In a study of Japanese schoolchildren, vitamin D supplements taken during the winter and early spring helped prevent seasonal flu and asthma attacks.

The idea for the study, study chief Dr. Mitsuyoshi Urashima, told Reuters Health, came from an earlier study looking at whether vitamin D could help prevent the bone-thinning disease osteoporosis. The researchers in that study noticed that people taking vitamin D were three times less likely to report cold and flu symptoms.

This led Urashima, of Jikei University School of Medicine, Tokyo, and colleagues to randomly assign a group of 6- to 15-year-old children to take vitamin D3 supplements (1,200 international units daily) or inactive placebo during a cold and flu season.

Vitamin D3, or cholecalciferol, is more readily absorbed by the body and more potent than vitamin D2, or ergocalciferol, the form often found in multivitamins.

During the study, conducted between December 2008 and March 2009, 31 of 167 children taking placebo caught influenza A, the most common form of the virus, compared with only 18 of 167 taking vitamin D.

The vitamin D group was 58 percent less likely to catch influenza A, the researchers report in the American Journal of Clinical Nutrition.

Vitamin D also appeared to suppress asthma attacks in children with a history of asthma. Two children taking vitamin D had asthma attacks during the study, compared to 12 children taking placebo. Urashima admitted to being a bit surprised by this finding and hopes to confirm it in a randomized trial targeting children with asthma.

Dr. Adit Ginde, of University of Colorado Denver School of Medicine, who was not involved in the study, told Reuters Health: “This is the first time a study has been done that rigorously shows that vitamin D supplementation can reduce a type of influenza in a dedicated clinical trial.” Ginde and colleagues published a study a year ago showing that asthmatics with lower vitamin D levels were at five times the risk for colds and flu.

In the Japanese study, vitamin D supplementation did not prevent influenza type B, which tends to appear later in the flu season than the “A” flu variety.

Ginde said there is no solid explanation for why vitamin D prevented influenza A and not influenza B. “The immune system fights different viruses in different ways. This finding needs to be explored in more detail,” Ginde said.

Based on the current study, giving kids vitamin D supplements during the winter may help reduce cases of influenza A, the researchers conclude. Urashima suggests that children could take 1,200 IU per day starting in September to prevent flu and asthma attacks during the flu season, but best for parents to check with their pediatrician first.

SOURCE: American Journal of Clinical Nutrition, online March 10, 2010.