In addition to different exercise methods, a variety of nutritional strategies are used to increase physical performance, which are particularly important in endurance disciplines. Up to 70% of endurance athletes experience gastrointestinal (GI) disturbances during prolonged exercise. As a gluten-free diet (GFD) is thought to reduce GI disturbances, systemic inflammation and increase physical performance, there is an increase in the number of athletes, without a confirmed diagnosis of coeliac disease or gluten intolerance, who choose a GFD. The aim of the study was to determine whether a GFD has an impact on cyclists performance during different modes of endurance exercise. Subjects were randomly assigned to one of four groups; (1) GFD and low-intensity training (LIT), (2) regular diet and LIT, (3) GFD and high-intensity training (HIT), and (4) regular diet and HIT. Before the start of the intervention, cyclists completed a modified cycling test and completed a food frequency questionnaire (FFQ) and a GI symptom assessment questionnaire. A 30-day intervention followed. Based on previous studies, which are still lacking in this area, we hypothesized that a GFD would have no effect on the cyclists' physical performance. However, we expected effects of HIT on VO2max and relative maximal power (RPP). At the end of the intervention, it was shown that a GFD had no effect on the improvement in physical performance. A statistically significant improvement in RPP occurred with HIT and conventional diet (p = 0.004). No statistically significant changes were found in VO2max. LIT and normal diet improved GI symptoms during activity after the intervention (p = 0.009). After activity, the improvement in GI symptoms was influenced by the LIT (p = 0.039). Due to the small number of subjects and the design of the study, we would suggest in the future to intervene on a larger number of cyclists and to perform the training in a more controlled environment.
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