One of the most frequent concerns of my patients is what to do about the flu, both seasonal and H1N1. Although the vaccines may be effective in preventing or lessening flu infections, medical science is showing that nature has provided us with some significant firepower of our own.

H1N1 aside, the most active time for the flu is in the late fall, winter and early spring. Researchers have wondered what happens to the flu for the rest of the year. The answer may be related to sun exposure and vitamin D - less in the winter and more in the summer.

Vitamin D is not a vitamin but a hormone. Starting in the skin, the body is able to manufacture it from the combination of cholesterol and sunshine. Research is demonstrating that vitamin D is needed for good immune function. During the winter, sunshine fades, vitamin D levels fall and the flu really kicks in. During the summer, with lots of sun, vitamin D levels increase and the flu seems to disappear. Is there a connection? The answer seems to be yes.

A 2007 study, published in the medical journal Epidemiology and Infection, established that people who did not take vitamin D had 10 times greater risk of flu during the winter than in the summer. Interestingly, the incidence of winter flu was not different from summer flu for those who took vitamin D year round. In addition, those who did not take vitamin D had a nine times greater risk of winter flu than those who took vitamin D. It seems that vitamin D may be a good way to prevent winter flu.

How can vitamin D reduce the incidence of flu? The answer may be that vitamin D activates immune system-based proteins that kill the flu virus. These are called antimicrobial peptides, or AMD. These proteins kill a wide range of pathogens including bacteria, some viruses, fungi and even some types of cancer cells. In the winter, low vitamin D levels may result in meager AMD production - and an increased risk of contracting the flu. In contrast, the summer sun results in more robust vitamin D levels, encourages AMD production and may prevent the flu. From my clinical perspective, though it's not definitive proof, patients with high vitamin D levels seem to have fewer infections than those with lower vitamin D levels.

The optimal level of vitamin D has not been determined. However, current adult recommendations from the Institute of Medicine (400-800 IU per day) are probably too low for most people living in northern Illinois. In the winter, the sun is not intense enough for the body to generate any vitamin D and many people need at least 5,000 to 6,000 IU of vitamin D per day. Usual dietary sources are inadequate and supplementation is the only reasonable option. Vitamin D is generally safe and toxicity is vanishingly rare. It is important to check blood levels because it is the only way to know if you are taking enough vitamin D.

There are many aspects to preventing the flu - vaccination, hand washing, cleaning surfaces - and now, vitamin D.

• Patrick B. Massey, M.D., Ph.D is medical director for complementary and alternative medicine for the Alexian Brothers Hospital Network.