Fat mass helps build bone mass in girls, study suggests; excessive fat reduction may increase osteoporosis risk

Lean mass is one of the strongest determinants of bone mass throughout life. Until now, it has been unclear whether fat mass and lean mass differ in how they influence bone development in boys and girls. Findings from previous studies have been inconsistent regarding whether fat mass has a positive or negative impact on bone development. This new study shows that fat mass is a strong stimulus for the accrual of cortical bone mass (hard outer layer of bone) in girls.

In this study, researchers used dual x-ray absorptiometry (DXA) to determine total body fat mass and lean mass, and peripheral quantitative computer tomography (pQCT) to measure cortical bone mass at the mid-tibia, in 4,005 boys and girls with a mean age of 15.5 years. Although lean mass was the major determinant of bone mass, fat mass also exerted an important positive influence, particularly in girls, in which the effect was approximately 70 percent greater than in boys.

"The effect of fat mass on bone mass appears to be strongest in girls," said Jonathan Tobias, PhD, of the University of Bristol in the United Kingdom, and lead author of the study. "Girls clearly have more fat mass than boys and our findings show that whereas the greater lean mass in boys contributes to their greater cortical bone mass, this effect is partly counteracted by the greater fat mass in girls."

"Fat mass in girls during puberty may have a long-term impact on bone health as they grow into adulthood," said Tobias. "Excessive reduction in fat mass could have adverse effects on the developing skeleton particularly in girls, leading to an increased risk of osteoporosis in later life."

Adrian Sayers of the University of Bristol in the United Kingdom also worked on this study.

The article, "Fat mass exerts a greater effect on cortical bone mass in girls than boys," will appear in the February 2010 issue of JCEM.