HONOLULU, HAWAII. There is, by now, ample evidence that vitamin D deficiency is associated with many disease conditions including hypertension, insulin resistance, diabetes, infections, influenza, autoimmune diseases, cancers, cardiovascular disease, stroke, and heart failure. A study carried out by a group of researchers at Harvard Medical School concluded that having a low blood plasma level of 25[OH]D – the first metabolite of vitamin D – increases the risk of suffering an ischemic stroke (stroke caused by a blood clot) by between 50 and 100%.

Now a group of researchers from the University of Hawaii reports that a low dietary intake of vitamin D is associated with an increased risk of suffering an ischemic stroke. Their study included 7385 Japanese-American men who were 45 to 68 years old when enrolled in the Honolulu Heart Program between 1965 and 1968. At time of enrolment, all study participants completed a detailed food frequency questionnaire. Dietary intake of vitamin D ranged from 0 to 212 micrograms/day (0 – 8500 IU/day) with the average being 3.62 micrograms/day (145 IU/day).

Over the 34 years following enrolment, 960 study participants suffered a stroke, of which, 651 were ischemic (thromboembolic), 269 were hemorrhagic (caused by a burst blood vessel), and 40 were of unknown type. The incidence of stroke in the lowest quartile of dietary vitamin D intake (0 – 45 IU/day) was 0.64%/year as compared to 0.51%/year in the highest quartile (165 – 8500 IU/day). After adjusting for potential confounding variables including age, total daily food consumption, body mass index, hypertension, diabetes, smoking, physical activity, cholesterol level and alcohol intake, the researchers conclude that a low dietary vitamin D intake is associated with a 27% increased risk of suffering an ischemic stroke. No association was found between vitamin D intake and the risk of suffering a hemorrhagic stroke. The researchers suggest that vitamin D supplementation may be beneficial for stroke prevention.
Kojima, G, et al. Low dietary vitamin D predicts 34-year incident stroke. Stroke, Vol. 43, August 2012, pp. 2163-67

Editor's comment: This study confirms the importance of vitamin D as an integral part of a stroke prevention program. The recently reported Nurses' Health Study observed a 2-fold reduction in the risk of suffering an ischemic stroke at a 25[OH]D plasma level of 95 nmol/L (38 ng/mL) as compared to a level of 50 nmol/L (20 ng/mL). To reach a level of 95 nmol/L would, for most people, require supplementation with about 4000 IU/day. Vitamin D comes in two different forms – vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is found in poorly formulated multivitamins, as an additive to some foods, and is the form preferably prescribed by many physicians. Unfortunately, it is about 10 times less effective than vitamin D3 and is, by some researchers, considered toxic.[1]
[1] Armas, LAG, et al. Vitamin D2 is much less effective than vitamin D3 in humans. Journal of Clinical Endocrinology & Metabolism, Vol. 89, No. 11, 2004, pp. 5387-91