**4. The beneficial effect of probiotics in physically active individuals**

Probiotics are currently defined as live microorganisms that have a beneficial health effect on the host when consumed in adequate amounts [81, 82]. They have an impact on the intestinal ecosystem through interactions with the host cells as well as intestinal microbiota regulating gut mucosal immunity. Among others, these interactions can contribute to improving the intestinal microenvironment, strengthening the intestinal barrier, modulating mucus secretion and the secretion of immunoglobulins or cytokines, as well as activating the innate immune response.

The beneficial role of probiotics relies on their ability to modulate the host's microbiota and to improve the barrier function of the gut mucosa [83, 84]. Probiotics produce broad-spectrum inhibitory bacteriocins and metabolites such as SCFAs inducing a decrease of the pH less favorable for bacterial growth [85]. Higher SCFA concentrations also reduce the differentiation of dendritic cells, thus decreasing proinflammatory cytokines production [86–88].

Probiotics improve the barrier function and tight junctions (TJs) between intestinal epithelial cells at the level of signaling pathways leading to the increase of the mucus layer or to the production of defensins as well as proteins of TJs. They regulate the expression of the TJs, where cellular contacts occur and thus maintain cell morphology. As have already been reviewed, several probiotic strains, like *Lactobacillus rhamnosus* GG, *Lactobacillus casei* DN-114001, *Escherichia coli* Nissle 1917, and different strains of *Lactobacillus plantarum*, have a protective effect against pathogen infections via the regulation of TJ proteins [89].

Other important components that build a protective barrier and avoid the adhesion of harmful bacteria to the epithelial cells are the mucus layer and cells of the intestinal epithelium and underlying lamina propria [90]. Each of them consists of several cell types preventing any direct contact with bacteria in the intestinal lumen. The intestinal epithelium consists of enterocytes responsible for absorbing molecules from the intestinal lumen. Paneth cells specialized in synthesizing and secreting antimicrobial peptides (AMPs) upon contact with enteric bacteria, Goblet cells, and entero-endocrine cells [90–92]. Goblet cells produce mucus and are mainly composed of high molecular weight glycoproteins called mucins. They are of two types: secreted mucins are responsible for the formation of the mucus layer, while transmembrane mucins are likely involved in signaling pathways [93–95]. A healthy mucus layer plays an important role in preventing inflammatory and infectious diseases. Altered

expression of specific mucins was associated with gastrointestinal diseases such as Crohn's disease [96] and ulcerative colitis [97] highlighting the importance of these proteins in the intestine. Several studies confirmed that specific strains of probiotic bacteria might affect the mucus barrier by regulating mucin expression. Thus, they can influence the properties of the mucus layer and indirectly regulate the gut immune system [89]. In multiple *in vitro* and in *vivo models*, it was shown that specific probiotic bacteria stimulate the gene expression levels of mucins. Among them, *L. plantarum* 299v, *E. coli* Nissle 1917, *L. casei* GG and *Lactobacillus acidophilus* LA1, and *Lactobacillus reuteri* R2LC or 4659 as well as probiotic mixture VSL#3 were confirmed to increase the level of mucins in the gut, therefore, influencing the properties of the mucus layer and indirectly regulate the gut immune system [95, 98–103]. It has also been evidenced that *Akkermancia muciniphila* increases the number of Goblet cells and the production of antimicrobial peptides, suggesting that it communicates with host cells and consequently stimulates the production of mucus [104].

Recent findings demonstrate that probiotics modulate the intestinal immune system by activating the immune response by recognizing specific receptors of innate immunity cells (epithelial cells, dendritic cells, and T cells). These receptors are called pattern recognition receptors (PRR) and include mostly Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain agents (NODs) [105]. They are recognized by MAMPs (microbe-associated molecular patterns). Their interaction with the gut epithelium stimulates the cells of the gut immune system at the lamina propria [106]. Differentiation of T helper lymphocytes and the activation of regulatory T cells stimulate the pro- or anti-inflammatory cytokines production. Probiotic bacteria, especially various *Bifidobacterium* strains, can act differently depending on the cytokine profile [107]. The effects may be systemic or local and limited to the stimulation of IgA secretion by Peyer's patch cells [84, 108].

Several studies have shown that probiotics supplementation could improve immune function in athletes [109]; reduce upper respiratory tract illness (URTI) [110], gastrointestinal symptoms [111–113], and gut permeability [114, 115]; as well as increase physical performance in elite and competitive athletes [113, 116].

Existing studies have shown an association between intestinal microbiota composition and physical activity, suggesting that modifications in the gut microbiota composition may contribute to the physical performance and exercise capacity of the host [117]. Probiotics may promote health through the improvement of the immune system and indirectly influence the performance of athletes by preventing illnesses that negatively affect healthy training [109, 118]. Recently, the International Society of Sports Nutrition (ISSN) provided a position stand on probiotics, concluding that probiotics have strain-specific effects in athletes [119]. Specific probiotic strains can improve the integrity of the gut barrier function in athletes after prolonged exercise, especially in the heat, which has been shown to increase gut permeability potentially causing systemic toxemia. Administration of selected probiotic strains has been linked to improved body composition and lean body mass, improved recovery from muscle-damaging exercise, normalizing age-related declines in testosterone levels, reductions in cortisol levels indicating improved responses to a physical or mental stressor, reduction of exercise-induced lactate, and increased neurotransmitter synthesis, cognition, and mood [reviewed in 119].

Generally, mid to long-term benefits (supplementation periods varying from 2 weeks to 3 months) of probiotics on physical performance have been studied [117]. In different studies, various probiotic strains and doses were examined, so it is *Regular Physical Activity Influences Gut Microbiota with Positive Health Effects DOI: http://dx.doi.org/10.5772/intechopen.110725*

difficult to compare the obtained results. Among them, the most studied bacteria are members of *Lactobacillus* and *Bifidobacterium* genera.

*Lactiplantibacillus plantarum* TWK10 is among the most studied probiotic strains in terms of physical performance outcomes. A dose-dependent increase in muscle mass was observed in a preclinical animal study [120] and was further confirmed in clinical studies [121]. Endurance performance in an exhaustive treadmill exercise was improved in healthy, untrained adult males, who were supplemented daily with TWK10 for 6 weeks, compared with those who received a placebo [122]. The postexercise blood glucose level was higher in TWK10 group compared with the control group suggesting improved energy harvest from gluconeogenic precursors during exhaustive exercise.

In male runners, supplementation with a multi-strain probiotic (*L. acidophilus*, *Lacticaseibacillus rhamnosus*, *Lacticaseibacillus casei*, *L. plantarum*, *Limosilactobacillus fermentum*, *Bifidobacterium lactis*, *B. breve*, *Bifidobacterium bifidum*, and *Streptococcus thermophilus*) for 4 weeks significantly increased the running time to fatigue [110].

Probiotic supplementation (*S. thermophilus* FP4 and *Bifidobacterium breve* BR03) was reported to likely enhance isometric average peak torque production, attenuating performance decrements and muscle tension in the days following a muscle-damaging exercise [123]. In a similar study design, *Bacillus coagulans* GBI-306086 significantly increased recovery at 24 and 72 h and decreased soreness at 72 h post-exercise [124]. Probiotic supplementation correlated with maintained performance and a small increase in creatine phosphokinase.

Probiotics, belonging to the *Veillonella* genus, isolated from a marathon runner, have recently shown promising results in mouse performance models [125]. These bacteria feed on lactic acid and produce propionate, which may increase endurance capacity.

In mice, oral administration of either *Bifidobacterium longum* subsp. longum OLP-01 [126] or *Ligilactobacillus salivarius* subsp. salicinius SA-03 [127], isolated from a female weightlifting Olympic medalist, was shown to significantly increase forelimb grip strength and endurance capacity in a swim-to-exhaustion test. Both bacterial strains significantly decreased blood lactate, ammonia, and creatine kinase levels after an acute exercise and increased hepatic and muscle glycogen stores, which indicated improved energy utilization and the attenuation of fatigue-related biomarkers in mice.

However, not all studies have shown enhancements in endurance performance following probiotic use in highly trained subjects or athletes [119]. It has been shown that the exhaustive endurance exercise was not affected in endurance-trained males after 4 weeks of *Lactobacillus fermentum* VRI-003 supplementation [128] or after *Lactobacillus helveticus* Lafti L10 in trained subjects [129]. Also, 3 months of supplementation with a probiotic formula containing bacteria of different species (*B. bifidum* W23, *B. lactis* W51, *Enterococcus faecium* W54, *L. acidophilus* W22, *Levilactobacillus brevis* W63, and *Lactococcus lactis* W58) did not have benefit in endurance performance in highly trained athletes [130]. However, after a 2-month intervention in female swimmers, probiotic yogurt with *L. acidophilus* SPP, *L. bulgaricus*, *B. bifidum*, and *S. thermophilus* improved the VO2max but had no impact on the 400-m swimming time [131]. Also after a 6-week intervention in competitive, high-level, female swimmers *B. longum* 35,624 did not enhance aerobic or anaerobic swimming performance or improve power or force production measurements [132]. After a 12-week multi-strain probiotic or probiotic + glutamine supplementation, no effects were observed on the time to complete an ultra-marathon race compared with controls [133].

Multi-strain probiotic supplementation (*L. acidophilus* CUL60 and CUL21, *B. bifidum* CUL20, and *Bifidobacterium animalis* subs p. lactis CUL34) for 28 days prior to a marathon race was associated with a limited decrease in average speed in the probiotics group compared to the control group [134]. However, there were no significant differences in finish times between the groups. *Bacillus subtilis* supplementation during training soccer and volleyball female players, in conjunction with postworkout nutrition, had no effect on physical performance [135]. However, body fat percentages were significantly lower in the probiotic group. *B. subtilis* DE111 did not improve either strength or performance in male [136] or female athletes [137] when combined with a training protocol involving resistance exercises.

Multi-strain probiotic supplementation for 12 weeks, combined with circuit training, improved muscular performance to a similar degree as circuit training alone in healthy, sedentary males [138], confirming the positive effect of resistance training on muscular outcomes, demonstrated well by other probiotic and exercise interventions among athletes [136, 137].

The well-established probiotic effects on gut health and immune system function may benefit endurance athletes, who perform high-intensity training and often encounter physiological challenges associated with GI and immune health during and after a competition. However, high-quality clinical studies, with adequate power, is necessary to uncover the impacts of probiotics on physical performance and the mechanisms of action through which probiotics affect exercise outcomes.
