Excessive Exercise and Immunity: The J-Shaped Curve

Exercise modulates the innate and specific arms of the immune system with a marked intensity-dependent response. This response might be influenced by sex differences and other factors including age, nutrition status, and overall level of psychological stress. Exercise immunology is the field that studies this area. In the 1990s, Dr. Nieman formulated the controversial “J-shaped hypothesis” to describe the relationship between exercise intensity and the risk of acquiring upper respiratory tract infections (URTI). This hypothesis suggests that moderate exercise has the ability to improve immune function above sedentary levels while high intensity exercise depresses the immune system. However, some methodological problems exist in studies of the J-curve which makes evidence more anecdotal than evidence-based regarding the role of moderate and intense exercise in the incidence of URTI. These limitations are presented in the chapter.