Give your immune system a boost this winter with a dose of exercise. The average individual can expect to experience 2-3 illnesses per year. A healthy immune system is needed to ward off invasions from pathogens, such as viruses, bacteria, and fungi, that you are exposed to on a daily basis.

Routinely engaging in moderate-intensity exercise can help to strengthen your body's defenses against infection and illness. Studies from the Appalachian State University in North Carolina have shown that walkers are up to 50% less likely to experience illnesses than their non-active counterparts. Moderate-intensity exercise has also been linked to less severe symptoms and faster recovery when an infection does take hold.

Factors that impair immune function:

• Smoking
• Sleep deprivation/fatigue
• Stress
• Poor diet
• Intense, exhaustive exercise/over-training
• Advanced age

Effects of acute bouts of moderate-intensity exercise on immune function:

• Boosts the activity of the "natural killer" (NK) cells (a type of white blood cell) which are your body's first line of defense against foreign bodies (germs). Without needing prior exposure to recognize foreign bodies, NK cells spontaneously inactivate pathogens that enter your body.
• Increases the activity of macrophages (a type of white blood cell) that attack foreign bodies.
• Effects from an acute bout of moderate-intensity exercise are temporary; but, when performed on a regular basis, will strengthen your immune function.

Effects of acute bouts of vigorous-intensity exercise on immune function:

• Intense, prolonged, exhaustive exercise (90 minutes or more) is associated with an impairment of NK cell activity, suppressing your immune system for up to 72 hours post-exercise, making you more susceptible to infection during this time period (the "open window" hypothesis).
• Increases stress hormones (cortisol and adrenaline) which are thought to play a role in decreased NK cell activity, as well as suppressing the activity of other types of white blood cells (T- and B-cells).

Regular and consistent participation in moderate-intensity exercise can have an accumulative effect that builds a healthy immune system. To minimize the deleterious effects of prolonged, intense exercise on your immune system, take care to include sufficient rest and recovery days to give your body a chance to restore itself. Making lifestyle changes that minimize mental stress, promote consumption of a well-balanced diet, ensure sufficient sleep, and promote smoking cessation will also help to maximize your immune function.

Resources:

ACSM Current Comment "Exercise and the Common Cold," Nieman, D.C., Weidner, T., and Dick, E.

Exercise Physiology Energy, Nutrition, & Human Performance, 6th edition, McArdle, W.D., Katch, F.I., and Katch, V.L., pp. 459-461.

About.com Sports Medicine, "Exercise and Immunity Can too Much Exercise Make You Sick?", Quinn, E.