Researchers See Link Between Vitamin D, Flu Immunity

In 2001, Dowell offered his own explanation: Annual light and dark cycles -- daylight getting longer and shorter -- affect biological rhythms, and somehow flu viruses have picked up on that and adjusted to those seasonal signals from humans.

"These seasonal rhythms have allowed influenza viruses to be more effective at infecting us in the winter," he said.

Studies Suggest Link
In addition, several intriguing studies involving vitamin D have accumulated in recent years:

• In February, researchers reported a link between vitamin D levels in the blood and upper respiratory infections in a group of nearly 19,000 people ages 12 and older. Those with deficient vitamin D levels -- less than 10 nanograms per milliliter -- were 55% more likely to have contracted a recent cold or upper respiratory infection than those with an adequate level, 30 nanograms per milliliter. The researchers, whose study was published in the Archives of Internal Medicine, noted that the current recommendation for vitamin D supplementation, between 200 and 600 International Units a day, is unlikely to produce optimal levels in the U.S. population. Indeed, several vitamin D researchers now say that adults should be taking up to 2,000 IU a day during the fall and winter, and that most children should take up 1,000 IU. 
• A 2006 review article in the journal Epidemiology of Infection laid out a detailed case for vitamin D and its protective effect against the flu. One of the arguments: About 2% of people have evidence of flu viruses in their systems during the summer, yet that seldom leads to outbreaks of the flu. The article also noted that the 1968 Hong Kong flu first showed up in Great Britain in August that year, but the virus did not cause any significant summertime illness. But as the sun became progressively lower in the sky and the winter solstice arrived, the first community outbreaks occurred. As spring arrived, flu cases ebbed and virtually disappeared after the summer solstice, only to rise again in September 1969 and explode in the days before the winter solstice. No vitamin D is made in the skin at latitude 52 degrees (the latitude of London) from about October to March," the article said.

Holick, a co-author of the article, said the only way the issue is going to be resolved is to do a large clinical trial using a placebo and vitamin D to see if it reduces flu rates.