Ultra-endurance athletes have been commonly endured extreme conditions during races and training sessions, resulting in exercise-associated clinical symptoms, including gastrointestinal symptoms, dehydration, and elevated oxidative stress. Although these alterations adversely affect sports performance and well-being, there is no certain treatment or consensus on alleviating the exercise-associated clinical symptoms in ultra-endurance athletes. The objective of this study is to determine the effects of oral rehydration salts supplemented with Lactobacillus Rhamnosus GG on exercise-induced gastrointestinal problems, dehydration and oxidative stress in ultra-endurance athletes. Additionally, we aimed to assess the exercise-induced alterations in oxidative stress determined by applying an acute strenuous exercise protocol ((a cycle ergometer (45 min at 65%VO2max) immediately followed by a treadmill test (75% VO2max to exhaustion)) before and after the supplementation period.

Inclusion Criteria:

• Participants who train at least 15 h per week,
• Participants who do not have any metabolic disease,
• Have had at least one gastrointestinal symptom determined by using a Gastrointestinal Symptoms Rating Scale and Bristol Stool Chart Form Scale,
• Being a non-smoker,
• Have participated in at least one ultra-endurance race/event (lasts > 4 hr),
• Taking no vitamins, minerals, dietary supplements, antibiotics, and any medication at least during the three months before the study,
• (For women) participants with a regular menstrual cycle of physiological length (24-35 days)

Exclusion Criteria:

• Regular tobacco use
• Inability to adhere to any of the study protocol requirements (i.e. alcohol, caffeine consumption, diet control)
• Having consumed any supplements and/or medicines for the preceding 3 months period.
• (For women) participants in menopause or using oral contraceptives