Quick quiz: What vital nutrient may protect against cancer, heart disease, stroke, bone fractures, and a host of other diseases? Experts on vitamin D are quick to answer, and, by their reckoning, many Americans are sorely lacking in the nutrient. Just this week, new data from a government-run health and nutrition survey found that most kids weren't getting enough vitamin D and that those with the lowest levels were more likely to have high blood pressure, high blood sugar, and low levels of "good" HDL cholesterol. At a meeting convened Tuesday by the Institute of Medicine, leading vitamin D researchers mentioned this study and many others as they tried to convince an IOM committee to raise the daily recommended intake (DRI) for the nutrient.
"When [the U.S. government] set the DRI in 1997, the amount of vitamin D recommended was based on the prevention of rickets," said Robert Heaney of Creighton University in Omaha. Rickets, a bone malformation condition, has long been linked to severe vitamin D deficiency, as Heaney knows from studying the effects of the vitamin on bones. Vitamin D-fortified milk now gives most people protection from rickets, but we may not be getting enough of the nutrient to benefit from its protective effects against cancer, stroke, and other diseases. Experts like Heaney consider the current recommendations—200 international units for children and adults up to age 50, 400 IUs for ages 50 to 70, and 600 IUs for those 70 and older—to be way too low to help us stay healthy. In fact, they say, most Americans are deficient, especially during the winter, when many Northerners don't get the exposure to the sun that lets their bodies make vitamin D.
Hundreds of studies have shown that people with high levels of vitamin D in their blood have lower rates of diseases and a lower death rate. However, that doesn't prove—and there really aren't any clinical trials showing—that people can lower their risk of illness by taking a supplement to raise their vitamin D level. That's a critical gap in the evidence, said Barry Kramer, editor in chief of the Journal of the National Cancer Institute. He cautions that previous research had suggested that high levels of beta carotene were associated with lower rates of cancer, but when people took beta carotene supplements as part of a clinical trial, they were more likely—not less—to die from cancer.
Clinical trials involving vitamin D have yielded only a few promising results. Older women who took a supplement containing 700 to 1,000 IUs a day wound up with fewer bone fractures and fewer falls, according to a review of the research conducted by the Agency for Healthcare Research and Quality. Research for other claims is lacking. For example, the Women's Health Initiative study found that breast cancer rates were unaffected when women took 400 IUs of vitamin D combined with a calcium supplement every day for seven years. And a study on pregnant women found that taking 1,200 IUs of vitamin D didn't help prevent a high blood pressure condition called preeclampsia.
How much to take. The vitamin D researchers at this week's meeting countered that these clinical trials may have used supplements that didn't contain enough of the nutrient for patients to achieve an optimal blood level of vitamin D. Michael Holick, a researcher at Boston University, says that most adults probably need to take about 2,000 IUs a day and that kids probably need about 1,000 IUs. Although vitamin D can be toxic at high doses, the latest research suggests that kids and adults can take 5,000 IUs or more a day in supplement form without any ill effects. (Our skin can make far more than that when exposed to sunlight, but any excess we make gets broken down by the body and doesn't cause any harm.)
After the IOM expert panel reviews the research, it most likely will raise the recommended levels of vitamin D, and that could mean more foods being fortified with D, says Bess Dawson-Hughes of Tufts University. She has researched using vitamin D supplementation to prevents falls and fractures. When these new recommendations will be issued and what they will be, however, is still unknown.