The Best D-fense against Colds

Forget C. It's vitamin D that appears to mount the strongest defense against the common cold. When investigators from the University of Colorado in Denver asked 19,000 people of all ages about their health habits and then took blood samples, they discovered that those with the lowest level of vitamin D reported 40% more respiratory problems, including more frequent colds, than those who had normal D levels. D-deficient asthmatics really struggled: They were five times more likely to have had a recent respiratory infection -- and among those with chronic obstructive pulmonary disease (COPD) who were also low on D, respiratory infections were twice as common.

While most people think it's vitamin C that wards off respiratory bugs, there's little science to back that up. Meanwhile there's growing evidence that vitamin D is a champion infection fighter. Besides the Colorado study, numerous other investigations have linked low blood levels of vitamin D to a higher risk of respiratory infections and one large Finnish study found that people low on D take twice as many sick days due to respiratory tract infections. According to medical experts, D protects against illness by regulating a protein that destroys microscopic invaders like bacteria, fungus and viruses and also prevents the immune system from releasing too many inflammatory agents called "chemokines" and "cytokines" into infected lung tissues.

Besides eating high D foods like oily fish, egg yolks and fortified cereals and popping a supplement, you can also build up your body's D stores by soaking up some rays. For most people, spending 10-15 minutes outdoors everyday, a few times weekly will do. However as we age, our bodies tend to rely more heavily on D to keep the immune system humming along. The Adequate Intake (AI) for vitamin D for anyone under 50 years is 5 micrograms per day; for people 50 and over, the AI is 10 micrograms per day. Those who are dark-skinned, housebound or who live in tall buildings, air-polluted neighborhoods, wear layers of clothing, or use sunscreens may also need an extra does of D.