To clarify the relationship between bone mineral density (BMD) and body composition in Japanese women aged 18–40 years with low forearm BMD.

The subjects were Japanese199 women who had been selected for inclusion in the study based on a low forearm BMD determined at the Annual Women’s Health Examination. The subjects’ mean (± standard deviation) age, body height, body weight, and body mass index (BMI) were 33.5 (±4.3) years, 158.1 (±5.1) cm, 49.6 (±5.7) kg, and 19.8 (±2.1), respectively. The BMD of the lumbar spine, total body, and left arm were measured using dual-energy X-ray absorptiometry (DXA). Fat mass (FM), bone-free lean tissue mass (LTM), and body fat percentage (BF%) were measured simultaneously with DXA.

In the structural equation model, the standardized regression weights for the path from BMI to BMD of all sites were 0.273–0.434. Conversely, the BF% to BMD of the total body and left arm were −0.192 and −0.296, respectively. In multiple regression analysis, the FM index (FMI) was significantly associated with the BMD of the lumbar spine as a weight-bearing site. The LTM index (LTMI) was significantly associated with the BMD of the total body and left arm as a non-weight-bearing site.

Young females with low forearm BMD had low body weight and BMI. Thinness was shown to be a risk factor for low BMD, in accordance with results reported elsewhere. A gain in body weight may have the effect of increasing BMD, but our results suggest that to increase BMD, the gain in body weight must include increases in LTM, and not FM alone.