The Truth About Vitamin D

Vitamin D has been touted as a magic bullet, protecting against bone fractures, heart disease and even cancer. Now, a government group takes a closer look at the data.

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Dorling Kindersley

There’s a lot of information out there about vitamin D, and most of it suggests that Americans need more of it. Studies have linked the so-called sunshine vitamin — because it’s synthesized in the body with sun exposure — to a host of health benefits, including lower risk of bone fractures, heart disease, cognitive problems in older adults, asthma and even cancer.

But now, a new review of the data conducted by the U.S. Preventive Services Task Force (USPSTF), advises tuning out the hype. The government-backed group looked at 19 clinical trials and 28 observational studies and found that vitamin D, in combination with calcium, can indeed help reduce bone fractures, but that as far as cancer and heart disease are concerned, the data just aren’t conclusive enough to suggest that vitamin D supplements have any benefit.

The USPSTF’s review follows a similar review in 2009, conducted by the Institute of Medicine (IOM). That review, which examined the link between vitamin D and 17 health outcomes, culminated in concrete recommendations for how much vitamin D people should get — 600 IUs daily for most Americans. The USPSTF’s review, in contrast, was not designed to make any daily-intake recommendations, but rather to take a more in-depth look at the question of bone fractures and cancer prevention. A separate group also studied whether vitamin D has any benefit for heart disease risk, and both reports are published in the Annals of Internal Medicine.

Mei Chung at the Tufts Medical Center Evidence-based Practice Center led the 2009 IOM review and reanalyzed the data for the current USPSTF report. In the interim, she says, only one new study of significance — concerning vitamin D and calcium’s effect on fracture risk — was published; otherwise, no new data were incorporated in the new review.

Overall, the conclusions on bone fractures have remained the same since 2009: using vitamin D supplements in combination with calcium appears to lower risk of fractures. Elderly people living in institutional settings like a hospice or hospital benefited most; their risk of fracture was lowered by 29%, compared with those taking placebo, according to USPSTF review. Those who were living in the community also benefited, but less so, reducing their risk of fracture by 11% compared with other non-institutionalized elderly taking a placebo.

When it comes to cancer risk, however, the data are not as clear. Some observational studies suggested a small protective effect of vitamin D, but others found that higher doses of the vitamin were actually associated with an increased risk of cancer. The murky findings are due in part to the fact that it’s not easy to measure exact levels of vitamin D in the body. The vitamin’s most common form, made by the kidneys, doesn’t live for very long in the body so it’s hard to detect. The metabolite of D that’s easier to measure is made by the liver, which is then broken down by the kidneys. So scientists must use these levels as a measure of the amount of vitamin D that cells actually get, but it’s still less than accurate.

The only true test of the vitamin’s effect, on cancer if any, would be to conduct a randomized controlled trial in which participants take various doses of vitamin D or a placebo, and are then tracked for cancer over many years. But, says Chung, given the difficulties in measuring vitamin D levels — and keeping track of its various other sources, including sun exposure and fortified foods — these studies are hard to conduct.

The data are just as muddy for vitamin D’s role in the heart. Some studies have linked low levels of the vitamin with a higher risk of heart problems. Geographic data seem to support this connection, as the incidence of heart disease appears to rise with distance from the equator: as sunlight exposure and, therefore, the body’s ability to make vitamin D wanes, risk of heart disease goes up. Also supporting this connection are biological studies that suggest vitamin D is important for maintaining the elasticity of blood vessels and for regulating blood pressure. But so far, only one major study has demonstrated that patients who take supplements of vitamin D actually have a lower risk of heart disease, so the question remains open.

“Even if we could conduct a randomized controlled trial, there will still be a lot of unanswered questions,” says Chung. “Everybody is exposed to diet, and there are no people who are not exposed to vitamin D. As opposed to a drug trial where if you give a certain drug to one group and a placebo to another group, you can be pretty sure that the placebo group has no exposure to the drug. But we don’t know how to measure background exposure to vitamin D, so we don’t know how much additional D — from sunlight, from diet — participants will have. There are too many unknowns.”

That’s why the USPSTF leaves open another key question: how much vitamin D do our bodies actually need? How much is enough? What constitutes too little? The IOM’s recommendation of 600 IU of vitamin D daily for most adults was based on the data for maintaining bone health, but not for the prevention of other conditions such as heart disease or cancer.

It might be a while before we get definitive answers about vitamin D’s effects on these diseases. “We need much more information and more improvements in how we measure D before we spend money on another randomized controlled trial to test vitamin D’s effect on cancer,” says Chung.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.

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