No doubt about it, vitamin D is enjoying its time in the sun.
In doctors' offices, testing for vitamin D levels, once a rarity, has become commonplace. And between 2002 and 2011, U.S. sales of vitamin D supplements jumped more than tenfold — from $42 million to $605 million — according to the Nutrition Business Journal. The issue of widespread vitamin D deficiency, says Michael F. Holick, M.D., author of The Vitamin D Solution, "has gotten the press's attention, the public's attention and — finally — the physicians' attention."
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Yet figures released this year by the U.S. Centers for Disease Control and Prevention (CDC) paint a different picture of the problem's scope: Just 8 percent of Americans are at risk of a significant vitamin D deficiency, although among African Americans and Mexican Americans that fraction is higher. Further, it's likely that some of the people taking supplements are getting too much of a good thing. An overload of vitamin D — though it would have to be an awful lot, says Holick — can cause kidney problems and tissue damage.
Strong bones, plus
How much D we need is a crucial question because the vitamin is important to so many aspects of health. In the past several years, studies have confirmed what we were all told as children — that it promotes strong bones. But studies have also shown that it may play a role in preventing cancers, dementia, diabetes and certain autoimmune diseases. There's also good evidence that vitamin D helps protect against the falls that can lead to fractures in older people who are unsteady on their feet.
The main source of vitamin D — throughout most of human history — has been the body's own manufacture of the vitamin when skin is exposed to sunlight. More recently, milk and other fortified foods have provided significant amounts of the nutrient.
But today several trends have led some experts to warn of a virtual pandemic of vitamin D deficiency. First, people are getting less sun because they're spending more time indoors and using more sunscreen when they're outdoors. They're also drinking less D-fortified milk.
Plus, the population is growing older (older people don't synthesize D as efficiently), as well as fatter. Scientists aren't sure why obese individuals are at increased risk for a D deficiency, but the fact that D is sequestered in fat tissue may play a role.
Still, even as health experts track these trends, they haven't yet reached a consensus about optimal levels. Indeed, whether your D level is adequate depends on whom you ask. According to the IOM's 2010 guidelines, most adults — even those who get little sun — can achieve the correct level of vitamin D by consuming about 600 to 800 international units (IUs) per day.
Meanwhile, other researchers and doctors have argued that those seeking to reduce bone-fracture risk and reap possible benefits for disease prevention should take supplements of 1,000 to 2,000 IUs daily.
And the Vitamin D Council, partly sponsored by supplement makers, recommends healthy adults with little sun exposure take 5,000 IUs of D daily — 1,000 IUs more than what the Institute of Medicine (IOM) deemed the safe upper limit of 4,000 IUs.
The IOM cautionary message that more vitamin D is not necessarily better has largely fallen on deaf ears, says JoAnn Manson, M.D., a professor of medicine at Harvard Medical School who helped draft the guidelines. "Very few people have been affected by that because there's so much marketing and promotion of vitamin D," she says.
She doesn't expect the promotional din to die down before the 2016 completion of a National Institutes of Health study of the vitamin. Led by Manson, it follows 20,000 people to test whether taking 2,000 units of D daily lowers the risk of cancer and cardiovascular disease. Until the results are in, she says, the public can be misled. "I think the enthusiasm for vitamin D supplements has outpaced the evidence."
Should you supplement?
So in the meantime, should you or your family members take a supplement? Be sure to speak with your doctor before buying any supplements. But one key factor to consider is skin color. Most African Americans should at least consider a supplement, says Susan Harris, a vitamin D expert at Tufts University. According to the CDC report, a majority of African Americans had insufficient blood levels of vitamin D, and nearly one-third had a frank deficiency, compared with 12 percent of Mexican Americans and 3 percent of whites. (The information the CDC collected for this report included only Mexican Americans, not American Hispanics in general.) Dark skin pigmentation serves as a natural sunscreen, she explains, blocking the ultraviolet rays that trigger vitamin D synthesis.
Some older people also should consider supplementing. In May the U.S. Preventive Service Task Force endorsed vitamin D supplements of 800 IUs a day for people who are at risk of falls.
You also can assume you're low on vitamin D if you live in the northern part of the country or spend most of your time indoors. Says Harris, "In Boston, I tell pretty much everyone that they should be taking a supplement during the winter."