This study will investigate the effect of postbiotics supplementation on gut microbiota (GM) composition, exercise-induced oxidative stress and performance following eccentrically biased exercise. The study will be a cross-over, randomized, double-blind, controlled study that will be conducted in two cycles. Participants will be randomly assigned into one of the two trials: i) Postbiotics supplementation for 4 weeks, ii) Placebo supplementation for 4 weeks. Participants will then perform a 45-min treadmill running at (-15% slope, \ 70% VO2max) followed by a time-trial (0% slope, \ 95% VO2max) until exhaustion. Before, as well as 24 h, 48 h and 72 h following exercise, assessment of GM composition and metabolites (short-chain fatty acids), exercise induced muscle damage (EIMD) (delayed onset of muscle soreness, creatine kinase), complete blood count (CBC), blood inflammatory status (tumor necrosis factor alpha, interleukin 6, C-reactive protein), gut inflammatory status (lipopolysacharides-binding protein, zonulin), blood redox status (total antioxidant capacity, catalase, protein carbonyls, reduced glutathione, oxidized glutathione, and performance (knee-extensors' and knee-flexors' isometric, concentric, eccentric torque, countermovement jump) will be performed. In addition, metabolism (lactic acid) will be assessed before and 4 min following exercise. Afterwards, the participants will receive the postbiotics supplement or the placebo for 4 weeks, and will repeat the exercise protocol and measurements of EIMD, CBC, blood inflammatory status, blood redox status and performance at the same time-points. At the second cycle, the participants will repeat the above procedures under the remaining condition. Between conditions, there will be a 14-day washout period. The results of the research will provide important information for coaches and physically active individuals, regarding the effectiveness of postbiotics to favorably change the gut microbiota composition and alleviate gut inflammation, exercise-induced inflammation and oxidative stress, and improve performance after intense exercise.