Researchers See Link Between Vitamin D, Flu Immunity

If the current strain of H1N1 swine flu is like flu strains of years past, it likely will mysteriously subside in the coming weeks.

For whatever reason, in temperate areas such as North America, flu largely is a fall and winter phenomenon.

A couple of theories have tried to explain that seasonality, but in recent years an intriguing new idea has emerged:

Levels of flu-fighting vitamin D reach their lowest point in the winter when ultraviolet light disappears.

Vitamin D, which is made in large amounts in the skin when it is exposed to solar radiation, is a hormone that regulates hundreds of genes. Some of those involve the body's innate immunity and its defenses against viruses, especially those affecting the respiratory system.

The idea is that if people increased their levels of vitamin D, it might help ward off outbreaks of flu.

For as far back as records exist, flu outbreaks have occurred around the planet when solar radiation was at its lowest. No one has been able to say why.

"It virtually disappears in the summer in temperate climates," said William Schaffner, chairman of the department of preventive medicine at Vanderbilt University School of Medicine. "In the tropics it kind of smolders all year long. Its seasonality is not nearly as pronounced."

Even the influenza pandemic of 1918 followed such a pattern, first showing up in Kansas in the winter of 1918 before subsiding and then returning with a vengeance in the fall.

Indeed, public health officials now will be watching the Southern Hemisphere as it heads into its winter, looking to see how swine flu and seasonal flu play out. And in the Northern Hemisphere, health officials are concerned about the virus coming back in the fall 2009 season. Even Mexico, particularly the northern regions, is far enough away from the equator to reflect the seasonal effect.

Part of the enthusiasm for vitamin D is based on research suggesting that it stimulates immune cells to produce anti-microbial substances that fight and help control the replication of viruses in the body.

At the same time, vitamin D helps tone down the immune system's response to invading viruses in the respiratory tract. It can prevent the overproduction of inflammatory substances that can lead to fluid buildup, more severe symptoms, even death. During the 1918 pandemic, flu produced severe inflammation in the lungs that caused many of its victims to drown in their own fluids.