The notion that prolonged, intense exercise causes an “open window” of immunodepression during recovery after exercise is well accepted. Repeated exercise bouts or intensified training without sufficient recovery may increase the risk of illness. Exercise increases circulating neutrophil and monocyte counts and reduces circulating lymphocyte count during recovery. This lymphopenia results from preferential egress of lymphocyte subtypes with potent effector functions. These lymphocytes most likely trans-locate to peripheral sites of potential antigen encounter (e.g., lungs and gut). This redeployment of effector lymphocytes is an integral part of the physiological stress response to exercise.
Current knowledge about changes in immune function during recovery from exercise is derived from assessment at the cell population level of isolated cells ex vivo or in blood. Evidence suggests that reduced immune cell function in vitro may coincide with changes in vivo and rates of illness after exercise. Among the various nutritional strategies and physical therapies that athletes use to recover from exercise, carbohydrate supplementation is the most effective for minimizing immune disturbances during exercise recovery.
Sleep is an important aspect of recovery, but more research is needed to determine how sleep disruption influences the immune system of athletes. The immune system is integral to the body's defense against infection. It also influences other physiological systems and processes, including tissue repair, metabolism, thermoregulation, sleep/fatigue, and mental health. Over the past 40 years, exercise immunology has developed into its own discipline based on the recognition that the immune system mediates many exercise effects and that stress responses mediated through the nervous and endocrine systems play a key role in determining exercise-induced immune changes. A classic paradigm in exercise immunology is that an “open window” of immunodepression can occur during recovery from intense exercise. In particular, this paradigm proposes that after intense exercise, some immune variables (e.g., lymphocyte and natural killer cell numbers and antibody production) transiently decrease below preexercise levels. As a result of this immunodepression, microbial agents, especially viruses, may invade the host or reactivate from a latent state, leading to infection and illness. If exercise is repeated again while the immune system is still depressed, this could lead to a greater degree of immunodepression and potentially a longer window of opportunity for infection.
Exercise-induced fatigue exists on a continuum. Repeated bouts of intense exercise on the same day or over several days may cause acute fatigue, as indicated by an inability to maintain exercise workloads. An athlete who trains intensely for 1−2 wk may experience a state of “functional overreaching,” which is associated with a temporary performance decrement, followed by improved performance. Intense training over an extended period without sufficient balance between training and recovery may lead to “nonfunctional overreaching” (NFOR). This condition is typically characterized by persistent fatigue, performance decrement, muscle soreness, and psychological and hormonal disturbances that can last for weeks or months. Depending on the time needed to recover from NFOR, an athlete may be diagnosed (retrospectively) as experiencing “overtraining syndrome”.
Recognition of the link between excessive training and risk of illness has stimulated interest in nutritional supplements and physical therapies to counteract immunodepression and to restore immune function after exercise training. In this mini-review, we update the current state of knowledge about the temporal changes in the immune system following exercise; how repeated bouts of exercise on the same day, extended periods of intense training, and sleep disruption influence the immune system; and the efficacy of various strategies for restoring immune function after exercise.