A Review on the Effect of Exercise on Obesity by Modulating the Immune System and Toll-Like Receptors

Obesity is considered a global epidemic, which, according to current evidence, is rising in military populations. It is now well established that the immune system and inflammation play a central role in the development of numerous chronic metabolic diseases including insulin resistance, type 2 diabetes, and obesity. Given the close relationships between adipose tissue and the inflammatory/immune system, obesity is considered an inflammatory chronic disease. Toll-like receptors (TLRs) are immune receptors that facilitate inflammation and induce insulin resistance, and are considered as a pivotal factor in the pathogenesis of obesity, possibly triggered by the activation of TLR4. There is evidence that TLRs may be involved in the link between a sedentary lifestyle, inflammation, and disease. Physical exercise elicits potent anti-inflammatory effects that are likely to account for many of the beneficial effects of regular exercise on chronic metabolic diseases. Recent studies have shown that exercise resulted in decreased monocyte cell-surface expression of TLRs. The precise physiological stimulus mediating an exercise-induced decrease in cell-surface TLR expression is not known; however, a number of possible signals have been implicated including anti-inflammatory cytokines, pituitary-hypothalamic-adrenal axis hormones, heat shock and acute-phase proteins and myokines secreted from active skeletal muscle (such as irisin). In this review, current knowledge about the TLRs, their signaling pathways, and their critical role in the innate immune system was summarized. Future human studies based on the close relationship between TLRs and types of sports activities may provide therapeutic methods to prevent the progression of obesity to metabolic syndrome and its complications.