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HALSON, S. L., G. I. LANCASTER, A. E. JEUKENDRUP, and M. GLEESON. Immunological Responses to Overreaching in Cyclists. Med. Sci. Sports Exerc., Vol. 35, No. 5, pp. 854–861, 2003.Acute bouts of prolonged strenuous exercise are often associated with immune suppression and an increased risk of infection. However, few studies have examined immunological responses to intensified training that results in overreaching or overtraining. We investigated the effects of intensified training on plasma cytokines, glutamine, glutamate, and other related immunological variables in endurance-trained cyclists.Eight male subjects (age 27.0 ± 3.0 yr, [latin capital V with dot above]O2max 58.0 ± 1.7 mL·kg-1·min-1, mass 73.7 ± 2.1 kg) completed 6 wk of training: 2 wk each of normal training (N, 7 ± 2 h·wk-1), intensified training (ITP, 14 ± 5 h·wk-1) and recovery training (R, 3.5 ± 2.5 h·wk-1). During the study period, subjects completed six graded cycle ergometer tests to exhaustion (MT), six simulated time trial tests (TT), and eight 2 × 10-min maximal effort bouts (IT). Subjects also completed questionnaires to assess mood state. Plasma concentrations of tumor necrosis factor-[alpha] (TNF-[alpha]) and interleukin-6 (IL-6), salivary IgA, plasma glutamine, glutamate, ammonia, urea, creatine kinase activity, and routine hematological measures were determined once per week.ITP resulted in overreaching in all subjects identified by a significant decline in performance and disturbances of mood state. Significant increases during the ITP were observed in creatine kinase activity and glutamate, whereas the glutamine/glutamate ratio (Gln/Glu ratio), red blood cell numbers (RBC), hemoglobin concentration (Hb), and packed cell volume (PCV) declined after ITP. No significant changes were observed in TNF-[alpha], IL-6, salivary IgA, glutamine, ammonia, urea and various routine hematological measures.Alterations in plasma cytokines do not appear to be related to the decline in performance and increased mood state characteristic of overreaching; however, the Gln/Glu ratio may be of use as a marker of overreaching and/or overtraining.