Study suggests excess body fat negatively affects women's bone health

Adjusting for muscle mass differences, researchers found participants with high body fat had bones 8% to 9% weaker than those with normal body fat.

Excess body fat in women may not have a beneficial effect on bone mass, and may actually negatively impact on adolescent bone, according to a study by researchers at the University of Georgia.

Richard D. Lewis, PhD, and colleagues used peripheral quantitative computed tomography (PQCT) to obtain three-dimensional bone scans on 115 women ages 18 to 19 years old who had either normal body fat composition — less than 32% body fat — or high body fat, greater than 32%.

The researchers found that participants with either normal- or high-body-fat both had comparable bone strengths. However, after adjusting for differences in muscle mass surrounding the bone, the researchers found that the bones of participants with high body fat were 8% to 9% weaker than those in the normal the body fat participants, according to a press release from the university.

Previous studies on bone health and obesity used a two-dimensional bone densitometer that is commonly used in osteoporosis screenings. Such measurements do not account for bone shape and geometry, which have a substantial influence on bone strength, the release reported.

"The fact that the two groups had similar bone strength measures is remarkable in itself, because you would expect it to be higher in the heavier person," Lewis stated in the release.

"When we corrected for the amount of muscle, we found that the obese person is not making as much bone as they should be for the amount of muscle that they have," Norman K. Pollock, a doctoral candidate at the university and first author of the study, noted in the release. "People haven't observed that in the past because they weren't using the three-dimensional scan."

The authors reported, "Consistent with the correlational data, after correcting for MCSA (muscle cross-sectional area) differences, the high-fat group had significantly lower tibial cortical bone area, cortical BMC (bone mineral content), and SSI (strength-strain index), as well as radial total bone cross-sectional area and SSI than did the normal-fat group.

"Collectively, our data suggest that, contrary to the idea that extra body weight is advantageous for the skeleton, excess weight in the form of fat mass does not provide additional benefits and may potentially be negative for adolescent bone," they said.

The exact mechanisms by which excess fat hinders bone strength are unclear, but studies of obese rats show that they produce more fat cells in the bone marrow and fewer bone cells. Since fat and bone cells originate from the same precursor, it may be that fat cell production is favored over bone cell production in obese people, Lewis reported in the release.

The study is published in the American Journal of Clinical Nutrition.

For more information:
Pollock NK, Laing EM, Baile CA, et al. Is adiposity advantageous for bone strength? A peripheral quantitative computed tomography study in late adolescent females. Am J Clin Nutr. 2007;86:1530-1538.