Why You May Not Need That Vitamin D Test After All

Doctors have embraced vitamin D testing and supplementation before there’s proof of health benefits.

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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.)

[Related: How Much Time in the Sun Do You Need to Get Your Vitamin D?]

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."