QUESTION: I would like to know if exercise helps decrease the number of colds a person gets or decreases the chance of getting the flu. With winter coming, I would like to think that I am less likely to get sick than my friends who don't exercise. I will look forward to your answer and I think others will be interested. Thanks.

ANSWER: According to an article in the September issue of "Running & FitNews" (American Running and Fitness Association), there is some evidence that exercise helps decrease the incidence of certain infectious diseases by strengthening the immune system. Immunity is a complex process whose function varies from sending alarm signals to fighting invading bacteria, viruses and toxic materials to healing injuries. In one study, women who exercised 45 minutes a day, five days a week for 15 weeks reported fewer symptoms of upper respiratory infections than women who didn't exercise. There were several "chemical and biological changes that suggested moderate exercise improved the women's immune systems, compared to the controls."Several studies looked at a family of proteins, called immunoglobulins , which can act as antibodies to resist infection. A group of high school basketball players was followed during a competitive season to determine the effect of this activity as measured by a specific antibody found in mucous and saliva called IgA. IgA levels in saliva increased both during rest and after any game. The researcher concluded that the general level of exercise during the season and the exercise during a specific game both protect against respiratory infections.

Another study found that IgA levels in a women's field hockey team were lower during intense competition than during training. The biggest drop occurring during the most difficult matches. This may indicate that immunity goes down if the work is too difficult or if stress levels are high.

One study looked at a group of specialized cells in the immune system called helper cells and suppressor cells. Using these cells as an indication of immunity, runners "took a beating during periods of heavy training, but not during easy periods of baseline training and after exercise."

From these and other studies, the article concluded that some aspects of the immune system appear to improve with moderate levels of exercise. However, very intense exercise can have negative effects.