Effects of Barley and Kefir on Gut Microbiota in Physically Active Adults

The purpose of this randomized controlled clinical trial is to investigate whether a four-week dietary intervention consisting of a daily meal with barley and kefir can influence gut microbiota diversity and composition in physically active adults participating in endurance sports. The study will further examine whether diet-induced changes in the gut microbiota are associated with changes in relevant circulating and urinary metabolites. Growing evidence indicates that the gut microbiota plays an important role in metabolism, immune function, and overall health. Dietary factors are among the most important modulators of gut microbial composition. Barley contains dietary fibre including beta-glucans and other prebiotic components that may promote the growth of beneficial bacteria, while kefir contains live probiotic lactic acid bacteria that may influence both microbial composition and function. However, there is limited knowledge regarding how practical and realistic dietary interventions affect gut microbiota and host biological responses in physically active adults. The primary objectives of the study are to determine whether daily consumption of a standardized meal consisting of barley porridge (70 g) and kefir (200 mL) for four weeks alters gut microbiota alpha diversity and gut microbiota composition compared with maintaining a habitual diet. Secondary objectives are to investigate whether changes in gut microbiota are associated with changes in circulating and urinary metabolite profiles related to gut microbiota activity and energy metabolism. Physically active participants aged 18 to 55 will be recruited and randomized to either an intervention group or a control group. Participants assigned to the intervention group will consume a daily breakfast consisting of barley porridge and kefir for four weeks. Participants assigned to the control group will maintain their habitual diet without receiving the dietary intervention. Both groups will be asked to maintain their usual dietary practices and training routines throughout the study period. Data collection will be performed at baseline and after four weeks and will include stool samples, blood samples, urine samples, dietary interviews, and recording of training volume and training load. Stool samples will be analyzed using Precision Microbiome Profiling (PMP) to characterize gut microbiota diversity and composition. Blood and urine samples will be analyzed using metabolomics approaches to assess circulating and urinary metabolites. The study aims to provide a comprehensive understanding of the relationships among diet, gut microbiota, metabolite profiles, and cellular responses in physically active adults, and to generate new knowledge regarding biological mechanisms that may be relevant for health, recovery, and physical performance.