Exercise and immune system

Does physical activity impact the immune system?

by Christina Body and Fitness

There is no doubt that exercise can alter the risk of infection, but whether this occurs through specific changes in the function of the immune system is often difficult to say for sure. For example, a sudden increase in intense physical training tends to result in increased rates of infections such as colds and flu. The increased risk of illness or sickness could be due to increased exposure to environmental stressors such as extremes of heat or cold, polluted air or water or altered sleep patterns.

Susceptibility to infection can also be modified, of course, by direct alterations in immune system function. The immune system is composed of a dizzying array of cellular, enzymatic and hormonal components that, when working properly, defend the body against invading organisms. Exercise can lower immune function by causing muscle damage, excessive catecholamine secretion (via both physical and mental stress), and over-training.

Measurement of the precise alterations in immune system function after exercise are complicated by several factors including intensity and duration of activity, state of training and basic day-to-day and person-to-person variations. White blood cell counts are typically elevated after acute bouts of exercise. The more intense and the longer duration the exercise is, the higher the elevations usually are. Marathon running is estimated to increase risk of infection by almost six-fold. Much of the increased white blood cell count is due to release of cells from tissues into the circulation and is rapidly corrected during recovery. Prolonged intense activity, however, may decrease white blood cell counts due to their migration from the circulation to areas of damage in the muscles. Exercise intensity plays an important role in immune system stimulation in that secretion of catecholamines, such as the stress-hormone cortisol, may initially stimulate white blood cell release from tissues, but in states of chronic over-training may actually suppress this action.

While excessive physical activity that leads to muscle injury and over-training can compromise immune function, moderate levels of exercise are thought to increase resistance to infection. Adequate nutrition is important, however, because inadequate levels of amino acids such as glutamine may impair growth and function of immune cells. Moderate levels of regular aerobic and resistance exercise have positive effects on immune system function. Chronic bouts of high intensity, long duration exercise, however, may lead to over-training, muscle injury and adverse effects in immune system function. Whether a given level of exercise is excessive for an individual depends on many factors including the athlete's state of training, the volume and intensity of training and recovery as well as the athlete's level of stress and overall state of nutrition.