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The reason why flu season breaks out every winter and then vanishes every summer has been a mystery long puzzling medical researchers.

Now, Japanese doctors may have found an answer to the enigma - low levels of vitamin D during the dark winter months.

In the first drug-style clinical trial giving relatively large doses of the vitamin to children, researchers found it was a potent flu-fighter. Those receiving the sunshine vitamin had a 42-per-cent reduction in the most severe type of flu, known as influenza A, compared to those not receiving the nutrient. The group not getting the vitamin also had six times more asthma attacks.

The experiment was conducted on about 350 otherwise healthy school-aged children, using doses of 1,200 International Units a day, six times the amount currently recommended by Health Canada.

The finding suggests that besides getting an annual flu shot, people might be able to reduce their odds of contracting the flu by popping a vitamin D pill.

The lead author of the study, Mitsuyoshi Urashima, a professor of epidemiology at Jikei University School of Medicine in Tokyo, hopes the discovery will present a new line of attack against the flu, but cautions that "we should have more scientific evidences to conclude" that taking the supplement works.

Even so, Dr. Urashima suspects the research is onto something and isn't waiting for additional studies. The researcher has been taking doses of 3,000 IU a day for the past two flu seasons and hasn't had a "fever, sore throat and fatigue since then," in contrast to previously having cold and flu bugs a few times a year. "I recommend to take it year round," Dr. Urashima said in an e-mail.

The Japanese study, published earlier this month in the American Journal of Clinical Nutrition, is one of a flurry of recent efforts to investigate the possible flu-fighting potential of vitamin D, triggered by a number of medical hints that the nutrient plays a role in boosting the immune system's ability to ward off infection.

The originator of the theory linking the flu to the vitamin is John Cannell, a U.S. psychiatrist who works at a state hospital for the criminally insane in California. Three years ago, he published an account of how an influenza epidemic swept through his hospital in 2005, but spared the ward where his patients had been receiving high doses of vitamin D.

"I noticed that the patients that I had on vitamin D just didn't get sick," said Dr. Cannell, who is also head of the Vitamin D Council, a non-profit U.S. advocacy group. He contends the Japanese research "confirms the theory" he holds that the nutrient plays a role in fighting the flu.

The view that vitamin D helps prevent the flu is an alternative to a competing theory among researchers that attributes the seasonality of the disease to the cold, low-humidity environment prevailing in winter. The winter weather, some believe, allows the flu virus to survive and be transmitted more effectively.

Those doctors suspecting a role for vitamin D believe it plays a part because the levels of the nutrient in people closely match the annual pattern of the flu. Vitamin D levels crash in fall and winter, coincident with the flu season. Amounts of the nutrient rise in the spring and summer, corresponding to the time of year when flu mysteriously disappears.

The reason for seasonality of the vitamin is that most of the nutrient that people have circulating in their bodies they make themselves in naked skin exposed to strong summer ultraviolet light, hence the reason it's often dubbed the sunshine vitamin.

Doctors have also found that many other diseases - ranging from cancer to multiple sclerosis, are more prevalent in high-latitude areas of the world such as Canada, where people experience pronounced seasonal fluctuations in vitamin D levels, leading to speculation that many chronic ailments are due in part to deficiencies in it.

Health Canada and U.S. authorities have appointed an expert panel to investigate these health claims, but it isn't expected to report findings until the summer.

The Japanese study involved children aged 10 on average, recruited at 12 hospitals and from patients of eight doctors in private practice. There were no serious adverse consequences to taking the vitamin, and doctors were sure of the precise number of flu cases because they tested swabs from the backs of the noses of those reporting symptoms.

The experiment ran during the four-month flu season beginning Dec. 1, 2008, to the end of March, 2009. Half the children received vitamin D, and half received placebos. The results were deemed statistically significant, and therefore unlikely to be due to chance.

For reasons that are unknown, the researchers found that vitamin D cut the risk of influenza A, the version of the bug that produces the most severe fevers and aches, but had no effect on influenza B, a less potent variety of the virus.

It's possible that different levels of vitamin D may be needed to fight each type. It's also possible that influenza B isn't susceptible to therapy using the nutrient.

Dr. Urashima was intrigued by the study's results showing a sharp reduction in asthma attacks, and plans another trial to see whether vitamin D prevents the common childhood ailment.

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