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Three-quarters of U.S. teens and adults are deficient in vitamin D, the so-called "sunshine vitamin" whose deficits are increasingly blamed for everything from cancer and heart disease to diabetes, according to new research.

The trend marks a dramatic increase in the amount of vitamin D deficiency in the U.S., according to findings set to be published tomorrow in the Archives of Internal Medicine. Between 1988 and 1994, 45 percent of 18,883 people (who were examined as part of the federal government's National Health and Nutrition Examination Survey) had 30 nanograms per milliliter or more of vitamin D, the blood level a growing number of doctors consider sufficient for overall health; a decade later, just 23 percent of 13,369 of those surveyed had at least that amount.

The slide was particularly striking among African Americans: just 3 percent of 3,149 blacks sampled in 2004 were found to have the recommended levels compared with 12 percent of 5,362 sampled two decades ago.

"We were anticipating that there would be some decline in overall vitamin D levels, but the magnitude of the decline in a relatively short time period was surprising," says study co-author Adit Ginde, an assistant professor at the University of Colorado Denver School of Medicine. Lack of vitamin D is linked to rickets (soft, weak bones) in children and thinning bones in the elderly, but scientists also believe it may play a role in heart disease, diabetes and cancer.

"We're just starting to scratch the surface of what the health effects of vitamin D are," Ginde tells ScientificAmerican.com. "There's reason to pay attention for sure."

But Mary France Picciano, a senior nutrition scientist in the National Institutes of Health's Office of Dietary Supplements, is skeptical that the dip is as deep or widespread as suggested, noting that there's disagreement on how much vitamin D is needed. She notes that the Institute of Medicine (IOM) defines insufficiency as less than11 nanograms per milliliter. Using that as a threshold, some 10 percent of U.S. adults are vitamin D deficient, according to a study published in November in the American Journal of Clinical Nutrition.

That earlier study, co-authored by Picciano, also found that vitamin D deficiency had become more common between the late 1980s and 2004, but that between half and 75 percent of that difference was due to changes in the test used to measure those blood levels and therefore wasn’t a true gauge. "The results are far overstated and their findings are not as accurate as ours," Picciano says. "There is some deficiency — I don't want to minimize that — but it's not as high as they're saying."

Ginde insists the results are reliable. "There's potential for methodology contributing to some of what we found," he says, but the magnitude of the change and other research "argue that this is the reality in the U.S. right now."

Ginde, who last month linked vitamin D deficiency to catching more colds, blames increasing use of sunscreen and long sleeves following skin cancer-prevention campaigns for the change. Using a sunscreen with as little as a 15-factor protection cuts the skin's vitamin D production by 99 percent, the study notes, and there are few sources of the vitamin in our diets. Some food sources are salmon, tuna, mackerel and vitamin D-fortified dairy products, such as milk.

IOM recommends that people get 200-600 International Units (IU) of vitamin D daily, but it's reviewing whether to increase that recommendation in the wake of new studies. An update is expected in May 2010. Ginde believes that whatever those recommendations turn out to be, blacks should take double the amount of vitamin D supplements, because they have more melanin or pigment in their skin that makes it harder for the body to absorb and use the sun's ultraviolet rays to synthesize vitamin D. He adds that people should also take greater amounts of vitamin D in the winter when there's less sunlight.

Jim Fleet, a professor of foods and nutrition at Purdue University who wasn’t involved in the study, agrees with Picciano that failing to consider differences in the vitamin D testing methods (used during the two survey periods) was "a fatal mistake." But he tells ScientificAmerican.com that real deficiencies in vitamin D exist, even when they're defined by the lower cutoff, and that some 40 percent of African Americans are vitamin D deficient according to that threshold.

"If you look at people in the categories that we worry about," he says, "that’s still a lot of people."