Experts differ on whether it's worthwhile to keep studying the popular vitamin supplement.
The evidence is in and vitamin D supplements, despite their popularity, do not seem to have significant benefits for heart health, cancer prevention or even bone health in healthy people, says a new review of previous studies from researchers in New Zealand.
The report, published today in the journal Lancet Diabetes & Endocrinology, is the latest to question the public's love affair with supplements and to suggest researchers should stop spending time and money trying to find out whether vitamins can prevent a host of chronic diseases. An editorial published in December in the Annals of Internal Medicine took the broadest swipe, saying the case was closed on the value of multivitamins and most other supplements for preventing chronic illnesses.
Not all experts agree, and studies on vitamins are continuing, even as the public spends $28 billion a year on them.
The new study, which reviews data from 40 previous studies, focuses only on vitamin D and concludes that "healthy people are very unlikely to get a benefit from it," says lead researcher Mark Bolland of the University of Auckland. It also concludes that additional studies are likely to be futile.
A leading vitamin D proponent dismissed the analysis, saying the previous studies were inadequate: "The scientific term for it is 'silly,' " says Michael Holick, a professor of medicine at Boston University. "There's nothing new here."
The new review focuses only on randomized trials, in which people were assigned to take vitamin D or placebos and then followed to see if their health differed as a result. It did not include observational studies, in which researchers simply asked people about vitamin D intake or measured levels in their blood and then looked at their health differences.
Those observational studies have found a link between low vitamin D levels and diseases ranging from cancer to heart disease and diabetes.
"The problem with those studies is that you can't determine whether there's a cause and effect," Bolland says. But groups with low levels "tend to be older, heavier, tend to exercise less and spend less time outside," which might help explain both their low levels and their poor health, he says. Outdoor time matters because the body produces vitamin D in response to sun exposure. Vitamin D also is available in foods such as fortified milk and salmon.
In the combined randomized trials, Bolland says, giving people vitamin D, even if they started with low blood levels, did not prevent heart attacks, strokes or cancer or lower the overall risk of bone fractures – or it lowered risk by less than 15%, the cutoff set in the new analysis. Vitamin D, combined with calcium, did help prevent hip fractures in nursing home patients. The researchers also could not rule out the possibility that vitamin D might lower the risk of premature death by 5% or so, the cutoff they set for findings on mortality.
The researchers did not look at falls. The U.S. Preventative Services Task Force says there is good evidence that vitamin D supplements help prevent falls in elderly people at high risk for them.
Holick, who has written books promoting vitamin D use, says the analysis is flawed by the fact that most previous studies have used vitamin D doses that were too low, often 200 to 400 international units (IU) a day. The Institute of Medicine, which reviewed research on vitamin D in 2010, says most adults should get 600 to 800 IUs, depending on age -- though those experts said most people could get that much without supplements.
Holick says there's reason to believe higher doses, such as the 2,000 IU a day now being tested in one large U.S. trial, are needed to produce benefits.
Bolland says he's skeptical: "We didn't find any evidence of that. In trials that did use high doses, people did achieve higher blood levels, but they still didn't seem to see benefits."
Holick also says the randomized trials have not explained away findings that certain diseases, such as colon cancer and multiple sclerosis, are more common at latitudes with low sun exposure. His theory is that low vitamin D levels are to blame.
"The debate is likely to continue," says an editorial accompanying the study, written by Karl Michaëlsson, a medical professor from Uppsala University in Sweden. But, he says, "the impression that vitamin D is a sunshine vitamin and that increasing doses lead to improved health is far from clear."