BOSTON, MASSACHUSETTS. There is convincing evidence that vitamin D, a hormone primarily involved in regulating calcium metabolism, reduces the risk of hypertension and diabetes. Since both of these conditions increase the risk of stroke, it is tempting to speculate that an adequate vitamin D status may also reduce the risk of stroke. A group of researchers from Harvard School of Public Health now confirms that low vitamin D levels are indeed associated with an increased risk of stroke, more specifically, ischemic stroke (stroke associated with a blood clot). Their study had two components:

• Correlating blood plasma level of 25-hydroxyvitamin D (25[OH]D) with incidence of ischemic stroke in a group of 33,000 female nurses taking part in the Nurses' Health Study started in 1976.
• Performing a meta-analysis of 6 studies evaluating the association between 25(OH)D levels and the risk of stroke.

Nurses' Health Study
The researchers prospectively identified and confirmed 464 cases of ischemic stroke. These cases were matched with 464 stroke-free controls of similar age, menopausal status, hormone replacement therapy status, race, and smoking status. After adjusting for a large number of nutritional, disease-related and lifestyle factors, the researchers concluded that women with an average plasma level of 25(OH)D of 35 nmol/L (range of 9.2 to 45.7) had a significantly increased risk of suffering an ischemic stroke (relative risk of 1.53) when compared to those with an average 25(OH)D level of 77.6 nmol/L (range of 65.5 to 264.3). More specifically, a woman with a 25(OH)D level of 55 nmol/L had twice the risk of suffering an ischemic stroke than did a woman with a 25(OH)D level of 95 nmol/L. They also noted that more than 40% of the study participants were deficient in vitamin D as defined as a plasma level of 25(OH)D below 50 nmol/L. Meta-analysis
Analysis of 6 studies involving 1214 stroke cases found a relative risk of 1.52 when comparing men and women with low vitamin D status with those with high levels of 25(OH)D. One study only considered ischemic stroke and when its results were combined with the results of the Nurses' Health Study, the relative risk of suffering an ischemic stroke associated with a low vitamin D level was 1.59.
Sun, Q, et al. 25-hydroxyvitamin D levels and the risk of stroke. Stroke, Vol. 43, June 2012, pp. 1470-77

Editor's comment: This study clearly links a high plasma level of 25(OH)D with a significantly reduced risk of suffering an ischemic stroke. Although the Nurses' Health Study involved women only, there is no reason to suspect that the results would not be applicable to men. It is noteworthy that the two-fold risk reduction observed at a 25(OH)D level of 95 nmol/L compared to a level of 50 nmol/L is comparable to the risk reduction observed with warfarin therapy. Unfortunately, the diet provides relatively little vitamin D. In the Nurses' Health Study the average vitamin D intake was 350 IU/day resulting in an average 25(OH)D level of 56 nmol/L. To reach a level of 95 nmol/L would require supplementation with about 4000 IU/day.