For instance, research suggests vitamin D levels are significantly higher in lean women and in those who are more physically active. Since vitamin D is stored in body fat, explains Chlebowski, those with less body fat may have more of the nutrient circulating in their bloodstream. Thinner, more physically fit folks tend to be healthier in general, which could—independent of their vitamin D levels—explain their lower incidence of diseases.
Chlebowski also worries about the harmful effects of taking D supplements. A recent review of 45 vitamin D studies conducted by the Cochrane Collaboration, a nonprofit research organization, suggested that some people who take vitamin D supplements might be susceptible to kidney stones or kidney disease. "People want tips and guidance," he says, "but at the moment there's insufficient evidence from research studies to guide them."
In fact, one study that came out this month suggests that vitamin D supplementation could be downright counterproductive—at least in African-Americans. The study involving 340 African-Americans with type 2 diabetes found that those with the highest vitamin D levels (many of whom were taking daily vitamin D supplements) were more likely to have dangerous plaque accumulation on their major heart arteries. "African-Americans have far lower rates of osteoporosis and tend to have denser bones than whites, yet they also tend to have lower vitamin D levels," says study author Barry Freedman, a professor of medicine at Wake Forest University School of Medicine. It could be, he adds, that they have some gene variation that enables them to have strong bones without needing as much of the bone-building nutrient. By the same token, blacks also tend to have little calcified plaque on their arteries—which lowers their risk of heart attacks—something that could be reversed if they take high doses of vitamin D supplements. "It's not known if the target vitamin D levels in the bloodstream should be the same in both blacks and whites," says Freedman.
Should you get a vitamin D test or wait until more studies are performed? "If your doctor is concerned about certain risk factors—you're obese, frail, have cognitive impairment, a lack of sun exposure—then I think it's reasonable," says Manson. All of these can be indicators of vitamin D deficiency. Testing "is one of the issues a national panel of experts is addressing in new guidelines that will be released this summer." Manson, who's serving on the panel convened by the Institute of Medicine, wouldn't comment on the specifics of the upcoming recommendations, which will define an optimal blood level for vitamin D and which are expected to raise the dietary intake recommendations for the nutrient. Currently, the recommendations are 200 IUs a day up to age 50, 400 IUs to age 70, and 600 IUs over 70.
For the time being, women over 65 and men over 60 can volunteer to participate in Manson's study, which will randomly assign either a daily placebo or a vitamin D pill (containing 2,000 IUs) along with a daily fish oil supplement (containing 1 gram of omega-3 fats) or a placebo to 20,000 individuals to see if either or both supplements reduce the risk of developing cancer, heart disease, and strokes. She says she hopes that one quarter of the participants recruited will be African-American. (Those who wish to participate can get free supplements by mail; go to www.vitalstudy.org for more information.)