352 Kekkonen and Korpela are with the Institute of Biomedicine, Pharmacology, University of Helsinki, Helsinki, Finland 00370. Vasankari is with the Dept. of Health and Exercise, University of Turku, Turku, Finland. Vuorimaa is with the Sports Institute of Finland, Vierumäki, Finland. Haahtela is with Helsinki University Hospital, Skin and Allergy Hospital, Helsinki, Finland. Julkunen is with the Dept. of Viral Diseases and Immunology, National Public Health Institute, Helsinki, Finland. International Journal of Sport Nutrition and Exercise Metabolism, 2007, 17, 352-363 © 2007 Human Kinetics, Inc. The Effect of Probiotics on Respiratory Infections and Gastrointestinal Symptoms During Training in Marathon Runners Riina A. Kekkonen, Tommi J. Vasankari, Timo Vuorimaa, Tari Haahtela, Ilkka Julkunen, and Riitta Korpela Heavy exercise is associated with an increased risk of upper respiratory tract infec- tions. Strenuous exercise also causes gastrointestinal (GI) symptoms. In previous studies probiotics have reduced respiratory tract infections and GI symptoms in general populations including children, adults, and the elderly. These questions have not been studied in athletes before. The purpose of this study was to investigate the effect of probiotics on the number of healthy days, respiratory infections, and GI-symptom episodes in marathon runners in the summer. Marathon runners (N = 141) were recruited for a randomized, double-blind intervention study during which they received Lactobacillus rhamnosus GG (LGG) or placebo for a 3- mo training period. At the end of the training period the subjects took part in a marathon race, after which they were followed up for 2 wk. The mean number of healthy days was 79.0 in the LGG group and 73.4 in the placebo group (P = 0.82). There were no differences in the number of respiratory infections or GI-symptom episodes. The duration of GI-symptom episodes in the LGG group was 2.9 vs. 4.3 d in the placebo group during the training period (P = 0.35) and 1.0 vs. 2.3 d, respectively, during the 2 wk after the marathon (P = 0.046). LGG had no effect on the incidence of respiratory infections or GI-symptom episodes in marathon runners, but it seemed to shorten the duration of GI-symptom episodes. Key Words: Lactobacillus rhamnosus GG, athletes, exercise Epidemiological studies suggest that strenuous acute or chronic exercise is associated with an increased risk of upper respiratory tract infections (15, 17, 20, 23, 26, 27). The risk appears to be especially high during the 2-wk period after a marathon-type race event (19). It is also known that during training periods with heavy exercise the risk of respiratory tract infection is increased (4, 6, 36), but mod- erate training seems to protect from respiratory tract infections (13, 16). Continuous heavy exercise suppresses certain immunological parameters such as neutrophil