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

The human gut microbiota is increasingly recognized as an important contributor to metabolic regulation, immune function, and overall health. A diverse and functionally balanced gut microbiota has been associated with favourable metabolic and health outcomes, while alterations in microbial composition have been linked to a range of physiological and pathological conditions. Emerging evidence suggests that the gut microbiota may also play a role in biological processes relevant to exercise adaptation, recovery, and physical performance.

Physically active individuals participating in endurance sports may represent a particularly interesting population for microbiome research. Previous studies suggest that highly trained individuals can exhibit distinct gut microbiota profiles compared with less active populations. At the same time, considerable inter-individual variation exists, and further research is needed to better understand how dietary factors may influence gut microbiota composition and function in this population.

Diet is one of the most important determinants of gut microbiota composition. The present study focuses on a practical dietary strategy based on two commonly available food products with potential microbiota-modulating properties. Barley is rich in dietary fibre, including beta-glucans and other prebiotic components that may stimulate the growth and metabolic activity of beneficial gut microbiota. Kefir is a fermented dairy product containing probiotic lactic acid bacteria and other microorganisms that may influence microbial composition and function. Both foods have demonstrated biological effects in previous studies and represent a realistic dietary intervention with potential applicability beyond a research setting.

The study is designed as a randomized controlled parallel-group intervention trial. Participants will be randomly allocated to either an intervention group or a control group using sex-stratified randomization to ensure balanced representation of men and women across study groups. The intervention group will consume a standardized breakfast consisting of barley porridge (70 g dry weight) and kefir (200 mL) daily for four weeks. Participants assigned to the control group will continue their habitual diet without receiving the dietary intervention. All participants will be asked to maintain their usual dietary habits and training routines throughout the study period.

Gut microbiota will be characterized using Precision Microbiome Profiling (PMP), a quantitative PCR-based analytical platform capable of quantifying approximately 100 microbial taxa representative of the human gut microbiome. Analyses will focus on changes in alpha diversity, assessed by the Shannon Diversity Index, as well as changes in the relative abundance of selected bacterial taxa.

Blood and urine samples will be analysed using metabolomics approaches to characterize metabolites associated with gut microbial activity and energy metabolism. Particular attention will be given to metabolites that may reflect interactions between the gut microbiota and host physiology. As gut microbiota-derived metabolites can enter the systemic circulation and act as signalling molecules between microbial communities and host tissues, metabolite profiling may contribute to a better understanding of potential biological mechanisms linking dietary intake, microbial activity, and systemic metabolism.

Data collection will be performed at baseline and after four weeks. Biological sampling will include stool samples for microbiota analyses and blood and urine samples for metabolomics analyses. Additional data collection will include dietary interviews, body weight measurements, and registration of training volume and training load. These measures will provide contextual information for the interpretation of microbiota and metabolomics data and help account for potential confounding factors.

Up to 80 participants will be included in the study. The planned sample size is based on expected changes in gut microbiota diversity reported in previous dietary intervention studies and takes anticipated participant attrition into account. The study is designed to evaluate the effects of a feasible and low-risk dietary intervention on gut microbiota diversity, microbial composition, and metabolite profiles in physically active adults.

Changes from baseline to week 4 will be compared between the intervention and control groups. Analyses will focus on measures of gut microbiota alpha diversity, microbial composition, and circulating and urinary metabolites. Repeated-measures statistical approaches will be used to account for within-participant changes over time and to evaluate differences between study groups.

The study seeks to improve understanding of the relationships among diet, gut microbiota, and systemic metabolism in physically active adults. The findings may contribute to the development of evidence-based dietary strategies aimed at supporting health, recovery, and physiological adaptation through modulation of the gut microbiota.