Ethnic and gender differences in bone mineral density and bone turnover in young adults: effect of bone size

Generally, the incidence of osteoporotic fracture is lower in black populations and in men. These effects of ethnicity and gender may result from differences in peak bone mineral density (PBMD) and bone turnover (BT), which in turn are affected by bone size. Therefore, the aims of this study were to examine the effects of ethnicity and gender on bone mineral density (BMD) and BT in young African-Caribbean and Caucasian adults, and to adjust for the effect of bone size on BMD and BT. BMD was measured at the lumbar spine, L2-L4 (LS), total body (TB) and femoral neck (FN) by dual-energy X-ray absorptiometry in 44 blacks (16 men, 28 women) and 59 whites (28 men, 31 women) ages 20-37 years. We measured serum bone-specific alkaline phosphatase (BAP) and serum osteocalcin (OC) as markers of bone formation and urinary immunoreactive free deoxypyridinoline (ifDpd) and crosslinked N-telopeptide of type I collagen (NTx) as markers of bone resorption. To adjust the data for any differences in bone size, we calculated: (a) bone mineral apparent density (BMAD), an estimated volumetric bone density which attempts to normalize BMD measurements for bone size; and (b) bone resorption markers as a ratio to total body bone mineral content (TB BMC). Two-way analysis of variance was used to compare the effects of race and gender, and to test for any interaction between these two factors. Blacks had higher BMD compared with whites at the TB (p<0.001), LS (p = 0.0001) and FN (p = 0.0005). This increase remained significant at the LS only after calculating BMAD. Men had higher BMD at all sites (except at the LS). This increase was no longer significant at the FN after calculating BMAD, and LS BMAD was actually greater in women (p<0.0001). Blacks and whites had similar concentrations of turnover markers, but men had higher bone turnover markers than women (BAP, p<0.0001; OC, p = 0.002; ifDpd, p = 0.03; NTx, p < 0.0001). This increase in bone resorption markers was no longer significant after adjusting for TB BMC (except for NTx in whites). We conclude that the skeletal advantage in blacks during young adulthood is not explained by bone size. However, it seems probable that bone size effects partially explain gender differences in BMD and bone turnover.