Of seven different bacterial phyla, two seem to dominate the colon, namely Firmicutes and Bacteroidetes (together, constituting approximately 90%) [1]. Gut microbiota modifications occur mainly through diet, physical activity and drug intake changes, whereas only 8.8% of the diversity and abundance of bacteria are shaped by genes [2]. Some dietary choices, such as low-fiber and high-fat diets, may alter the gut microbiota within 24 h [3]. The disturbed ratio of Bacteroides to Firmicutes (B:F ratio) is an example of a microbial shift in obesity.

Exercise affects the gut microbiota and the intestinal environment. In general, a high level of physical activity is accompanied by an increase in gut microbiota diversity and health-promoting bacterial abundance (e.g., Akkermansia muciniphila and Feacalibacterium prausnitzii) [4]. However, there is high heterogeneity in the responsiveness of the human gut microbiota to the current lifestyle. It has been proposed that some people may be identified as good responders, while others are called non-responders because of a lack of microbial adaptations to lifestyle changes known to promote a healthy gut [5]. There are also indications that the microbiota accounts for ca. 58% of the variation in circulating metabolite levels in humans [6], which suggests that bacteria may be involved in nutrient and bioactive compound metabolism and absorption from not only food but also dietary supplements.

There is a growing body of knowledge on the impact of different diets (HPD, high-protein diet; HCHD, high-carbohydrate diet) and pre- and probiotics on athlete gut microbiota; however, nutrient intake level is not the only driver of the differences between the sport classification groups [7]. Little is known about the potential effects of specific supplements dedicated to different sport disciplines (e.g., citrulline in “anaerobic” exercise or sodium bicarbonate in “aerobic” exercise) on microbiota composition. Creatine and caffeine are two of the most effective sport supplements; however, the actions of these compounds are universal for sport disciplines of both types. In this resviearchw, besides the pre- and probiotics for athletes, typical anaerobic- and aerobic-type exercise supplements will be described, namely protein, citrulline, arginine, and nitrates for anaerobic disciplines and carbohydrates, sodium bicarbonate, and beta-alanine for aerobic disciplines.

The modification of athletes’ microbiota for performance enhancement remains under investigation. Clinical strategies for gut disorders may be used to enhance recovery after exercise and during increased training load season. Intestinal bacteria are known to affect sleep, appetite, mood, pain and cognition ^{[8][9][10][11][12]}[8,9,10,11,12]. There are multiway connections between the gut microbiota and the brain, as well as peripheral organs, such as the lungs, skeletal muscles, liver and skin ^{[13][14][15][16]}[13,14,15,16]. Mechanisms underlying many regulatory functions of the microbiota are neuroimmonologically based, as gut bacteria that are known to promote health influence production and modulate the biological activities of immune cells and neurotransmitters [17].

Regular exercise modulates the gut microbiota, but it was found that endurance training affects the intestinal microbiota in a specific way [18]. Long-distance running or cycling increases abdominal organ ischemia, which may have detrimental effects on the intestinal epithelium [19]. Along with local ischemia or hydration status changes, dietary recommendations for endurance athletes are an additional potentially harmful factor that could affect the gut microbiota. It is recommended that runners or cyclists consume a very high-carbohydrate diet (>45% of calories and >6 g of carbohydrates (CHO) per kg of body weight) to maintain muscular glycogen stores and sustain energy levels during long-lasting training or competition [20]. It is also recommended for athletes in these disciplines to avoid excessive fiber consumption, as it may slow down digestion and cause gastrointestinal distress during exercise. These two recommendations make the diet for endurance athletes similar to a Western diet, which is known to promote weight gain and metabolic disorders.

However, high-carbohydrate, so-called agrarian diets promote microbial biodiversity and richness. It is hypothesized that this effect is mostly mediated by fiber and resistant starch consumption. Diets based on grains, fruits, and vegetables may decrease Bacteroides abundance, while increasing probiotic strains of Bifidobacteria [21]. While cyclists, runners, or swimmers avoid excess fiber, it could be a risk factor for a healthy gut. In theour previous study, researcherswe did not observe any significant differences in Bacteroides spp., Bifidobacterium spp., Akkermansia muciniphila, and Feacalibacterium prausnitzii abundance between amateur cyclists consuming a high-carbohydrate diet and controls with a more sedentary lifestyle consuming a diet containing more protein and fat [22]. Although the carbohydrate consumption of study participants was high (mean of 4.48 g/kg b.w. (body weight) in precompetition season vs. 5.18 g/kg b.w. in competition season; p < 0.05), it was not as high as in professional athletes (6–10 g/kg b.w.). The cyclists in the study also consumed appropriate amounts of fiber (approximately 26.8 g daily). These observations confirm the results of a study conducted by other authors who used a different method of bacterial genome sequencing and collected more samples. They found higher abundances of Bacteroides and Blautia in marathon and half-marathon runners and competitive cyclists than in controls, higher Veillonella in runners than in controls, and higher Prevotella in cyclists with a high training load than in cyclists with a low training load [23]. The same effect for Prevotella was observed in marathon runners and cross-country skiers (as compared to sedentary controls) but not for Bacteroidetes [24].

Glucose (dextrose) and maltodextrin (hydrolyzed starch) are among the most popular supplements for endurance athletes. Using CHO-based sports drinks at doses up to 90 g per training sessions (or 1.2 g/kg b.w.) enhances glycogen stores and maintains hydration and speeds up the recovery process [25]. However, high sugar consumption is known to exert negative effects on the gut mucosa and epithelium and, therefore, on the microbiota. This, in turn, may promote low-grade inflammation related to cardiovascular disease and other disorders [26]. The authors of these findings called maltodextrin “the modern stressor of the intestinal environment”. Gut dysbiosis a type 2 diabetes mediator, and high sugar intake was observed to decrease Bacteroidetes and increase Proteobacteria [27]. Drinks with glucose and/or maltodextrin, which contain 10–15% of daily carbohydrates, may significantly increase total carbohydrate intake and potentially interfere with both the gut protective barrier and the microbiota. Carbohydrate mouth rinse could be considered to maintain hydration during exercise, but it does not allow for larger doses of carbohydrates [28].

Athletes use sodium bicarbonate (NaHCO₃) to buffer excessive hydrogen ion accumulation in muscles during exercise. Some data suggest that it enhances endurance performance, but the outcomes of meta-analyses have yielded conflicting results [29]. To date, there have been no studies on the potential impact of continuous sodium bicarbonate intake on the gut microbiota composition and metabolome of athletes. However, it was found that NaHCO₃ at doses over 0.2 g/kg b.w. can acutely trigger adverse gastrointestinal symptoms [30]. These symptoms include diarrhea, which not only leads to dehydration but also microbiota disturbances. In one study, researchers assessed gut microbiota changes in patients with liver steatosis using water with standardized sodium bicarbonate, calcium, magnesium, and sulfate contents. They found a decrease in the abundance of Blautia strains and an increase in Subdoligranulum, both of which have potential probiotic properties [31]. In patients with type 2 diabetes, it was found that drinking bicarbonate-enriched water is associated with changes in metabolites related to carbohydrate breakdown and an increase in Dehalobacteriaceae bacteria strains [32]. It is unknown whether NaHCO₃ has any effect on the gut microbiota of athletes.

Beta-alanine is one of the compounds that make up carnosine in the body and effectively increases its level when consumed with diet [33]. Carnosine is a peptide that is mostly stored in muscle tissues. Among its most important functions are to neutralize reactive oxygen species, reduce glycation and chelate metal ions. It also blocks the accumulation of hydrogen ions in skeletal muscles during high-intensity physical activity, which is why athletes often use it [34]. In ergogenic doses, that is, 4–6 g daily divided into 4–5 portions, beta-alanine is considered well-tolerated [35]. No human studies have considered the potential effects of beta-alanine on the gut microbiota.

Resistance exercise (especially in the eccentric phase of a given exercise) and sprints (especially during downhill running) are known to cause muscle damage. To recover from the most intense strength and speed training, athletes consume a high-protein diet with special focus on branched-chain amino acids (BCAAs; mainly leucine, isoleucine, and valine) from sources such as whey protein, eggs and meat, which stimulates muscle protein synthesis through the mTOR pathway.

Different sources of dietary protein (animals, plants, mushrooms and yeasts) may have different impacts on the gut microbiota due to different fiber and antioxidant contents. However, protein supplements such as concentrates and isolates have most of the fat, carbohydrates, and fiber removed and are therefore easily digestible. Among athletes, whey products seem to be the most popular [40]. A systematic review of eight randomized controlled trials showed that, contrary to yogurt or kefir, whey and casein isolates (from milk) do not significantly affect the gut microbiota composition in healthy people [41]. In a randomized clinical trial, it was found that protein supplementation during caloric restriction leads to greater visceral fat mass reduction and increased microbial diversity, especially in participants with low baseline diversity, as compared to a diet without additional protein [42]. In infants (1–3 years old), whey protein hydrolysate induced an increase in the production of probiotic bacteria counts and metabolites (short-chain fatty acids; SCFAs), which suggests prebiotic functions of hydrolyzed protein [43]. However, in a pilot trial of the impact of protein supplements on athlete gut microbiota, a team of researchers found a decrease in probiotic strains of Blautia and Bifidobacterium (Bifidobacterium longum) and an increase in Bacteroidetes [44]. The authors concluded that long-term protein supplementation may have detrimental effects on the gut microbiota; however, this study was conducted on a small group (protein supplementation, n = 12; control, n = 12) of endurance rather than resistance athletes. In another randomized, double-blind trial examining the effects of multicompound products based on whey protein on sleep quality and the gut microbiota of people with sleep problems, researchers observed an increase in Bifidobacterium abundance [45]. However, it is not clear whether this effect was achieved through whey protein or galacto-oligosaccharides, which are known for prebiotic properties and were a part of the tested product. The impact of soy protein and peptides on gut microbiota seems to be more unequivocal. In a mini review based on both animal and human studies, the authors found that soy derivatives stimulate the growth of microbial diversity, especially bacteria with probiotic properties [46]. There is evidence that soy peptides stimulate Lactobacilli and Bifidobacteria and simultaneously decrease Bacteroidetes, which is why athletes should consider mixing their protein sources in the diet.

Bodybuilders specifically value supplements such as citrulline and arginine because these amino acids promote vasodilation through increased nitric oxide (NO) production. This effect (so-called muscle pump) increases the transport of oxygen to working muscles. Citrulline is an amino acid derivative whose metabolism is related to the protein amino acid arginine [47]. When consumed in the diet, citrulline is broken down into arginine molecules. In turn, this amino acid is involved in the synthesis of nitric oxide in the endothelial cells of blood vessels. It delays the onset of fatigue during strength training and reduces muscle soreness on the first day after intense exercise. Another mechanism of action of citrulline or citrulline malate is the excretion of excess ammonia, which is formed during muscle activity and contributes to fatigue [48]. In addition to ammonia clearance, citrulline can improve gut homeostasis. In a double-blind, crossover study of 10 healthy men, citrulline supplementation prior to exercise attenuated splanchnic hypoperfusion, thereby protecting the mucosa from exercise-induced damage [49]. It has been proposed that this effect is mediated by increased arginine bioavailability. Citrulline and arginine participate in the urea (ornithine) cycle. Moreover, citrulline is a diagnostic tool for assessing short bowel function, as it is produced mostly in the gut [50]. While citrulline and arginine are recommended in both types of sport disciplines (aerobic and anaerobic), athletes such as bodybuilders and weightlifters tend to use much higher doses than runners and cyclists. In aerobic disciplines, athletes should consume approximately 1.5–2.0 g of arginine per day, while athletes in anaerobic disciplines may take more advantage of doses up to 10–12 g per day [51]. There is evidence that arginine, similar to glutamine, contributes to SCFA levels, thereby reducing the ratio of Firmicutes to Bacteroidetes [52]. Owing to its alkalizing properties, arginine is used as a prebiotic agent in dental care [53].

Citrulline and arginine, as well as dietary nitrates, are consumed by athletes for the same reasons. While the abovementioned amino acids increase NO indirectly, dietary nitrates (for example, from beetroot and rocket) do so directly. Nitrate pathways are mediated by microbial communities in the gut and provide a respiratory substrate [54]. Nitrate supplementation is one of the most effective methods to enhance exercise performance. Nitrate reduction begins in the mouth and is induced by specific bacteria [55]. While its properties in the muscular system, gastrointestinal tract, and oral microbiota are well known, its potential impact on the gut microbiota of athletes of different sports remains unknown. In one study conducted on human fecal samples, it was found that NO may decrease health-promoting Faecalibacterium prausnitzii biomass [56]. In this experiment, researchers used an in vitro fermentation model to mimic the natural gut environment.

Prebiotics are a group of substances resistant to enzymes present in the human digestive tract and capable of stimulating the growth of health-promoting microorganisms. These substances improve the colonization of the host organism, which is a desirable phenomenon from the point of view of the functioning of many areas of the entire body.

Another type of prebiotic that could be helpful for athletes is beta-glucans (i.e., from mushrooms and oats), which may promote Lactobacilli and Bifidobacteria abundance and elevate the Firmicutes/Bacteroidetes ratio [64]. There are similarities in the prebiotic properties of inulin and beta-glucans [65]. Surprisingly, beta-glucan supplementation at doses of 2 g/day for 4 weeks was found to increase athletes’ grip strength [66]. Improvements in VO₂max and 1 min double rocking jumps were also reported in this researchtudy. In healthy people exercising in the heat, beta-glucan (from yeast) was found to decrease inflammatory markers levels, which may preserve intestinal mucosa and microbiota in prolonged exhaustive activities [67].

Probiotics are live microorganisms that have a mutualistic relationship with human cells when they are delivered to the gastrointestinal tract (supplements, fermented vegetables and dairy). Inhabiting the intestines, they produce protective compounds that strengthen the physical barrier between the lumen of the digestive tract and the bloodstream, as well as the microbiological barrier, by secreting compounds that inhibit other microorganisms [68]. By influencing the “lining” of the intestines, probiotic bacteria facilitate the absorption of electrolytes, controlling the state of hydration, and also improve the breakdown of proteins, fats and carbohydrates, modifying the nutritional state [69]. Much attention is also paid to the fact that bacteria produce vitamins, especially B vitamins, and enhance absorption of iron and calcium ^[70][71][70,71].

Both germ-free and rodent models with antibiotic treatments are often used to determine potential modulators of host gut microbiota. Germ-free animals are housed in a sterile environment, which allows for detection of non-environmental factors affecting intestinal bacteria [76]. In a recent experiment comparing the effects of endurance and resistance exercise on murine (C57BL6N mice) gut microbiota, endurance training was found to promote higher bacterial diversity. Four weeks of different training programs led to higher relative abundance of Desulfovibrio species in endurance exercise and Clostridium sp. (namely C. cocleatum) in resistance exercise [77]. These results support hypotheses on the different effects of different exercise characteristics on the intestinal microbiome. However, ergogenic supplements and their interactions with the gut microbiota in trained rodents were not studied extensively.

Supplementation of inulin in male wild-type Groningen rats for 2 weeks increased Bacteroidetes and decreased Firmicutes abundances, along with increased acetate and succinate production [90]. In hyperuricemia mice, inulin enrichment of the diet led to a decrease in toxin levels and an increase in health-promoting Akkermansia bacteria, as well as SCFAs [91]. In a longer period of time, pectins from different food (beet, citrus, and soy) were found to increase Firmicutes and Lactobacillus and decrease Bacteroidetes in male Wistar rats (7 weeks of supplementation), and this shift was found to increase butyrate and propionate production [92]. Beta-glucans reversed gut barrier dysfunction in obese mice fed with a high-fat diet. This phenomenon was accompanied by regulation of Bacteroidetes and Proteobacteria levels, as well as cognitive changes [93]. Beta-glucans were also found to promote Blautia and Alistipes and inhibit Proteus and Lachnospiraceae and to be beneficial in the ulcerative colitis mouse model [94].

As gut training becomes more popular among athletes, it is necessary to describe future directions in mapping the interactions between different prebiotics, probiotics, and the most popular ergogenic aids. Very little is known about the modulatory effects of the gut microbiota on the ergogenic actions of most supplements.

Considering the immune-boosting effects of probiotics, it can be concluded that ergogenic effects are achieved through a decrease in the number of forced days off (rest days related to infections). Meeting the dietary requirements for fiber (different prebiotic fractions) consumption is a possible way to avoid gastrointestinal tract disturbances during the training and competition season and may protect from potentially harmful nutritional extremes, such as a very high-protein or very high-carbohydrate diet. More studies are needed in the field of both pre- and probiotic supplementation for athletes, as well as ergogenic supplementation of the gut microbiota. Before consuming any nutritional supplement, the person concerned should consult a reliable and qualified professional, who must base his claims on scientific evidence, i.e., a sports doctor or a dietician/nutritionist specializing in sports nutrition.