Bone density in black and white South African women: contribution of ethnicity, body weight and lifestyle

Ethnic differences in bone mineral density (BMD) between healthy adult black and white South African women were studied. Higher BMD was only noted at the femoral sites in black women. Body weight significantly impacted these findings. A lower fracture risk at all skeletal sites cannot be assumed in black South African (SA) women.

Bone mineral density (BMD) varies amongst women of different ethnicities. African-Americans have higher BMD at all skeletal sites compared with whites. On the African continent, bone density studies suggest site-specific ethnic differences in BMD. To examine the contribution of body weight and lifestyle characteristics to ethnic differences in BMD between adult black and white South African women, we assessed lumbar spine (SBMD), femoral neck (F_NBMD) and total femoral BMD (F_TBMD) by dual-energy X-ray absorptiometry (DXA) in 184 black and 143 white women aged between 23 and 82 years.

BMDs were compared amongst pre- and postmenopausal blacks and whites before and after adjustment for covariates with significant univariate association with BMD. Volumetric bone mineral apparent density (BMAD) of the spine and femoral neck was also calculated to account for ethnic differences in bone size.

Before adjustment, SBMD was lower (p < 0.05), F_TBMD similar and F_NBMD (p < 0.01) higher in premenopausal black women. Similar SBMD, but significantly higher BMD at the femoral sites (p < 0.01), was noted in postmenopausal blacks compared with whites. Amongst anthropometric measures and lifestyle factors, only adjustment for weight significantly altered these observed ethnic differences in bone density. After adjustment for weight, SBMD remained lower in premenopausal blacks and became lower in young postmenopausal blacks. Weight adjustment eliminated all ethnic differences in proximal femoral BMD measurements, with the exception of F_NBMD that remained higher in younger postmenopausal blacks. Before adjustment, calculated SBMAD was similar and F_NBMAD consistently higher in blacks in all the menstrual groups. Adjustment for weight did not alter these findings.

Most of the observed ethnic difference in BMD was explained by differences in body weight between black and white SA women. The higher femoral BMD in older blacks may explain, the lower hip fracture prevalence in black South African women. The lower SBMD in pre- and postmenopausal black women in this study suggests that factors other than BMD should be considered to explain a lower vertebral fracture prevalence in blacks, if a lower fracture prevalence does indeed exist.