McAllister: What vitamin is best for colds?

If you’re fending off a winter cold or battling a bout with the flu, you may be taking extra vitamin C to hasten your recovery. While boosting your intake of vitamin C may be beneficial, the results of a new study show that taking extra vitamin D may be even better.

Vitamin D, known as the sunshine vitamin, is produced in the skin in the presence of ultraviolet light from the sun. During the darker days of winter, vitamin D levels in the body typically dwindle, leaving many Americans with a seasonal deficiency.

Mounting scientific evidence suggests that low blood levels of the vitamin are at least partially responsible for the higher prevalence of respiratory tract infections during the coldest months of the year, when sunshine is in short supply.

The results of a study published in the Feb. 23 issue of Archives of Internal Medicine offer strong support for vitamin D’s role in reducing the risk of colds and the flu. After examining the relationship between vitamin D and respiratory illness in nearly 19,000 adolescents and adults, researchers found that those with the lowest blood levels of the vitamin were about 40 percent more likely to have suffered a recent respiratory infection than those with the highest levels.

The association was even stronger among individuals with pre-existing lung ailments. Among study volunteers with chronic obstructive pulmonary disease, including emphysema, researchers found that those with low blood levels of vitamin D were twice as likely to have experienced recent respiratory infections. Asthma patients with the lowest vitamin D levels were five times more likely to have suffered a recent infection than those with the highest levels.

Although its exact mechanism of action isn’t completely understood, scientists theorize that vitamin D may help ward off infection by bolstering the protective powers of the immune system. In laboratory experiments, the vitamin has been shown to dramatically enhance the germ-killing potential of white blood cells.This action is most apparent in the lining of the respiratory tract, which plays a critical role in protecting the throat, lungs and sinuses from invasion by disease-causing viruses and bacteria.

While vitamin D helps the body fend off colds and the flu, it also appears to arm the immune system against more serious illnesses, including cancer. In a four-year study of more than 1,000 healthy menopausal women, researchers at Creighton University found that those who took daily doses of calcium plus 1,100 international units of vitamin D experienced a 60 percent reduction in breast cancer compared to women taking placebo pills.

Adequate intakes of vitamin D have long been associated with a lower risk of colorectal cancer. Harvard investigators recently reported that men with low blood levels of the vitamin were significantly more likely to develop colorectal cancer than those with higher levels.

Findings from several small clinical trials suggest that vitamin D also may thwart the progression of prostate cancer. In men diagnosed with the disease, supplementation was shown to slow the rise in blood levels of prostate specific antigen (PSA), a biological marker for the condition.

Vitamin D deficiency was once believed to be relatively rare among Americans, but recent epidemiological studies suggest that the condition is becoming a widespread problem in the United States, prompting many health professionals to push for an increase in the recommended daily allowance. Although some physicians now suggest daily doses of at least 1,000 international units to 2,000 international units for most adults, the current recommended daily allowance is substantially lower.

In young, healthy individuals, about 10 to 15 minutes of daily sun exposure is usually sufficient to keep levels of the vitamin within the normal range. With age, however, the skin manufactures vitamin D less efficiently, leaving many older people at risk for a deficiency.

Sunscreen, smog, dark complexions and clothing interfere with the absorption of the sun’s rays, as well as the skin’s ability to produce vitamin D. While certain foods, including milk, orange juice and a few grain products are fortified with the vitamin, the levels are rather low -- typically less than 100 international units of vitamin D per serving.

With about 350 international units per serving, oily fish, including salmon and mackerel, are excellent food sources. For Americans whose diets are less than wholesome, taking a vitamin supplement makes good sense.

Getting adequate amounts of Vitamin D may help prevent colds and the flu during the darker days of winter. Even better, it could reduce the risk of developing other, more serious illnesses all year long.

Rallie McAllister is a board-certified family physician, speaker and the author of several books, including “Healthy Lunchbox: The Working Mom’s Guide to Keeping You and Your Kids Trim.” Her Web site is www.rallieonhealth.com. Copyright 2009 Creators Syndicate Inc.