Site-specific differences in bone mineral density in black and white premenopausal South African women

We examined ethnic differences in bone mineral density (BMD) and the contribution of body composition, lifestyle and socioeconomic factors in South African women. Femoral neck and total hip BMD were higher, but lumbar spine BMD was lower in black women, with body composition, lifestyle and socioeconomic status (SES) factors contributing differently in ethnic groups. There is a paucity of data on the relative contribution of body composition, lifestyle factors and SES, unique to different ethnic groups in South Africa, to BMD. We examined differences in femoral neck (FN), total hip (TH) and lumbar spine (LS) BMD between black and white premenopausal South African women and the associations between BMD and body composition, lifestyle factors and SES in these two ethnic groups. BMD and body composition were measured in 240 black (27 +/- 7; 18-45 years) and 187 white (31 +/- 8; 18-45 years) women using dual-energy X-ray absorptiometry. Questionnaires were administered to examine SES, physical activity and dietary intake. After co-varying for age, FN and TH were higher in black than white women (FN 0.882 +/- 0.128 vs. 0.827 +/- 0.116 g/cm(2), P < 0.001; TH 0.970 +/- 0.130 vs. 0.943 +/- 0.124 g/cm(2), P = 0.018). When adjusting for ethnic differences in body composition, LS was higher in white than black women. In black women, fat-free soft tissue mass, SES and injectable contraceptive use explained 33-42% of the variance in BMD at the hip sites and 22% at the LS. In white women, fat-free soft tissue mass and leisure activity explained 24-30% of the variance in BMD at the hip sites, whereas fat mass, leisure activity and oral contraceptive use explained 11% of the variance at the LS. FN and TH BMD were higher, but LS BMD was lower in black than white South African women with body composition, lifestyle and SES factors contributing differently to BMD in these women.