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This study is basically saying that vitamin D3 supplementation was given to a relatively small group of children to see if there would be a lower incidence of Infuenza A.  What they found was that the group taking vitamin D had a lower incidence of contracting the illness, plus the children that had a past history of asthma had less attacks!  Read below for the details....

Vitamin D3 Supplements in Winter May Help Protect Against Influenza A

Vitamin D3 Supplements in Winter May Help Protect Against Influenza A

April 2, 2010 - Vitamin D3 supplementation during the winter is linked to lower incidence of influenza A, particularly in specific subgroups of schoolchildren, according to the results of a randomized, double-blind, placebo-controlled trial reported online in the March 10 issue of the American Journal of Clinical Nutrition.

"To our knowledge, no rigorously designed clinical trials have evaluated the relation between vitamin D and physician diagnosed seasonal influenza," write Mitsuyoshi Urashima, MD, PhD, from Jikei University School of Medicine, Minato-ku, in Tokyo, Japan, and colleagues. "We investigated the effect of vitamin D supplements on the incidence of seasonal influenza A in schoolchildren."

Schoolchildren were randomly assigned to receive vitamin D3 supplements (1200 IU/day) or placebo from December 2008 through March 2009. The main endpoint of the study was the incidence of influenza A, diagnosed by influenza antigen testing (rapid influenza diagnostic test [RIDT]) on a nasopharyngeal swab specimen.

In the vitamin D3 group, 18 (10.8%) of 167 children had influenza A, as did 31 (18.6%) of 167 children in the placebo group (relative risk [RR], 0.58; 95% confidence interval [CI], 0.34 - 0.99; P = .04). The association of decreased influenza A incidence with vitamin D supplements was stronger in children who had not been taking other vitamin D supplements (RR, 0.36; 95% CI, 0.17 - 0.79; P = .006) and in those who started nursery school after age 3 years (RR, 0.36; 95% CI, 0.17 - 0.78; P = .005).

Among children with a previous diagnosis of asthma, 2 children in the vitamin D3 group vs 12 children in the placebo group had asthma attacks as a secondary outcome (RR, 0.17; 95% CI, 0.04 - 0.73; P = .006).

"This study suggests that vitamin D3 supplementation during the winter may reduce the incidence of influenza A, especially in specific subgroups of schoolchildren," the study authors write. "....Moreover, asthma attacks were also prevented by vitamin D3 supplementation."

Limitations of this study include small sample size, lack of data on serum 25-hydroxyvitamin D or on urinary calcium data, and lack of information on the presence or development of influenza A antibodies.

"Future studies should include a larger sample size of schoolchildren without comorbidities to determine the optimal dose and duration of vitamin D supplementation by measurement of serum 25-hydroxyvitamin D, serum and urinary calcium, and titers of antibody to influenza levels," the study authors conclude.

Jikei University School of Medicine supported this study. The study authors have disclosed no relevant financial relationships.

Am J Clin Nutr. Published online March 10, 2010. Abstract