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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Sun exposure is one way of getting vitamin D, but don't overdo it. — Photo by Jupiter Images/Getty Images

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.

Worrisome trends

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.

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