A little over three years ago, a San Francisco-area psychiatrist and several colleagues in other fields floated a provocative hypothesis: that a deficiency in vitamin D — the sunshine vitamin — might render people vulnerable to infections, including the flu. Now Japanese researchers offer tangible support for that idea. They show that vitamin D supplementation dramatically cut the incidence of seasonal flu among the children they followed.

Their double-blind, placebo-controlled trial was small. Only 334 children completed a three-month course of six pills per day — a dropout rate of almost one-quarter. Then again, that’s a lot of pills to make kids take each day to get just 1,200 international units of the vitamin. (Especially since the teeny capsule I down contains almost twice that much.)

But the supplementation certainly looked promising. Incidence of influenza A was 10.8 percent among the 167 kids who received vitamin D pills. That's in contrast to a flu rate of 18.6 percent among an equal number of children getting identical looking inert pills. Doctors monitoring the trial confirmed flu cases using a test to assay for the influenza-A germ.

The study has just been published online, ahead of print, in the American Journal of Clinical Nutrition.

Children with asthma may have benefited especially. Two asthma attacks occurred during the trial among kids getting the vitamin, compared to 12 in the unsupplemented group. Then again, the study doesn’t note how many kids with a history of asthma had been randomized into each arm of the trial. So it’s therefore possible that a comparable number of susceptible kids had not been present in each group.

Incidence of another strain of infection — influenza B — did not vary by supplement group, according to team leader Mitsuyoshi Urashima of the Jikei University School of Medicine in Tokyo, and colleagues. Then again, these researchers argue that any benefits of the vitamin might have been dampened by the timing of its administration. The trial began in December 2008, after the flu season had begun, and the researchers acknowledge that it may take almost three months “to reach a steady state of vitamin D concentrations by supplementation. Thus, December might be theoretically too late to start [a flu prevention regimen].”

John Cannell, the doc who came up with the idea that vitamin D might trammel flu, reports in his Vitamin D Newsletter, which came out Sunday, that "I hear through the grapevine that the CDC [Centers for Disease Control and Prevention] has discovered that, of the 329 American children who have died so far from H1N1, vitamin D levels in the dead children were lower than in children who survived the swine flu." He offers no additional details.

Vitamin D — itself a misnomer since the active chemical is actually a hormone — seems to play a pivotal role in total body health. It not only helps build bone and muscle but has been linked to lower risk of gum disease, diabetes, chronic obstructive pulmonary disease, cancer and autoimmune disease.

And the kicker: Most people in the developed world are chronically deficient. What’s more, those who have heavily pigmented skin, are overweight or who live in high latitudes face an aggravated risk of deficiency.

Indeed, a second new study due to appear in the same journal — this one by a U.S. team of scientists — reports that among a random sample of more than 3,000 postmenopausal women, almost 60 percent were vitamin-D deficient.

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