Occasionally, we invite experts to write about the subject they know best. The following main content was written by:

Shawn Dassie, MS, CSCS, USAW, NASM-PES
Director of Fitness and Sports Performance
360 Health Club

According to the CDC, the average adult has two to three upper respiratory infections each year. We are all exposed to viruses all day long, but some people seem more susceptible to catching colds or the flu.

However, there are some things that seem to protect us from picking up colds. One of those things appears to be moderate, consistent exercise. More and more research is finding a link between moderate, regular exercise and a strong immune system.

More recent studies have shown that there are physiological changes in the immune system as a response to exercise. During moderate exercise, immune cells circulate through the body more quickly and are better able to kill bacteria and viruses. After exercise ends, the immune system generally returns to normal within a few hours, but consistent, regular exercise seems to make these changes a bit more long-lasting.

Research indicates that moderate exercise repeated on a near-daily basis leads to a long-term immune response. This research also indicates that those who walk at 70-75 percent of their HR Max (See HR article) for 40 minutes per day had half as many sick days due to colds or sore throats as those who don't exercise.

Although most research indicates that exercise has a beneficial response on the immune system, there is also evidence that too much intense exercise can reduce immunity. This research is showing that more than 90 minutes of high-intensity endurance exercise can make athletes susceptible to illness for up to 72 hours after the exercise session. This is important information for those who compete in longer events such as marathons or triathlons.

Intense exercise seems to cause a temporary decrease in immune system function. Research has found that during intense physical exertion, the body produces certain hormones that temporarily lower immunity.

If you are already ill, you should be careful about exercising too intensely. Your immune system is already taxed by fighting your infection, and additional stress could undermine your recovery. In general, if you have mild cold symptoms and no fever, light or moderate exercise may help you feel a bit better and actually boost your immune system. Intense exercise will only make things worse and likely extend your illness.

Sources:
Acute exercise stimulates macrophage function: possible role of NF-kappaB pathways. Cell Biochemistry and Function. 2006 Aug 14;

MedLine Plus Exercise and Immunity.

Nieman DC, Henson DA, Austin MD, Brown VA. The immune response to a 30-minute walk. Med Sci Sports Exerc 37:57-62, 2005. David Nieman, of Appalachian State University

Nieman DC. Risk of Upper Respiratory Tract Infection in Athletes: An Epidemiologic and Immunologic Perspective. Journal of Athletic Training 1997 Oct.