Editing Exercise (section)

=== Immune system ===
Although there have been hundreds of studies on physical exercise and the [[immune system]], there is little direct evidence on its connection to illness.<ref name="Gleeson2007" /> [[Epidemiology|Epidemiological]] evidence suggests that moderate exercise has a beneficial effect on the human [[immune system]]; an effect which is modeled in a [[J curve]]. Moderate exercise has been associated with a 29% decreased incidence of [[upper respiratory tract infection]]s (URTI), but studies of marathon runners found that their prolonged high-intensity exercise was associated with an increased risk of infection occurrence.<ref name="Gleeson2007" /> However, another study did not find the effect. Immune cell functions are impaired following acute sessions of prolonged, high-intensity exercise, and some studies have found that athletes are at a higher risk for infections. Studies have shown that strenuous stress for long durations, such as training for a marathon, can suppress the immune system by decreasing the concentration of lymphocytes.<ref>{{cite journal | vauthors = Brolinson PG, Elliott D | title = Exercise and the immune system | journal = Clinics in Sports Medicine | volume = 26 | issue = 3 | pages = 311–319 | date = July 2007 | pmid = 17826186 | doi = 10.1097/01893697-200220010-00013 | s2cid = 91074779 | doi-access = free }}</ref> The immune systems of athletes and nonathletes are generally similar. Athletes may have a slightly elevated [[natural killer cell]] count and cytolytic action, but these are unlikely to be clinically significant.<ref name=Gleeson2007>{{cite journal | vauthors = Gleeson M | title = Immune function in sport and exercise | journal = Journal of Applied Physiology | volume = 103 | issue = 2 | pages = 693–699 | date = August 2007 | pmid = 17303714 | doi = 10.1152/japplphysiol.00008.2007 | s2cid = 18112931 }}</ref>

Vitamin C supplementation has been associated with a lower incidence of [[upper respiratory tract infection]]s in marathon runners.<ref name=Gleeson2007 />

[[Biomarker]]s of [[inflammation]] such as [[C-reactive protein]], which are associated with chronic diseases, are reduced in active individuals relative to sedentary individuals, and the positive effects of exercise may be due to its anti-inflammatory effects. In individuals with heart disease, exercise interventions lower blood levels of fibrinogen and C-reactive protein, an important cardiovascular risk marker.<ref name=Swardfager2012>{{cite journal | vauthors = Swardfager W, Herrmann N, Cornish S, Mazereeuw G, Marzolini S, Sham L, Lanctôt KL | title = Exercise intervention and inflammatory markers in coronary artery disease: a meta-analysis | journal = American Heart Journal | volume = 163 | issue = 4 | pages = 666–676 | date = April 2012 | pmid = 22520533 | doi = 10.1016/j.ahj.2011.12.017 }}</ref> The depression in the immune system following acute bouts of exercise may be one of the mechanisms for this anti-inflammatory effect.<ref name=Gleeson2007 />