New findings call into question the idea that being overweight or obese might protect people from developing brittle bones.

Dr. Jean-Marc Kaufman of Ghent University Hospital in Belgium and his colleagues found that fattier men had smaller, thinner bones, while those with more lean mass had larger, denser skeletons.

High body mass index (BMI) has been thought to protect both men and women against the bone-thinning disease osteoporosis, while being thin boosts a person's likelihood of fractures due to this condition. The idea behind the theory is that extra weight stresses the bone, stimulating the formation of new bone tissue.

But new evidence suggests that fat mass might affect bones differently than muscle mass does, Kaufman and his team note in the Journal of Clinical Endocrinology and Metabolism.

To investigate the issue further, the researchers looked at bone density and volume, as well as lean and fat mass, in 768 men aged 25 to 45, including 296 pairs of brothers.

After the researchers adjusted for weight, they found that men's bone mass and volume fell steadily as their percentage of fat mass increased, while bone size rose in tandem with lean mass. Fat in the trunk area had a stronger influence on bone size than fat on the arms and legs.

"Lean mass," the researchers conclude, "is the major determinant of bone size, providing further evidence that bone size is adapted to the dynamic load imposed by muscle force rather than passive loading" by fat.