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When scientists at the Australian Institute of Sport recently decided to check the Vitamin D status of some of that country’s elite female gymnasts, their findings were fairly alarming. Of the 18 gymnasts tested, 15 had levels that were “below current recommended guidelines for optimal bone health,” the study’s authors report. Six of these had Vitamin D levels that would qualify as medically deficient. Unlike other nutrients, Vitamin D can be obtained by exposure to ultraviolet radiation from sunlight, as well as through foods or supplements. Of course, female gymnasts are a unique and specialized bunch, not known for the quality or quantity of their diets, or for getting outside much.

But in another study presented at a conference earlier this year, researchers found that many of a group of distance runners also had poor Vitamin D status. Forty percent of the runners, who trained outdoors in sunny Baton Rouge, Louisiana, had insufficient Vitamin D. “It was something of a surprise,” says D. Enette Larson-Meyer, an assistant professor in the Department of Family and Consumer Sciences at the University of Wyoming and one of the authors of the study.

Vitamin D is an often overlooked element in athletic achievement, a “sleeper nutrient,” says John Anderson, a professor emeritus of nutrition at the University of North Carolina and one of the authors of a review article published online in May about Vitamin D and athletic performance. Vitamin D once was thought to be primarily involved in bone development. But a growing body of research suggests that it’s vital in multiple different bodily functions, including allowing body cells to utilize calcium (which is essential for cell metabolism), muscle fibers to develop and grow normally, and the immune system to function properly. “Almost every cell in the body has receptors” for Vitamin D, Anderson says. “It can up-regulate and down-regulate hundreds, maybe even thousands of genes,” Larson-Meyer says. “We’re only at the start of understanding how important it is.”

But many of us, it seems, no matter how active and scrupulous we are about health, don’t get enough Vitamin D. Nowadays, “many people aren’t going outside very much,” Johnson says, and most of us

assiduously apply sunscreen and take other precautions when we do. The Baton Rouge runners, for instance, most likely “ran early in the morning or late in the day,” Larson-Meyer says, reducing their chances of heat stroke or sunburn, but also reducing their exposure to sunlight.

Meanwhile, dietary sources of Vitamin D are meager. Cod-liver oil provides a whopping dose. But a glass of fortified milk provides a fraction of what scientists now think we need per day. (A major study published online in the journal Pediatrics last month concluded that more than 60 percent of American children, or almost 51 million kids, have “insufficient” levels of Vitamin D and another 9 percent, or 7.6 million children, are clinically “deficient,” a serious condition. Cases of childhood rickets, a bone disease caused by lack of Vitamin D, have been rising in the U.S. in recent years.)

Although few studies have looked closely at the issue of Vitamin D and athletic performance, those that have are suggestive. A series of strange but evocative studies undertaken decades ago in Russia and Germany, for instance, hint that the Eastern Bloc nations may have depended in part on sunlamps and Vitamin D to produce their preternaturally well-muscled and world-beating athletes. In one of the studies, four Russian sprinters were doused with artificial, ultraviolet light. Another group wasn’t. Both trained identically for the 100-meter dash. The control group lowered their sprint times by 1.7 percent. The radiated runners, in comparison, improved by an impressive 7.4 percent.

More recently, when researchers tested the vertical jumping ability of a small group of adolescent athletes, Larson-Meyer says, “they found that those who had the lowest levels of Vitamin D tended not to jump as high,” intimating that too little of the nutrient may impair muscle power. Low levels might also contribute to sports injuries, in part because Vitamin D is so important for bone and muscle health. In a Creighton University study of female naval recruits, stress fractures were reduced significantly after the women started taking supplements of Vitamin D and calcium.

A number of recent studies also have shown that, among athletes who train outside year-round, maximal oxygen intake tends to be highest in late summer, Johnson says. The athletes, in other words, are fittest in August, when ultraviolet radiation from the sun is near its zenith. They often then experience an abrupt drop in maximal oxygen intake,

beginning as early as September, even thought they continue to train just as hard. This decline coincides with the autumnal lengthening of the angle of sunlight. Less ultraviolet radiation reaches the earth and, apparently, sports performance suffers.

Concerned now about your Vitamin D status? You can learn your status with a simple blood test. An at-home version is available through the Web site of the Vitamin D Council. (Use of the tests is restricted in some states, including New York. See the website for details.) Be sure that any test checks the level of 25(OH)D in your blood. This level “should generally be above 50 nanograms per milliliter,” Larson-Meyer says.

If your levels are low, talk to your doctor about the best response. Sunlight is one easy, if controversial, fix. “Most dermatologists will still tell you that no amount of sun exposure is safe,” Johnson says.

But Larson-Meyer and other Vitamin D researchers aren’t so sure. “There’s no good, scientific evidence that five to thirty minutes of sunlight a few times a week is harmful,” she says.

Or try supplements. “1,000 IU a day and much more for people who are deficient” is probably close to ideal, Larson-Meyer says. This, by the way, is about double the current recommended daily allowance. Most experts anticipate that this allowance will be revised upward soon. Consult with your doctor before beginning supplements. Overdoses of Vitamin D are rare, but can occur.

Finally, stay tuned. “In the next few years, we’re going to be learning much more” about the role of vitamin D in bodily function and sports performance, Larson-Meyer says.