Can Exercise Boost Your Immune System

Can exercise "boost" your immune system and help protect your body against infections?

What about supplements that claim to boost your immune system? Do they work? EndurElite Chief Endurance Officer Matt Mosman, MS, CISSN, CSCS has your answer in this 60-second brain bomb!

Exercise And Resistance To Infection

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.

Moderate Aerobic Exercise Protects Against 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.

• First, each bout of moderate aerobic exercise causes an increase in blood levels of natural killer cells, neutrophils, and antibodies (9).
• Therefore, an acute bout of exercise provides a positive boost to both the innate immune system (natural killer cells and neutrophils) and the acquired immune system (antibodies).
• Note that this exercise-induced boost in immune function is transient and that the immune system returns to pre-exercise levels within three hours.
• Nonetheless, it seems that each bout of exercise improves immune function against pathogens over a short period, which results in a reduced risk of infection (22).
• Other factors might also contribute to the positive impact of routine exercise on the reduced risk of infection. For example, people who engage in regular exercise may also benefit from an improved psychological well-being (i.e., less emotional stress), good nutritional status, and a healthy lifestyle (e.g., adequate sleep).
• Each of these factors has been linked to a reduced risk of infection and, therefore, may also contribute to the connection between regular exercise and lowered risk for URTI.

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.

High-Intensity/Long-Duration Aerobic Exercise Increases the Risk of Infection

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:

1. Decreased blood levels of B-cells, T-cells, and natural killer cells
2. Decrease in natural killer cell activity and T-cell function
3. Decrease in nasal neutrophil phagocytosis
4. Decrease in nasal and salivary IgA levels
5. Increase in pro- and anti-inflammatory cytokines

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 (see Fig. 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.

• For example, athletes engaged in intense training may also be exposed to other potential stressors, including a lack of sleep, mental stress, increased exposure to pathogens due to large crowds, air travel, and inadequate diet to support immune health.
• Each of these factors has been reported to have a negative impact on immune function and, therefore, could contribute to the increased incidence of URTIs in athletes (9).
• Figure 6.5 summarizes the factors that may explain why athletes engaged in intense training are at a greater risk of infection.

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.

IN SUMMARY

• Exercise can have both a positive and negative effect on the risk of infection. The relationship between the intensity/amount of exercise and the risk of developing a URTI is described as a J-shaped curve (Fig. 6.3). This J-shaped curve illustrates that moderate-intensity aerobic exercise decreases the risk of infection, whereas high-intensity/prolonged exercise increases the risk of infection.
• Regular aerobic exercise can reduce the risk of infection in several ways. In particular, an acute bout of moderate exercise increases blood levels of antibodies, natural killer cells, and neutrophils that provide a positive boost to the immune system.
• The fact that moderate aerobic exercise provides a transient boost to the immune system is another example of how regular exercise can enhance a control system in the body to preserve homeostasis. Indeed, by protecting the body against infection, the immune system plays an important role in maintaining homeostasis.
• High-intensity/long-duration exercise has been shown to have a temporary depressive effect on the immune system. This acute immune suppression following an intense exercise session provides an “open window” during which viruses and bacteria can grow, resulting in an infection.

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