Racial/Ethnic Differences in 25-Hydroxy Vitamin D and Parathyroid Hormone Levels and Cardiovascular Disease Risk Among Postmenopausal Women

Background-Recent evidence suggests that racial/ethnic differences in circulating levels of free or bioavailable 25-hydroxy vitamin D (25[OH]D) rather than total 25(OH) D may explain apparent racial disparities in cardiovascular disease (CVD). We prospectively examined black-white differences in the associations of total, free, and bioavailable 25(OH)D, vitamin D-binding protein, and parathyroid hormone levels at baseline with incident CVD (including nonfatal myocardial infarction, nonfatal stroke, and CVD death) in postmenopausal women. Methods and Results-We conducted a case-cohort study among 79 705 postmenopausal women, aged 50 to 79 years, who were free of CVD at baseline in the WHI-OS (Women's Health Initiative Observational Study). A subcohort of 1300 black and 1500 white participants were randomly chosen as controls; a total of 550 black and 1500 white women who developed incident CVD during a mean follow-up of 11 years were chosen as cases. We directly measured total 25(OH)D, vitamin D-binding protein, albumin, parathyroid hormone, and calculated free and bioavailable 25(OH)D. Weighted Cox proportional hazards models were used to examine their associations with CVD risk. Although vitamin D-binding protein and total, free, and bioavailable 25(OH)D were not significantly associated with CVD risk in black or white women, a significant positive association between parathyroid hormone and CVD risk persisted in white women (hazard ratio comparing the highest quartile with the lowest, 1.37; 95% CI, 1.061.77) but not in black women (hazard ratio comparing the highest quartile with the lowest, 1.12; 95% CI, 0.79-1.58), independent of total, free, and bioavailable 25(OH)D or vitamin D-binding protein. Conclusions-Circulating levels of vitamin D biomarkers are not related to CVD risk in either white or black women. Higher parathyroid hormone levels may be an independent risk factor for CVD in white women.