Prolonged intense exercise has been associated with transient suppression of immune function and an increased risk of infections. The catabolism of tryptophan via kynurenine may play an important role. Researchers examined the effect of probiotic supplementation on the incidence of upper respiratory tract infections (URTI) and the metabolism of aromatic amino acids after exhaustive aerobic exercise in trained athletes during three months of winter training. Thirty-three highly trained participants were randomly assigned to either the probiotic group, receiving 1 X 1010 colony forming units (CFU) of a multi-species probiotic or the placebo group using double blind procedures, once daily for 12 weeks. Serum concentrations of tryptophan, phenylalanine and their primary catabolites kynurenine and tyrosine, as well as the concentration of the immune activation marker neopterin were determine at baseline and after 12 weeks, both at rest and immediately after exercise. After 12 weeks, post-exercise tryptophan levels were lowered by a significant 11% in the placebo group compared to baseline, but remained unchanged in the probiotic group. The ratio of subjects taking the placebo who experienced one or more URTI symptoms was increased 2.2- fold compared to those on probiotics. The data indicates reduced exercise-induced tryptophan degradation rates in the probiotic group, and that daily supplementation with probiotics limited exercise-induced drops in tryptophan levels and reduced the incidence of URTI.\r\n\r\nStrasser B., et al. Probiotic Supplements Beneficially Affect Tryptophan-Kynurenine Metabolism and Reduce the Incidence of Upper Respiratory Tract Infections in Trained Athletes: A Randomized, Double-Blinded, Placebo-Controlled Trial. Nutrients 2016, 8, 752; doi:10.3390/nu8110752