The benefits of exercise are no secret in the realms of physical and mental health. In November 2020, the World Health Organization (WHO) published new recommendations regarding how much physical activity is necessary for good health for different age groups and for those with certain health conditions. To summarize their recommendations, adults ages 18-64 and adults age 65 and older should aim for at least 150-300 minutes of moderate-intensity aerobic activity, 75-150 minutes of vigorous-intensity activity, or an equivalent combination of both per week, while an increased amount may provide additional benefits. Muscle-strengthening exercises at least moderate in intensity that focus on the major muscle groups are recommended for adults at least twice weekly, while older adults are recommended to perform combination exercises that focus on functional strength and balance on at least 3 days per week. The WHO states physical activity as benefiting cardiovascular health, aiding in the prevention and management of non-communicable diseases (e.g., diabetes, heart disease, and cancer), reducing depression and anxiety symptoms, and enhancing thinking and learning. This leaves a lingering question: what is the effect of exercise on the immune system?
Such a discussion prompts an overview of immunology. The immune system is divided into two types of immunity—innate immunity and adaptive immunity. Innate immunity can be considered as the body’s first line of defense when it encounters a pathogen, and it acts in a non-specific way, meaning that it does not distinguish the type of pathogen that it encounters. The components of the innate immune system are the skin and mucous membranes that act as physical barriers, macrophages that digest foreign particles, white blood cells that act as macrophages and stimulate other cells of the immune system, mast cells that activate an inflammatory response, natural killer cells that attack certain substances, and dendritic cells that present the pathogen to cells of the adaptive immune system. The activation of the innate immune system triggers an inflammatory response and the release of chemical messengers called cytokines that also signal that an immune response is needed.
Adaptive immunity takes over if the innate immune system is unable to rid the pathogen. It is much slower to respond because it needs to first identify the specific pathogen. This involves T lymphocytes (T cells), B lymphocytes (B cells), and antibodies that are produced by B cells. Simply put, T cells and B cells are involved with signaling between each other, destroying cells that have become infected, and remembering the identity of the pathogen so that an immune response can occur immediately if it is encountered again. Antibodies circulate in the blood to prevent healthy cells from becoming infected and are also pathogen-specific.
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The field of exercise immunology has been present since the early 1900s; however, since 1990, the field has noted that there are multiple facets to be taken into consideration when considering the effect of exercise on the immune system. What is known is that the immune system does respond to exercise, and it is dependent on the exercise extent and duration. Acute exercise, defined as moderate-intensity and less than 60 minutes in duration, is believed to enhance the immune system by stimulating the circulation of various immune cells and anti-inflammatory cytokines, an additive effect with regular exercise. In contrast, prolonged exercise of moderate- to high-intensity without appropriate rest can trigger a decrease in immune cell activity. Regular moderate-intensity exercise also decreases inflammation and reduces stress hormones. By increasing circulating immune cells and decreasing systemic inflammation, regular exercise helps to improve the body’s ability to protect against infection. Data also suggests that regular exercise may delay the onset of immune system dysregulation that is associated with aging, solidifying the importance of regular exercise as one ages. It must be noted to be mindful of the intensity and duration, due to a potential window of opportunity for infection after prolonged, high-intensity exercise, as anti-inflammatory cells increase to repair muscle damage to the extent of possible immunosuppressive levels.
If prolonged moderate- to high-intensity exercise can decrease immune cell activity, why would anyone consider competitions or endurance events? Some studies suggest other conditions must be taken into consideration, such as psychological stress, sleep quality and duration, and nutritional deficiency that may also decrease the immune system in those individuals. In addition, studies of a group of elite athletes actually had fewer missed work days due to illness than the general population, suggesting possible adaptation to training demands. As for the amateur population digesting this information, it would be wise to train using common sense, adjusting training volume and intensity when necessary and making only incremental increases in workload, while being mindful of other factors that may decrease the immune system, such as frequency and timing of rest days, quality and duration of sleep, nutrition, and stress.
In a time when overall health and immune system function is especially important, studies suggest exercise plays an important role. Not mentioned above was the impact of exercise on obesity and on disease states that may decrease the immune system’s ability to fight disease. By following exercise guidelines, the benefits of physical activity play a vital role in overall health, overlapping in many ways to help keep the body healthy and happy.
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