**Background:** Sport disciplines with different metabolic characteristics (anaerobic vs. aerobic) require distinct dietary approaches. Anaerobic athletes (bodybuilders, sprinters) typically consume high-protein diets (HPD) and nitric oxide enhancers (citrulline, nitrates), while aerobic athletes (runners, cyclists) prefer high-carbohydrate diets (HCHD) and buffering agents (sodium bicarbonate, beta-alanine). Gut bacteria and their metabolites influence nutrient absorption, neurotransmitter production, immune function, and muscle recovery. However, the impact of these specialized diets and supplements on gut microbiota composition remains poorly understood. This mini-review summarizes human and animal studies on popular supplements and their effects on gut homeostasis and sport performance, drawing on the authors' previous research in amateur bodybuilders and cyclists.
**Methods:** The authors conducted a narrative review of human and animal studies, supplemented by results from their own previous research. They examined two groups: amateur bodybuilders on HPD (mean calories from protein: 33.6% vs. 22% in controls, p < 0.05; mean calories from fat: 27.6% vs. 36.4%, p < 0.05) and amateur cyclists on HCHD (mean carbohydrate consumption 4.48 g/kg b.w. in precompetition season vs. 5.18 g/kg b.w. in competition season, p < 0.05). The review covers supplements categorized by exercise type: for aerobic disciplines—carbohydrates (glucose, maltodextrin), sodium bicarbonate (NaHCO₃), and beta-alanine; for anaerobic disciplines—protein (especially whey), citrulline, arginine, and dietary nitrates. Prebiotics (pectins, inulin, fructo-oligosaccharides, beta-glucans) and probiotics are also reviewed.
**Key Results:** In the authors' study of amateur cyclists on HCHD, no significant differences were observed in Bacteroides spp., Bifidobacterium spp., Akkermansia muciniphila, or Faecalibacterium prausnitzii abundance compared to sedentary controls. Similarly, in amateur bodybuilders on HPD, no significant differences were found in colony-forming unit counts of these same bacteria. Other studies cited show: marathon runners and competitive cyclists had higher abundances of Bacteroides and Blautia than controls; higher Veillonella in runners; and higher Prevotella in cyclists with high training load. High sugar consumption (including maltodextrin) is noted to decrease Bacteroidetes and increase Proteobacteria. Sodium bicarbonate at doses over 0.2 g/kg b.w. can trigger adverse gastrointestinal symptoms. A systematic review of eight RCTs found whey and casein isolates do not significantly affect gut microbiota in healthy people. Citrulline supplementation prior to exercise attenuated splanchnic hypoperfusion in a double-blind crossover study of 10 healthy men. A sport-specific systematic review and meta-analysis found that probiotics increased time to exhaustion, specifically when single-strain probiotics (e.g., L. plantarum, L. casei, B. longum) were administered at doses over 3 × 10⁹ CFU for less than 4 weeks. Probiotic supplementation also resulted in a decrease in the risk of developing infections and symptom severity. Beta-glucan supplementation at 2 g/day for 4 weeks increased athletes' grip strength, VO₂max, and 1-minute double rocking jumps.
**Clinical Implications:** The review underscores that current evidence is insufficient to recommend specific ergogenic supplements for gut microbiota modulation in athletes. For anaerobic athletes, citrulline may protect gut mucosa by attenuating splanchnic hypoperfusion, but nitrate effects on gut microbiota remain unclear. For aerobic athletes, sodium bicarbonate and beta-alanine have not been studied for gut effects. Prebiotics (inulin, FOS, beta-glucans, pectins) may protect gastrointestinal homeostasis but are not specific to exercise type. Only beta-glucans show preliminary evidence as ergogenic agents via creatine metabolism. Probiotics appear most promising for endurance athletes, with single-strain formulations improving time to exhaustion and reducing infection-related training days lost. The authors conclude that meeting dietary fiber requirements may protect against gut disturbances from extreme diets (very high-protein or very high-carbohydrate) and that more well-designed human trials are needed, particularly in resistance-trained athletes and on the gut-mediated effects of sodium bicarbonate, beta-alanine, and nitrates.