Studies show limited evidence for vitamin D

Two meta-analyses, published in the March 2 Annals of Internal Medicine, found only limited associations between vitamin D supplementation and cardiovascular health.

The first analysis included 13 observational studies and 18 trials. The majority of the studies found no effect of vitamin D on development of glycemia or diabetes. In the observational studies, lower vitamin D levels or intake was associated with increased risk for hypertension and possibly also cardiovascular disease, but the association was not as strong as previous studies have reported, the authors noted. This could mean that the previous findings are due to reverse causation (healthier people taking more vitamin D), or other health benefits of vitamin-D-heavy diets, the meta-analysis authors suggested.

The other analysis included 17 prospective studies and randomized trials of patients on dialysis and the general population. The analysis found a consistently strong inverse association between vitamin D use and cardiovascular mortality in the patients on dialysis, but the results are not generalizable to healthy populations without further study, the authors said. The randomized trials found a slight but statistically insignificant reduction in cardiovascular risk in people taking moderate to high doses of vitamin D.

The authors of both studies, and an accompanying editorial, called for large randomized trials of vitamin D supplementation. The evidence for vitamin D is more promising than that for other vitamins or mineral supplements, the editorialist said, but it's insufficient to support widespread high-dose vitamin D supplementation at this time. Research to determine the optimal dose, benefits and potential harm should be a public health research priority, the editorialist concluded.

ACP Internist covered the controversy over vitamin D supplementation in November 2009.