Ethnic differences in bone density in female South African nurses

In the United States, the higher prevalence of osteoporosis and the higher incidence of fractures in whites than in blacks may be attributed to the finding of lower bone density (BD) in both white children and adults. In South Africa, osteoporosis and fractures also occur more frequently in whites than in blacks. Appendicular BD has been found to be similar in black and white children in South Africa, but there is little information available on BD of adults in South Africa. This cross-sectional study aimed to assess changes in BD with age in adult females in South Africa and to assess possible differences in peak BD and in the rate of postmenopausal bone loss between blacks and whites. Data for 180 black and 184 white female nurses aged 20–64 years were analyzed. The distal radius bone density (RBD) was measured by single photon absorptiometry. The lumbar spine bone density (SBD) and the femur bone density (FBD) were measured by dual-energy X-ray absorptiometry. Blacks were shorter than whites (p = 0.0001), and blacks' weight, body mass index, and skinfold thickness increased with age. Peak SBD and RBD were similar in blacks and whites, but peak FBD was higher in blacks (p = 0.0001). This ethnic difference in peak FBD became apparent in the fourth decade. Peak FBD was similar in black and white subjects with normal body mass indices (p = 0.09), but in overweight subjects peak FBD was higher in blacks than in whites (p = 0.0001). SBD (p = 0.03), RBD (p = 0.0002), and FBD (p = 0.04) in whites, but not in blacks, decreased after the menopause. Weight had a positive association with FBD in premenopausal blacks (R = 0.24, p = 0.0001) but not in whites. In the postmenopausal group, adiposity had a positive association with SBD and RBD in blacks and with FBD in both blacks and whites. These results suggest that weight-bearing is important in the attainment of peak FBD and that adiposity may be important in decreasing postmenopausal bone loss. The finding of higher FBD in blacks than in whites may explain the lower rate of femoral fractures in black South Africans. However, the similarity in SBD between blacks and whites in this study suggests that factors other than bone density might contribute to the lower vertebral fracture rate in black compared with white South Africans.