By Barbara A. Brehm, Ed.D.

If you need another reason to stick to your exercise program, here it is: Regular physical activity appears to improve functioning of the immune system. Recent research suggests that people who exercise regularly come down with fewer colds and flus than their sedentary friends. Regular physical activity seems to strengthen the immune system so that it is better able to fight the foreign invaders who would like to set up house in our bodies and make us miserable in the process.

Like many physical attributes, the immune system is affected by both genetic and environmental factors. And like other factors, it changes as we age, becoming less effective in protecting us from bacteria and viruses, and less adept at recognizing and disabling cancerous cells. Age-related changes are why it takes older folks longer to recover from infections and infectious diseases, and why malignancies become more common with age.

The good news is that much of the immune system decline previously attributed to age is in fact influenced by lifestyle. Older folks are less likely to eat well or be physically active - factors that have a lifelong cumulative effect on immune system status.

What kind of exercise is best?

Scientists still do not know exactly how exercise preserves immune strength, so it is really too early to say what types or amounts of exercise maximize immune function. The evidence so far suggests that moderately vigorous exercise appears to be the most helpful. One study examined women (67 to 85 years old) who had been exercising for more than one hour a day for the previous five years. In laboratory tests, their immune response was stronger than that of women who had only been walking for the previous 12 weeks, and almost as responsive as the immune systems of much younger women.

The immune response of the group that started a walking program did not improve over the course of the 12-week experiment, as measured in the lab. Twelve weeks may not have a very strong effect on an aging immune system, while a lifetime of physical activity can slow the aging process. But before you decide it is too late for you to reap the immunity benefits of exercise, consider this: The walkers still came down with fewer colds than a control group of healthy women who remained sedentary. This suggests that even moderate exercise confers important immune benefits.

And while vigorous exercise appears to be helpful, beware: Over-training can decrease immunity. Exercising too much weakens many physical systems as your body becomes energy depleted. Frequent colds and flus are often a sign of trying to do too much.

Since exercise improves immunity, should you exercise if you are sick?

It depends on how sick you are. Exercise strengthens the immune system over the course of many years, but one bout of exercise does not appear to enhance immune function, and may even make you sicker by drawing on your already exhausted energy stores. Some people seem able to exercise when they are only mildly sick, but, in general, rest is what you need when you're not feeling well. Sickness is your body's way of saying, “Slow down!”

If the immune system becomes less effective as we age, why are my kids always sick while my parents hardly ever come down with a cold?

While the strength of an immune response declines with age, the memory the immune system retains of previous viral encounters does not. Each time your immune system mounts a defense against a bacteria or virus, it “remembers” and carries a record of what that invader looks like. The next time those bacteria or viruses get into your body, the immune system recognizes them as targets for immediate attack, and you don't get sick. Immune system memory is why vaccines work; your immune system learns to recognize viruses you have been immunized against, like polio, diphtheria and so on.

Scientists have identified more than 200 viruses that cause cold symptoms. A newborn's immune system hasn't learned to recognize any of them. Each time a new virus comes along, babies and young kids get sick. If we get two or three colds every year, by the time we are 60 or 70, our immune systems recognize many cold viruses, and unless we are rundown or have other health problems, we rarely catch a cold.