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

Abstract

Background: Prolonged intense exercise has been associated with transient suppression of immune function and an increased risk of infections. In this context, the catabolism of amino acid tryptophan via kynurenine may play an important role. The present study examined the effect of a probiotic supplement 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.

Methods: Thirty-three highly trained individuals were randomly assigned to probiotic (PRO, n = 17) or placebo (PLA, n = 16) groups using double blind procedures, receiving either 1 × 10¹⁰ colony forming units (CFU) of a multi-species probiotic (Bifidobacterium bifidum W23, Bifidobacterium lactis W51, Enterococcus faecium W54, Lactobacillus acidophilus W22, Lactobacillus brevis W63, and Lactococcus lactis W58) or placebo once per day for 12 weeks. The serum concentrations of tryptophan, phenylalanine and their primary catabolites kynurenine and tyrosine, as well as the concentration of the immune activation marker neopterin were determined at baseline and after 12 weeks, both at rest and immediately after exercise. Participants completed a daily diary to identify any infectious symptoms.

Results: After 12 weeks of treatment, post-exercise tryptophan levels were lowered by 11% (a significant change) in the PLA group compared to the concentrations measured before the intervention (p = 0.02), but remained unchanged in the PRO 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 (PLA 0.79, PRO 0.35; p = 0.02).

Conclusion: Data indicate reduced exercise-induced tryptophan degradation rates in the PRO group. Daily supplementation with probiotics limited exercise-induced drops in tryptophan levels and reduced the incidence of URTI, however, did not benefit athletic performance.

Keywords: intense exercise; kynurenine; probiotics; tryptophan; upper respiratory tract infections.

Figure 1

Flow of participations through each stage of the trial.

Figure 2

Training loads for endurance training (h/week) over the study period for the participants who completed the study. Graph shows mean ± standard error of the mean (SEM); * p < 0.05 (Mann-Whitney U test). Asterisks depict weeks with significant differences between PRO (—) and PLA (···) groups. PRO: probiotics-supplemented group; PLA: placebo group.

Figure 3

Resting energy expenditure (REE; (kcal/day)) in trained athletes before and after 12 weeks of treatment. PRO: probiotics-supplemented group (n = 14); PLA: placebo group (n = 15). Graph shows mean + SEM; * p < 0.05 (ANOVA).

Figure 4

Tryptophan concentrations before and after exhaustive exercise in the probiotic (n = 14) and placebo (n = 15) group of trained athletes before and after 12 weeks of treatment (four blood draws per athlete). Graph shows mean ± SEM; * p < 0.05: Wilcoxon, # p < 0.05: week 0, before exercise placebo vs. probiotics: Mann-Whitney-U, n.s. = not statistically significant.

Figure 5

Incidence of upper respiratory tract infections (URTIs) in trained athletes before and after 12 weeks of treatment. The share of subjects on placebo (gray columns, 0.79) who experienced 1 or more URTI symptoms was 2.2-fold greater than those on probiotics (black columns, 0.35; * p = 0.016).