Gender differences in volumetric bone density: a study of opposite-sex twins

Gender difference in bone size is a potential confounder when comparing bone density between males and females. A comparison of volumetric BMD (vBMD) between men and women, which is a measure of bone mass relative to three-dimensional bone volume (g/cm(3)) as opposed to areal bone density (g/cm(2)), may be a more accurate reflection of gender differences in bone density. The aims of this study were to examine gender differences in bone mass (BMC), areal BMD (aBMD), volumetric BMD (vBMD) by comparing twins of opposite sex in whom the effects of age, genes and environment are partially controlled for. DEXA derived BMC, aBMD, vBMD at the third lumbar vertebra (L3), femoral neck (FN) and forearm (1/3 radius) were compared between 82 opposite sex pairs aged 18-80. BMC was significantly higher in males at all three sites (26-45.5%). For aBMD the gender differences remained significant at all sites except the spine. The average differences in aBMD were not as great as the differences in BMC (2.2-20.5%). The differences in vBMD, however, followed a different pattern. FN and L3 vBMD were significantly higher in females (4.8 and 0.6%, respectively), while radial BMD was not significantly different between the sexes. Comparing aBMD values between males and females, when females in general have a smaller skeleton than males may not be a true indication of gender differences in bone density. A comparison of vBMD between men and women shows only small differences in bone density between the sexes.