The gut microbiota plays a crucial role in athletic performance and the health of athletes. Fermented foods have proven to be effective in personalizing the gut microbiota. This dissertation aimed to investigate the effect of sauerkraut intake on the composition and functionality of the gut microbiota in active athletes to assess its potential application in personalizing their gut microbiota. Additionally, the microbial community of sauerkraut was examined, and laboratory parameters and digestive function were monitored during the intervention. Three studies were conducted on active athletes: a proof-of-concept study on professional athletes, a follow-up study on recreational athletes, and a longitudinal study on a single participant. We demonstrated that a 10-day intake of 250g of sauerkraut can lead to significant changes in the composition and functionality of the gut microbiota in professional athletes, though not in terms of α-diversity. In recreational athletes the same short-term effects were not not observed. We found that the changes induced by the intervention depend on the initial composition of the gut microbiota. We have found that a 10-day intake of 250g of sauerkraut can induce significant long-term changes in the composition and functionality of the gut microbiota Through the longitudinal study and monitoring of digestion and side effects, we determined that a minimum of one week of 250g daily sauerkraut intake is necessary. It was established that a 10-day intake of 250g of sauerkraut results in significant changes in differential blood counts and a decrease in vitamin B12 concentration. In conclusion, sauerkraut has the potential to personalize the gut microbiota of athletes.