Sex Difference between Body Composition and Weight-Bearing Bone Mineral Density in Korean Adult Twins: Healthy Twin Study

We performed a monozygotic (MZ) cotwin–control study using the MZ twin pair difference in bone mineral density (BMD) to assess the relationship between body composition and BMD at weight-bearing sites. This study controlled for common genetic factors and applied only to environmental factors, using 185 MZ twin pairs aged 30–50 years (140 male subjects, 230 female subjects). As expected, total lean mass (TLM) was greater in males and total fat mass (TFM) was greater in females. In male twins, TLM was associated with BMD at the legs, pelvis, and spine, with percent BMD increases of 0.41 (95% confidence interval [CI] 0.17–0.64), 0.62 (95% CI 0.35–0.89), and 0.27 (95% CI 0.01–0.54) for every 1 kg. In female twins, TFM was associated with BMD at the legs and pelvis, with percent BMD increases of 0.10 (95% CI 0.03–0.17) and 0.10 (95% CI 0.02–0.18) for every 1 kg. The results support the hypothesis that skeletal muscle and bone mass in middle-aged men are linked. In contrast, this association was not shown in women, and the impact of TFM on BMD was significant. Therefore, there were sex differences in the relationship of body composition on BMD.