Overtraining effects on immunity and performance in athletes

Overtraining is a process of excessive exercise training in high-performanc e athletes that may lead to overtraining syndrome. Overtraining syndrome is a neuroendocrine disorder characterized by poor performance in competition , inability to maintain training loads, persistent fatigue, reduced catecho lamine excretion, frequent illness, disturbed sleep and alterations in mood state. Although high-performance athletes are generally not clinically imm une deficient, there is evidence that several immune parameters are suppres sed during prolonged periods of intense exercise training. These include de creases in neutrophil function, serum and salivary immunoglobulin concentra tions and natural killer cell number and possibly cytotoxic activity in per ipheral blood. Moreover, the incidence of symptoms of upper respiratory tra ct infection increases during periods of endurance training. However, all o f these changes appear to result from prolonged periods of intense exercise training, rather than from the effects of overtraining syndrome itself. At present, there is no single objective marker to identify overtraining synd rome. It is best identified by a combination of markers, such as decreases in urinary norepinephrine output, maximal heart rate and blood lactate leve ls, impaired sport performance and work output at 110% of individual anaero bic threshold, and daily self-analysis by the athlete (e.g. high fatigue an d stress ratings). The mechanisms underlying overtraining syndrome have not been clearly identified, but are likely to involve autonomic dysfunction a nd possibly increased cytokine production resulting from the physical stres s of intense daily training with inadequate recovery.