Vitamin D's impact on cancer and heart health needs more research

Evidence linking vitamin D supplementation to cancer and cardiovascular disease prevention remains murky at best, say two separate studies published in the Dec. 20, 2011, Annals of Internal Medicine.

Until further research is done, physicians should be wary when relying on the nutrient in treatment regimens, researchers warn.

One of the studies, which looked at the effects of vitamin D and calcium supplementation in cancer and bone fracture, will help inform a new recommendation on the topic from the U.S. Preventive Services Task Force. The agency is expected to release the draft recommendation for public comment by the end of February.

Vitamin D has been linked to a number of conditions, but conflicting evidence has shown that "the nutrient falls into the category of something that has both benefits and harms," said Virginia A. Moyer, MD, MPH, professor of pediatrics at the Baylor College of Medicine and chair of the task force.

"We are deeply committed to following the scientific evidence wherever it leads," she said.

The task force asked researchers at Tufts Medical Center Evidence-based Practice Center in Boston to review existing randomized clinical trials and observational studies examining vitamin D and cancer and fracture risk, with a focus on older adults and the dose-response relationship.

Researchers found that combined vitamin D and calcium supplementation can reduce fracture risk, but with smaller effects among independent, older adults than with institutionalized elderly persons. However, appropriate dosing regimens require further study, they said.

Evidence regarding the benefits or harms of vitamin D supplementation for the prevention of cancer was limited and inconsistent, according to the analysis. A recent trial using an annual mega-dose of the nutrient showed harmful effects, while other studies using lower doses showed mixed effects. Some data came up with varying results across different types of cancer and populations.

Part of the difficulty in understanding the efficacy of vitamin D supplements, with or without calcium, comes from limitations in accurately measuring dietary intake and background exposure, said Mei Chung, PhD, MPH, lead author of the study on cancer and fracture risk.

"Future research should really try to look at what would be a safe and effective dose in a regimen," also taking into account frequency, Chung said. Until that is confirmed, "doctors need to be cautious about any recommendation."

There is not enough solid evidence from clinical trials to show that vitamin D plays a role in the prevention of cardiovascular disease, said a second study in Annals.

Some trials found that moderate to high doses of vitamin D supplements may reduce heart disease risk, with benefits seen mainly in dialysis patients. Few studies, however, showed consistent or conclusive associations with cardiovascular outcomes, in particular endothelial dysfunction, hypertension and atherosclerosis.

"There is a school of thought which suggests that vitamin D may merely be a surrogate marker for poor health status ... but more well-designed, large studies are needed," said Dr. Cora McGreevy of the Royal College of Surgeons in Ireland, lead author of the second Annals study. "Given the growing worldwide burden of cardiovascular disease, any new potential treatments, such as vitamin D supplementation, necessitate further evaluation. The confirmation of an association could have far-reaching implications for both patient care and health policy."