Kefir, Gut Microbiota, and Athletic Performance in Soccer Players

This randomized controlled trial aims at evaluating the impact of kefir on the participants' gut microbiota and athletic performance among professional female soccer players of 18-29 years. The present study is intended to reveal the potential of kefir, a fermented milk product that is packed with probiotics , in altering the diversity and profile of gut microbiota, physical performance, and parameters of body composition.

This trial was carried out for a period of four weeks and the subjects were divided into two parallel groups; the kefir group in which the participants consume 200ml of kefir per day and the control group in which the participants continued with their normal diet without kefir supplementation. The subjects were assigned to either of the two groups at the beginning of the study. Since the study involved a dietary intervention, the trial was conducted without blinding the subject and the researcher. Measurements were made at the beginning and at the end of the intervention in order to evaluate the outcomes.

The main goals of the study were to observe the changes in the diversity and the composition of the gut microbiome with the help of 16S rRNA sequencing and to determine the changes in the athletic performance indices including VO2max and finishing speed which was measured by 30-15 Intermittent Fitness Test. Other objectives were to establish the effects of kefir on other body composition measures such as body fat percentage, fat free mass and total body weight , to examine the patterns of dietary intakes and their relationships with the microbiota and performance measures and to examine the changes in certain gut microorganisms including Akkermansia muciniphila and Faecalibacterium prausnitzii which are known to have positive effects on human health and performance.

The alpha diversity of the gut microbiome was measured with the Shannon and Chao1 indices and the beta diversity was calculated with the Bray-Curtis dissimilarity. The taxonomic analysis was done at the phylum, genus and species level in order to identify the changes in the microbial community due to consumption of kefir. The participants' athletic performance was evaluated with IFT Test to explore the changes in VO2max and finishing speed. Changes in the body composition were assessed by Bioelectrical Impedance Analysis (BIA) and skinfold callipers and the dietary intake data were collected through three-day food records, one off day and two training days to examine the macronutrients and micronutrients intake and their relationship with the microbiota and performance.

This trial seeks to fill an important research gap in the current literature on functional foods in sports nutrition with focus on professional female athletes. In this way, the current study seeks to link dietary interventions with the gut microbiota as well as performance parameters in order to provide empirical support for how kefir can improve the gut health and performance of athletes and to facilitate the development of personalised nutrition for athletes.