Vitamin D Supplementation and Cardiovascular Outcomes in Blacks

V itamin D metabolism involves the skin (as one of the sources), the liver (for the conversion of vitamin D to 25 (OH) vitamin D), and the kidneys (for the conversion of 25 (OH) vitamin D to 1, 25 (OH)2 vitamin D, the active metabolite). Black skin, as compared with Caucasian skin, has been shown to be an inefficient source of vitamin D, and hence blacks are more likely to be vitamin D deficient compared with Caucasians. Vitamin D deficiency has been associated with cardiovascular risk1 and the excess cardiovascular risk seen in the black population has been attributed partly to vitamin D deficiency.2 Despite the fact that vitamin D deficiency is common in black populations, current data on interventional trials aimed at assessing whether vitamin D supplementation would improve cardiovascular risk are limited.3 The idea of noninvasively measuring endothelial function using ultrasonography of the brachial artery was introduced in the 1990s. Celermajer et al. showed that this noninvasive measure was associated with cardiovascular risk factors and cardiovascular events.4 Since then, numerous studies have associated brachial flow-mediated dilation (FMD) with incident cardiovascular events in populations with established cardiovascular diseases and also in asymptomatic populations.5 Furthermore, brachial FMD has been shown to improve with cardioprotective therapies such as HMG CoA reductase inhibi tors, angiotensin-converting enzyme inhibitors, exercise and weight loss; and worsen with cigarette smoking and emotional stress. Improvement of brachial FMD with cardioprotective therapies has been associated with a reduction in cardiovascular outcomes. Brachial FMD is considered a barometer of vascular health and postulated to capture risk over and beyond the cumulative cardiovascular risk that can be assessed using traditional risk factors. Unfortunately, the application of brachial FMD has been plagued since its introduction by an unacceptable variability despite published guidelines.6 This high variability has reduced the attractiveness of brachial FMD and has greatly impaired its progression from a research tool to a clinical tool for measuring vascular health. Harris et al.7 should be commended for performing this much needed interventional trial assessing the effect of oral supplementation of vitamin D on vascular health in blacks. However, the authors used brachial FMD as the outcome instead of hard cardiovascular events. In addition, the sample size of their cohort was very small and follow-up was relatively short. Despite these limitations, the findings of their study7 were consistent with similar studies in Caucasian populations. This study7 should be interpreted with caution and should not be equated with a reduction in cardiovascular outcomes with vitamin D supplementation in blacks with vitamin D deficiency. In addition, other aspects, such as safety, sustainability of the improvement in brachial FMD with this therapy and the ideal duration of therapy need to be addressed. Larger interventional trials with longer follow-up periods assessing the effect of oral vitamin D supplementation on hard cardiovascular outcomes in blacks with vitamin D deficiency are needed.