the two groups for age, size and levels of body fat. Volunteers in both groups ranged from lean to heavy.

The team, led by endocrinologist David E. Kelley, performed at least 10 cross-sectional computed-tomography scans of each man's thigh, gauging the density of lean tissue--primarily skeletal muscle -- by the degree to which it reduced the transmission of X-rays. Levels of normal-density lean tissue did not correlate with diabetes or obesity, they found. However, obesity was strongly linked to the presence and quantity of unusually low-density lean tissue, which tended to deposit around a core of normal-density skeletal muscle. Obese diabetics deposited the lightest form of this excess lean tissue.

These findings represent the first indications of a difference in the composition of lean tissue in normal-weight and obese people, Kelley and his Co-workers assert in the September AMERICAN JOURNAL OF CLINICAL NUTRITION. "One intriguing possibility," they write, "is that [the normal-density tissue] represents the residual thin man within the obese individual."

"It's an excellent study," comments endocrinologist Stephen Lillioja, who says the new findings may lead "to insights on why obese people are getting insulin resistant." Lillioja, who works at the Phoenix office of the National Institute of Diabetes and Digestive and Kidney Diseases, suggests that a microdispersion of fat cells in skeletal muscle or an excess of triglycerides (fat) within individuals muscle cells might account for the excess lean tissue's unusually light density. If so, he says, this might help explain the impaired glucose metabolism seen in type II diabetics.

Indeed, Kelley says his analyses lead him to suspect that the unusual muscle tissue observed in heavier men may represent precisely the type Lillioja describes.