Exercise is like a medication. In the correct dose, it can be beneficial, but in excess, it can be harmful. Different athletes have different tolerances for stress. A grueling workout for one person might be a routine workout for another. The key is to find the right level of exercise for you that challenges your body to maintain and increase fitness without over-stressing it and increasing your chances of getting an infection due to a temporarily weakened immune system.
Strengthening Immune System
It is possible to exercise vigorously, but not excessively, without stressing the immune system. In fact, between 60 percent and 90 percent of dedicated amateur and professional athletes who do not over-train report they have fewer colds than people who don't exercise, according to a June 2001 report by the President's Council on Physical Fitness and Sports Research. Dr. Mark Jenkins, associate team physician for Rice University, writes on the SportsMedWeb site that improved immune function through moderate exercise is "especially true in older athletes, and it appears that regular exercise can help reduce the age-related decline in immune function."
Weakening Immune System
The President's Council concluded that "Most endurance athletes should experience low-to-normal URTI, or upper respiratory tract infection, risk during periods of regular training, with URTI risk rising during periods of overreaching/overtraining and competition." While cumulative challenging workouts can lower immunity, even a single session of particularly excessive exercise can put an athlete at risk. For example, Jenkins refers to the observation that two-thirds of participants developed URTIs shortly after completing an ultramarathon." He concludes that ". . . cumulative overtraining weakens the athlete's immune system, leading to frequent illness and injury."
Effects of Too Much Exercise on Immunity
More than one component of the immune system may be weakened by excessive training -- for example, by more than 90 minutes of intense exercise. These include changes in the number and function of immune system cells, such as white blood cells, antibodies, and pro- and anti-inflammatory biochemicals such as cytokines. During the 3 to 72 hours following an overly intense workout, there may be an "open window" during which "viruses and bacteria may gain a foothold, increasing the risk of subclinical and clinical infection. Thus, risk of upper respiratory tract infections can increase when athletes push beyond normal limits," according to Jenkins.
Avoiding Exercise Burnout
It's important to monitor how you feel to prevent overstressing your body. Jenkins suggests you do this by keeping an exercise training log. Recording your subjective feelings of how tired you are after each workout can help you adjust your training intensity if you feel you aren't recovering between workouts. In addition to a well-planned, flexible training program, Jenkins recommends recording your heart rate each morning; if it increases day-to-day, you may be overstressing your body. Eating a healthy diet, getting enough rest and getting an annual flu shot may also decrease immune system stress. See your doctor if you have frequent URTIs or constant fatigue. They could be signs that something more than too much exercise is stressing your immune system.