Prolonged intense exercise causes immuosuppression

A research posted on U.S. National Library of Medicine shows that prolonged intense exercise causes immunosuppression, whereas moderate-intensity exercise improves immune function and potentially reduces risk and severity of respiratory viral infections. These findings have given rise to the `J-shaped' hypothesis relating exercise dose with infection risk.

Source:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2803113/figure/F1/

Respiratory viral infections represent the most prevalent and pathogenic form of infectious disease, accounting over 7% of all deaths in both men and women in 2004. Infection occurs when a host comes in contact with infected aerosolized droplets or contaminated surfaces, following which the virus invades and infects the host's upper and/or lower respiratory mucosal tissues. Illness duration typically lasts 7–14 days, and the usual symptoms include: cough, nasal congestion, fever, body aches, malaise, and in severe cases death. Deaths associated with respiratory viral infection occur most often in children, elderly, and other immune-compromised individuals, as their immune systems are incapable of handling the elevated viral load.

Cross-sectional and longitudinal data suggests persons who engage in regular moderate intensity exercise maintain a reduced risk of self-reported respiratory symptoms.

An early study published by Heath et al. found runners in the upper two quartiles of yearly mileage (>866 miles run during the 12 month follow-up period) had a significantly higher risk of self-reported upper respiratory tract infection (URTI) symptoms compared to runners in the lowest quartile (<486 miles). A recent study found low-to-moderate frequency of exercise reduced the risk of influenza-associated mortality in adults living in Hong Kong during 1998, while a high frequency of exercise (>4d/week) failed to reduce the risk of mortality when compared to the sedentary referent group (34).

moderate intensity exercise performed prior to infection or infectious symptoms reduces respiratory virus-associated mortality in animals. In contrast, intense exercise before or during viral infection has been associated with greater morbidity and mortality

These findings have given rise to the `J-shaped' hypothesis (Fig. above) relating exercise dose with infection risk

This finding has provided evidence to support the hypothesis that moderate intensity exercise reduces inflammation and improves the immune response to respiratory viral infections. We hypothesize that acute and chronic moderate exercise induces a level of stress hormones that down-regulates excessive inflammation within the respiratory tract and aids in activating innate anti-viral immunity shifting the immune response towards a Th2 profile, thereby balancing the Th1/Th2 responses to prevent an excessive Th1 immune reaction to these pathogens. Prolonged, intense exercise may do this as well but may shift the balance too much towards Th2 and away from Th1 actually allowing the virus to gain a better foothold and cause greater pathology. Further research is necessary to examine cellular and molecular mechanisms through which exercise modulates immune function. Additionally, human studies should attempt to elucidate the most common respiratory pathogens responsible for infections associated with high intensity exercise training and athletic competitions, and the methods they employ to evade immune response, as well as attempt to translate mechanistic studies to a human experimental model. Based upon the available evidence, moderate intensity exercise training should be used as an adjunct to other preventative measures against respiratory tract viral infection.