Exercise affects the immune system

Resistance and endurance training are commonly seen as healthy activities to engage in – but can exhaustive exercise increase the likeliness of getting ill? The human body is a complex physical structure of organic tissue – an isolated event can affect tissues in several other locations. The immune system is an adequate ambassador of our complexity and is essential for maintaining health. Dysfunction can lead to a wide variety of diseases. A common perception by elite athletes is that frequent and intense exercise leaves the body predisposed to infections. However, an inconsistency exists when health care professionals suggest that regular exercise is beneficial to the immune system by providing some resistance to infections. So, who’s right?

The Mayo Clinic is a world-renowned medical practice dedicated to the diagnosis and treatment of virtually every type of complex illness. Medical staff outlined seven benefits of regular physical activity, which include: improving mood; combating chronic disease; assisting in weight management; strengthening the heart and lungs; promoting better sleep; increasing intimacy and a better sex life; and – drum roll – it can be fun! This all sounds wonderful and highly prolific for physical health, however a contradiction occurs when comparing occasional and recreational activities to chronic and elite fitness endeavors. Periods of frequently exhaustive training constantly push a trainee’s fitness threshold through progressive overloads in strength, power and endurance.

Sports immunology is a relatively new field of study; focusing on physical, psychological and environmental stress on immune function. Over the last 100 years, medical journals have published over 600 articles on the subject; the majority since 1990. Most recently, medical professionals and scientists have begun to understand the interaction between exercise and immune function – as well as the problems associated with subjective symptoms. A large collection of scientific, clinical and epidemiological data supports the concept of positive and negative impacts of exercise on the immune system, to include the American College of Sports Medicine position papers and the Surgeons General’s report on physical activity and health.

In February, the Journal of Applied Physiology published research examining immune function in sport and exercise. In his research, Michael Gleeson states that “exercise can have both positive and negative effects on immune function and susceptibility to minor illnesses. The relationship between exercise and susceptibility to infection has been modeled in the form of a ‘J’-shaped curve.” There is fairly convincing evidence that moderate and regular physical activity is associated with decreased incidences of infection. More specifically, it has been reported that around two hours of moderate exercise per day is associated with a 29 percent risk reduction for picking up an upper respiratory tract (URT) infection, compared to a sedentary lifestyle. On the other hand, research using elite and aggressively-trained athletes suggests a discord in exercise and health. Reports following a competitive ultra-endurance running event indicate a 100-500 percent increase in risk of infection for several weeks. This is a concern for elite athletes since even minor infections can result in a drop in performance and the ability to sustain a heavy workload. Severe infections can cause persistent fatigue.

Periods of heavy training do suppress several aspects of immunity but athletes do not become clinically immune deficient. Moreover, many reported symptoms of respiratory tract infections (sore throat, runny nose, congestion, fever) are subjective, and as such, easily over-exaggerated. URT symptoms are easily confused with allergies, inhalation of air pollutants and airway inflammation.

In April, the American College of Sports Medicine published a five-month surveillance study comparing 32 highly-trained elite athletes with 31 recreationally-competitive athletes and 20 untrained sedentary controls. During the 2005 training period, a total of 37 URT infections were reported by 28 subjects. Of these episodes, nine were controls, seven recreationally-competitive exercisers and 21 elite athletes. Gleeson’s J-curve response was evident. Of the 37 episodes, and equally interesting, pathogens were identified in only 11 individuals! Infectious agents were confirmed in two controls and three recreationally-competitive exercisers. Of the 21 episodes reported by the highly-trained elite athletes, only six were cause by an actual pathogenic infection – suggesting that reported URT problems in elite athletes are seldom caused by infectious microorganisms.

Many nutritionists suggest dietary supplementation to enhance the body’s immune system; such as vitamin C, vitamin E and zinc. Several studies have shown how 500-600mg of vitamin C per day minimizes occurrence of URT infections following ultra marathons. While vitamin E at doses around 400IU support immune function, excessive amounts appear detrimental by increasing the oxidative stress on cells. Consumption of carbohydrate during exercise supports energy levels, proper immune function and lowers circulating levels of cortisol, the main adrenal stress hormone that acts as an anti-inflammatory agent.

However, once again, another contradiction exists: it’s possible that antioxidant supplementation and carbohydrate ingestion during exercise could limit adaptations to exercise – adaptations dependent on inflammation and the otherwise suppressed immune system – as well as negating fat burning affects from an increase in blood glucose levels. Nutritional interventions may reduce the risk of infection but they could also limit performance and hard-earned training adaptations. It can also be argued that carbohydrate intake during exercise allows the athlete to train harder and longer. Extreme exercise may impair immune function, but this may not be entirely harmful to an otherwise healthy host, partially due to how these events also lead to reduced whole-body inflammation – events also benefiting long-term health.

Exercise certainly affects the immune system. Recreationally active people engaged in moderately-intense physical activity will boost their immune system function, as compared to sedentary individuals. Athletes habitually performing exhaustive training protocols can impair immune system function. It is likely that overreaching, and subsequently overtraining, can lead to a longer lasting immune dysfunction. During periods of frequently exhaustive training, it may be advantageous to supplement with antioxidants to help support immune function; while concurrent carbohydrate intake should only be considered when the goal is maximum performance (such as long competitive endurance events) versus accumulated training adaptations to resistance exercise or mobilizing stored fat.

More research regarding immune system responses to sports and exercise is being conducted in today’s clinical and scientific settings. Today, it’s a lot of this-for-that information but it’s likely some innovative and interesting conclusions will follow in the future.

P. Brolinson, D. Elliott, Exercise and the Immune System. Clinics in Sports Medicine, Volume 26, Issue 3, Pages 311-319

Gleeson M., Immune function in sport and exercise. J Appl Physiol. 2007 Aug;103(2):693-9. Epub 2007 Feb 15.