The link between exercise and perceived health is supported by a considerable amount of research, with additional studies appearing regularly to add weight to the claim. A study published this year in the Physical Therapy journal investigated the effectiveness of 30 minutes of resistance training on the physical condition, emotional status, and quality of life of pregnant women. Participants reported increased self-confidence, a reduction in pregnancy-related issues such as nausea, fatigue, back pain, and headaches, and improvements to sleep. However, research into the direct impact to the immune system has obtained mixed results.
An investigation by Nieman, Henson, Austin and Sha (2010) into the effect of physical activity on upper respiratory tract infection (URTI) noted that during the autumn-winter flu season individuals who participated in frequent aerobic activity and reported good levels of physical fitness were significantly less likely to experience illness. Additionally, for those individuals affected by URTI, the length and severity of symptoms increased for subjects who reported largely sedentary lifestyles. Physical activity is thought to trigger the production of macrophages (cells that attack URTI-causing bacteria), and physiological changes in the immune system during exercise increase the speed at which these cells circulate through the system. The body returns to a normal state within a few hours after physical activity, but a regular exercise schedule appears to extend this period of immunity, resulting in decreased susceptibility to infection.
A second study, published in the 2013 edition of the Razi Journal of Medical Sciences, supported the connection between exercise and resistance to illness, finding a positive correlation between regular exercise and cell-mediated immunity (even more so when combined with the ingestion of carbohydrates). These benefits are even greater to the elderly. The increased life span in the human population has revealed that some infections, cancer, and autoimmune phenomena occur more frequently as age increases, and growing evidence has revealed that regular and moderate exercise can promote an improvement in the innate immune function. It has been demonstrated that the incidence and mortality rates for certain types of cancer are lower among active individuals. As noted in the April 2008 edition of the European Review of Ageing and Physical Activity, a moderate exercise program (for example, 7,000 steps a day) can be extremely valuable in reversing the physiological changes in immune function that occur with ageing.
Further research has shown clear influence of a six-month endurance exercise program on the immune function of prostate cancer patients undergoing treatment. The study, published in BMC Cancer, highlighted the minimising effects of exercise on the mortality risk and treatment related side effects. Although knowledge about the impact of physical activity on tumour relevant growth factors is still rudimentary, the study will continue to investigate other immunological effects on patients during palliative care.
Conversely, exercising during illness can have a negative effect on the immune system, especially when the illness is systemic. A study in 2009 at the University of Illinois found that the inflammation caused by cellular response to viruses could be prolonged by intensive exercise. The initially helpful inflammatory response eventually becomes counterproductive in fighting the infection and can instead cause damage to its host. The general advice given is to wait two weeks after symptoms subside before engaging in any intensive training, and to begin gradually rather than jumping straight back into your former routine. For endurance athletes especially, allowing the body a chance to rest is key to avoiding an impaired immune system. A program of exercise that consistently and progressively challenges the body without severely compromising it is the most practical training approach.
Although the relationship between physical activity and the suppression or stimulation of the immune system is not fully understood, there is certainly evidence to show that regular, moderate-intensity exercise helps to protect against some diseases, specifically those involving the upper respiratory tract. It can also improve cardiovascular health, lower blood pressure, and help control body weight. However, too much intense physical activity can reduce immunity. The key is therefore knowing how much exercise is enough and recognising when to rest.