Gastrointestinal Function, Damage and Symptoms During Exercise and the Potential Therapeutic Role of Probiotic Supplementation

Gastrointestinal (GI) symptoms are widely reported in athletes participating in prolonged endurance events including; cyclists, triathletes and marathon runners, although there is a large estimated range of between 4–96% of participants affected (Costa et al., 2017a). Two potential causes of exercise-associated GI symptoms are exercise-induced increases in GI permeability and damage, and the consumption of carbohydrate (CHO) during exercise (Costa et al., 2017b, de Oliveira, 2017, de Oliveira and Burini, 2014). To date, the association between markers of GI damage or barrier disruption and GI symptoms during endurance exercise has been equivocal. Additionally, while CHO consumption enhances exercise capacity and performance (Stellingwerff and Cox, 2014), ingestion also appears to increase the risk of GI symptoms, although data is again equivocal (Costa et al., 2017a). Given this prevalence of GI symptoms, probiotics may offer a convenient and practical strategy to reduce exercise-associated GI symptoms, with evidence showing their capability to alter CHO absorption and metabolism along with attenuating exercise-induced GI permeability and damage. The aim of this thesis is to investigate the relationship between markers of GI permeability and damage, measured in the blood, and GI symptoms during endurance exercise, up to marathon distance running. The role of CHO intake in GI symptoms will be examined, as well as the use of probiotics to reduce such symptoms. To realise these aims, the following objectives will be addressed: 1) Investigate the prevalence and severity of exercise-associated GI symptoms (Chapters 4 and 7) 2) Explore the role of different circulatory markers to measure GI permeability and damage, and their relation to GI symptoms (Chapters 5, 6, and 7 ) 3) Investigate the use of probiotics in reducing GI symptoms exacerbated by CHO during prolonged exercise (Chapter 6 and 7) 4) Investigate the efficacy of probiotics to reduce GI permeability and damage during exercise (Chapter 6 and 7) The main findings from this thesis imply that a moderate prevalence of GI symptoms exists in recreational runners, with 27-58% of marathon participants having reported mild to severe GI symptoms during the race. Laboratory-based exercise protocols exhibited much lower symptom prevalence compared with marathon races. When markers of GI permeability and damage were assessed, there appeared to be no association between these and GI symptoms. One systemic measure that correlated to a number of GI symptoms during a marathon was CD14, a non-specific marker of monocyte activation. Probiotics had no measurable effect on markers of GI permeability and damage during a marathon race, however, probiotic supplementation had a small but significant effect on exercise metabolism during steady state cycling; with an increase in CHO oxidation and subsequent reduction in lipid oxidation. Probiotic supplementation was also associated with less severe GI symptoms during both a marathon race, and in the four weeks leading to it. Furthermore, beneficial effects of probiotics on GI symptoms during the marathon were associated with better maintenance of running speed during the final third of the race. Taken together, GI symptoms are prevalent in endurance athletes. Symptom aetiology can vary but probiotics appear to be a promising strategy that can be used to not only reduce GI symptoms, but may even indirectly improve exercise performance.