Right now, at this very moment, your vitamin D levels are probably at their lowest levels of the year, since your body can't make the nutrient from sunshine during the dark winter months. You could be facing an increased risk of heart disease, osteoporosis, dementia, diabetes, and a variety of cancers. Scared yet?
Many people are, especially after having their levels tested and being told that they're deficient. With a spate of recent studies touting the prevalence of D deficiency—half of all Americans—and the disease-prevention benefits of having high vitamin D levels, doctors have rushed to test and treat low levels with supplements. Some experts, though, are starting to sound alarms about the boom in testing, which has been increasing by 80 to 90 percent per year, with several million people expected to be checked in 2010, according to Mayo Clinic pathologist Ravinder Singh. He says he's concerned about the lack of standardization among testing labs after seeing results vary greatly from lab to lab using the same blood sample. (In a separate matter, Quest Diagnostics, the nation's largest medical laboratory, last year revealed that it provided "questionable" vitamin D level results to thousands of patients that may have indicated levels were higher or lower than they actually were.)
The bigger problem, though, is that experts still don't agree on what an "optimal" level of vitamin D might be or whether raising levels actually prevents disease. Vitamin D researcher Robert Heaney, a professor of medicine at Creighton University in Nebraska, says most researchers agree that blood levels should be at least 30 nanograms/milliliter to protect bones, but some scientists think these levels should higher—perhaps 40 ng/ml or more to provide protection against diabetes, heart disease, and cancer. Others, though, question whether it's truly beneficial to drive blood levels into this range using supplements. "I think there's great potential for vitamin D to reduce the burden of chronic disease, but I also think there's reason to be cautiously optimistic," says JoAnn Manson, chief of preventive medicine at Brigham and Women's Hospital in Boston, who is conducting a large clinical trial of vitamin D supplementation. "Let's not jump on the bandwagon and take megadoses before we have results from research trials." After all, a host of supplement studies—on vitamin C, vitamin E, selenium, beta carotene—found that those who were given supplements fared no better, and sometimes worse, than those who took placebos. This, despite the fact that previous population studies had shown that those with high levels of these particular nutrients had lower rates of heart disease, cancer, and other diseases.
Could the same hold true for vitamin D? Those promising benefits of high vitamin D amounts were demonstrated in studies that simply looked at vitamin D levels in various populations and then correlated them with disease. Only a handful of intervention studies have shown a modest benefit to giving vitamin D supplements—mostly for lowering the risk of bone fractures in elderly nursing home residents when vitamin D is taken along with a calcium supplement. Heaney and his colleagues found in a 2007 study of nearly 1,200 postmenopausal women that those who were randomly assigned to take a vitamin D and calcium supplement for four years had a 60 percent lower risk of developing any kind of cancer than those who were randomly given placebos. But while that finding is intriguing, argues cancer researcher Rowan Chlebowski, a professor of medicine at David Geffen School of Medicine at the University of California–Los Angeles, the study was small and needs to be replicated before it's possible to conclude that raising vitamin D levels prevents cancer. "I think it's reasonable to do further studies," he says. "But people need to realize that high levels of vitamin D in the bloodstream may simply be due to good genes or some other factor beyond sun exposure or dietary intake."
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.)