Background: Black individuals are at greater risk of stroke and vitamin D deficiency than white individuals. Epidemiologic studies have shown that low 25-hydroxyvitamin D concentrations are associated with increased risk of stroke, but these studies had limited representation of black individuals. Methods: We examined the association of 25-hydroxyvitamin D with incident stroke in the Reasons for Geographic and Racial Differences in Stroke (REGARDS) study, a cohort of black and white adults ≥45 years of age. Using a case-cohort study design, plasma 25-hydroxyvitamin D was measured in 610 participants who developed incident stroke (cases) and in 937 stroke-free individuals from a stratified cohort random sample of REGARDS participants (comparison cohort). Results: In multivariable models adjusted for socio-demographic factors, co-morbidities and laboratory values including parathyroid hormone, lower 25-hydroxyvitamin D concentrations were associated with higher risk of stroke (25-hydroxyvitamin D >30 ng/mL reference; 25-hydroxyvitamin D concentrations 20-30 ng/mL, hazard ratio 1.33, 95% confidence interval (95% CI) 0.89,1.96; 25-hydroxyvitamin D <20 ng/mL, hazard ratio 1.85, 95% CI 1.17, 2.93). There were no statistically significant differences in the association of lower 25-hydroxyvitamin D with higher risk of stroke in black vs. white participants in fully adjusted models (hazard ratio comparing lowest vs. highest 25-hydroxyvitamin D category 2.62, 95% CI 1.18, 5.83 in blacks vs. 1.64, 95% CI 0.83, 3.24 in whites, Pinteraction = 0.82). The associations were qualitatively unchanged when restricted to ischemic or hemorrhagic stroke subtypes or when using race-specific cut-offs for 25-hydroxyvitamin D categories. Conclusions: Vitamin D deficiency is a risk factor for incident stroke and the strength of this association does not appear to differ by race.
