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The concept that exercise can have both a positive and a negative effect on the risk of infection is almost two decades old. This idea was introduced by Dr. David Nieman when he described the risk of a URTI as a J-shaped curve that changed as a function of the intensity and amount of exercise being performed.
People engaging in regular bouts of moderate exercise are at a lower risk of a URTI compared to sedentary individuals and people who engage in intense and/or long-duration exercise sessions.
For example, marathon runners represent those at the high end of the exercise intensity and duration scale. Indeed, running a marathon increases the risk of a URTI in the days following the run (24).
In the next two segments, we discuss the reasons why moderate- and high-intensity exercise have different effects on the risk of infection.
Although controversy exists, several studies support the concept that people who engage in regular bouts of moderate aerobic exercise catch fewer colds (i.e., URTIs) than both sedentary individuals and people who engage in high-intensity/long-duration exercise (22,23).
For example, both epidemiological and randomized studies consistently report that regular exercise results in an 18% to 67% reduction in the risk of URTI (9).
Interestingly, this exercise-induced protection against infection can be achieved with many types of aerobic activity (e.g., walking, jogging, cycling, swimming, sports play, and aerobic dance).
In general, it appears that 20 to 40 minutes of moderate-intensity exercise (i.e., 40% to 60% of V˙O2 max) per day is adequate to promote a beneficial effect on the immune system.
Importantly, this exercise-induced reduction in the risk of URTI occurs in both young adult and middle age men and women (22). Further, regular aerobic exercise appears to benefit the immune system in older individuals as well (28).
The explanation as to why moderate-intensity exercise protects against URTI remains a topic of debate. Nonetheless, there are several possible reasons to explain this.
Although it appears that aerobic exercise provides protection against colds, it remains unknown if resistance exercise training provides the same level of protection against infection.
This is because few studies have systematically investigated the impact of resistance exercise on immune function.
Nonetheless, based on the available evidence, it appears that an acute bout of resistance exercise results in a transient increase in natural killer cells (15).
However, this immune boost is temporary because blood levels of primary immune cells return to normal within a short period following a resistance training session (5).
To summarize, it appears that regular bouts of resistance exercise might protect against infection, but additional research is required to firmly establish that resistance exercise alone is effective in providing protection against URTIs.
The idea that athletes engaged in intense training are more susceptible to infections originated from anecdotal reports from coaches and athletes.
For instance, the marathon runner Alberto Salazar reported that he caught 12 colds in 12 months while training for the 1984 Olympic marathon (17). Because Salazar was engaged in intense exercise training, many people reasoned that the high level of exercise training was responsible for his increased number of colds.
However, anecdotal reports do not prove cause and effect, and scientific studies were required to determine if intense exercise training leads to an increased risk of infection.
Although controversy exists (31), several studies support the concept that athletes engaged in intense endurance training suffer a higher incidence of URTI compared to sedentary individuals or people engaged in moderate exercise (Fig. 6.3) (9, 21, 24).
For example, compared to the general population, evidence indicates that sore throats and flu-like symptoms are more common in athletes involved in intense training (11,24).
Indeed, the risk of developing a URTI is two- to sixfold higher in athletes following a marathon compared to the general public (24). This increased risk of illness is a concern for athletes because even minor infections can impair exercise performance and the ability to sustain intense exercise training (27).
Further, prolonged viral infections are often associated with the development of persistent fatigue, which poses another threat to the athlete (7).
There are several reasons why high-intensity and long-duration exercise promotes an increased risk of infection (9). First, prolonged (>90 minutes) and intense exercise has a temporary depressive effect on the immune system. For example, after a marathon, the following major changes occur in immune function:
Collectively, these changes result in a depression of the immune system’s ability to defend against invading pathogens.
It has been argued that this immune suppression following a marathon provides an “open window,” during which viruses and bacteria can gain a foothold and increase the risk of infection (seeFig. 6.4).
The biological reason to explain why intense exercise promotes immune depression is probably related to the immunosuppressive effects of stress hormones such as cortisol (8).
High cortisol levels have been reported to depress immune system function in several ways (8).
For example, high levels of cortisol can inhibit the function of specific cytokines, suppress natural killer cell function, and depress both the production and function of T-cells (20).
Although strenuous exercise can depress immune function, other factors may also contribute to the increased risk of infection in athletes engaged in intense training.
Finally, do weeks of intense exercise training result in a chronic state of immune depression? The answer to this question is no because following an acute bout of exercise, circulating leukocyte number and function return to pre-exercise levels within 3 to 24 hours (8).
Further, comparisons of leukocyte numbers and other markers of immune function between athletes and nonathletes do not differ markedly (8). Therefore, in the resting state, immune function is not different between athletes and nonathletes.