To our knowledge, this analysis is one of few studies that identified correlates of BMD among postmenopausal women of West African ancestry residing outside the U.S. Of importance, Tobagonian women had 10–18% and 29% higher BMD compared to non-Hispanic Black and White women in the U.S., respectively [4
]. To our knowledge, these African Caribbean women have the highest BMD on a population level reported to date for women.
It is well established that BMD is significantly higher among U.S. Black women compared to White women. Greater skeletal size and bone size (depth) may partially explain the higher BMD that was observed among postmenopausal Tobagonian women compared to U.S. women of similar age. Areal BMD accounts for only bone length and width, not bone depth [25
]. Reference data did not allow for direct comparison of volumetric BMD [4
], which takes bone depth into consideration, between Tobagonian and U.S. women. Research suggests that people of African ancestry may have genetic potential for higher BMD compared to people of European ancestry [7
]. This high BMD potential may not be realized among native Africans due to nutritional and lifestyle factors [12
]. It is reasonable to consider that the lower levels of genetic European and East Indian admixture among Tobagonian women, in the context of better nutrition and lifestyle factors, may be a possible reason for the higher BMD that we observed in this analysis compared to American Blacks, where European admixture is more prevalent [7
Correlates explained 35% and 36% of the variability in areal BMD measured at the femoral neck subregion and total hip, respectively. The amount of variability accounted for in this analysis is slightly higher than other studies of American Whites, Blacks and Asians where only 20–30% of the variability in hip BMD was explained by risk factors [16
]. A recent study by Robbins et al. investigated the association between BMD and demographic, health-related, and functional status variables for Black men and women, aged 67–96 years, in the Cardiovascular Health Study [16
]. The model that best predicted BMD in Black women included weight, age, and income group, which accounted for 28% of the variability in hip BMD. Weight was the strongest correlate of BMD, explaining 21% of the variability in BMD. Our results confirm these observations and extends the findings to a slightly younger study population of African Caribbean women [16
]. Wang et al. reported that anthropometric, lifestyle, and medical factors accounted for 12–32% of the variability in BMD for the Tobago Family Health Study [7
Low BMI and cigarette smoking were identified as risk factors for low BMD among U.S. non-Hispanic Black women based on data from NHANES III [30
]. In this analysis, we were unable to demonstrate an association between BMD and smoking, which is probably due to the low prevalence of current smoking (0.3%) or ever smoking (2.7% - data not shown). Similarly, statin use has been commonly associated with higher BMD and we observed a positive correlation between statin use and BMD but the effect was not significant after age-adjustment, likely reflecting low power. Current statin use was only significant in the femoral neck BMAD multiple linear regression model.
We confirmed the findings of previous studies that thiazide diuretics use [31
], aspirin use [31
], and history of diabetes [34
] were significant predictors of higher BMD. The association between diabetes and higher BMD was independent of body weight and thus, other factors may contribute. For example, insulin and insulin-like growth factors have been shown to have an anabolic effect on bone [36
] and Type 2 diabetics have hyperinsulinemia. Lower BMD was associated with increasing age [16
], history of thyroid disease [37
], hip of fracture [38
], and use of beta-blockers. The predominance of medication use in our final model is novel. We limited our analyses to current use which was verified against medication bottles. It is also possible that medication use is a surrogate for the severity of the disease and we had greater power to detect associations with medications because prevalence of use, especially thiazide diuretics and beta blockers, was quite high. This high prevalence contrasts with the low prevalence of smoking, alcohol consumption, and previous fracture.
A major strength of our study was the focus on a unique homogeneous group of women of Western African descent. We examined a large number of possible correlates that have previously been linked to BMD in other studies. Our results add to the body of literature examining the epidemiologic correlates of BMD in White women [40
], White men [41
], and Asian men [42
]. There are, however, several limitations. Ethnicity was based on self-report and represents a crude surrogate for biological, environmental, cultural and behavioral differences among individuals. However, this would have less impact on our study because of the homogeneous nature of the Tobago population. We had no information on possible biological correlates of BMD including sex steroids, biochemical markers of bone turnover and Vitamin D levels. Self-reported data are subject to inaccurate recall, resulting in possible biased estimates of measures of association. Due to the lack of automated medical history data, we were unable to verify medical history. Research has shown, however, that self-reported medical history tends to be consistent with medical records [43
]. We attempted to partially correct for any dependence of areal BMD on the volume of bone (i.e., bone size) by calculating femoral neck BMAD, an estimate of volumetric density. Finally, this study focused on a rural, community-based sample of postmenopausal women and our correlates of BMD may not be generalizeable to other postmenopausal women of African ancestry.
In conclusion, African Caribbean women have the highest BMD on a population level reported to date for women. This may reflect low European admixture. Correlates of BMD among Caribbean women of West African ancestry were similar to those reported for U.S. Black and White women. Our findings provide support for heterogeneity in BMD among persons of West African ancestry. We observed that body weight was a strong predictor of BMD in African Caribbean women. The amount of variability in BMD explained by these correlates was considerably greater than other studies of White and Black women [16
]. We believe our findings contribute to the limited research that has been completed on etiology of fracture risk among women of West African ancestry.