An investigation into the relationship between soft tissue body composition and bone mineral density in a young adult twin sample

The purpose of this study was to investigate the relationship of fat mass (FM) and lean mass (LM) with bone mineral density (BMD) independent of genetic effects. We also assessed the extent to which genetic and environmental influences explain the associations between these phenotypes. Body composition and BMD were measured using dual‐energy X‐ray absorptiometry in 57 MZ and 92 same‐sex DZ twin pairs, aged 23‐31 years, chosen to represent a wide range of intrapair differences in BMI (0 to 15.2 kg/m²). Heritability estimates were adjusted for height and gender. In multiple linear regression analysis, intra‐pair differences in both FM and LM were independently associated with intra‐pair differences in BMD at most skeletal sites after adjustment for gender and differences in height. Within MZ and DZ pairs, LM was a significantly stronger predictor of whole‐body BMD than FM (p<0.01). Additive genetic factors explained 87% (95% CI: 80‐ 91%), 81% (95% CI: 70‐88%) and 61% (95% CI: 41‐75%) of the variation in whole‐body BMD, LM and FM, respectively. Additive genetic factors also accounted for 69‐88% of the covariance between LM and BMD and for 42‐72% of the covariance between FM and BMD depending on the skeletal site. The genetic correlation between LM and whole‐body BMD (r_g=0.46, 95% CI 0.32‐0.58) was greater than that of FM and whole‐body BMD (r_g=0.25, 95% CI 0.05‐0.42). In conclusion, our data indicate that peak BMD is influenced by acquired body weight as well as genetic factors. In young adulthood LM and BMD may have more genes in common than do FM and BMD. © 2010 American Society for Bone and Mineral Research