The microorganisms in the gastrointestinal tract play a significant role in nutrient uptake, vitamin synthesis, energy harvest, inflammatory modulation, and host immune response, collectively contributing to human health. Important factors such as age, birth method, antibiotic use, and diet have been established as formative factors that shape the gut microbiota. Yet, less described is the role that exercise plays, particularly how associated factors and stressors, such as sport/exercise-specific diet, environment, and their interactions, may influence the gut microbiota. In particular, high-level athletes offer remarkable physiology and metabolism (including muscular strength/power, aerobic capacity, energy expenditure, and heat production) compared to sedentary individuals, and provide unique insight in gut microbiota research. In addition, the gut microbiota with its ability to harvest energy, modulate the immune system, and influence gastrointestinal health, likely plays an important role in athlete health, wellbeing, and sports performance. Therefore, understanding the mechanisms in which the gut microbiota could play in the role of influencing athletic performance is of considerable interest to athletes who work to improve their results in competition as well as reduce recovery time during training. Ultimately this research is expected to extend beyond athletics as understanding optimal fitness has applications for overall health and wellness in larger communities. Therefore, the purpose of this narrative review is to summarize current knowledge of the athletic gut microbiota and the factors that shape it. Exercise, associated dietary factors, and the athletic classification promote a more “health-associated” gut microbiota. Such features include a higher abundance of health-promoting bacterial species, increased microbial diversity, functional metabolic capacity, and microbial-associated metabolites, stimulation of bacterial abundance that can modulate mucosal immunity, and improved gastrointestinal barrier function.

Position statement: The International Society of Sports Nutrition (ISSN) provides an objective and critical review of the mechanisms and use of probiotic supplementation to optimize the health, performance, and recovery of athletes. Based on the current available literature, the conclusions of the ISSN are as follows:

Variations in gut microbiota might impact metabolism leading to body weight excess. We assessed the impact of a probiotic supplementation in pediatric obesity on weight, metabolic alterations, selected gut microbial groups, and functionality. Cross-over, double-blind, randomized control trial (BIFI-OBESE trial; NCT03261466). 101 youths (6-18 years, Tanner stage ≥2) with obesity and insulin-resistance on diet were randomized to 2 × 109 CFU/AFU/day of Bifidobacterium breve BR03 (DSM 16604) and B. breve B632 (DSM 24706) (51) or placebo (50) for 8 weeks with a 4-weeks wash-out period. All subjects (M/F 54/47) completed the first 8 weeks, and 82 (M/F 43/39) the last part without adverse events. Mixed-effects models revealed a carry-over effect on many variables in the entire study, narrowing the analysis to the first 8 weeks before the wash-out periods. All subjects improved metabolic parameters, and decreased weight and Escherichia coli counts. Probiotics improved insulin sensitivity at fasting (QUICKI, 0.013 CI95%0.0-0.03) and during OGTT (ISI, 0.654 CI95%-0.11-1.41). Cytokines, GLP1, and target microbial counts did not vary. Of 25 SCFAs, acetic acid and acetic acid pentyl-ester relative abundance remained stable in the probiotics, while increased in the placebo (p < 0.02). A signature of five butanoic esters identified three clusters, one of them had better glucose responses during probiotics. An 8 weeks treatment with B. breve BR03 and B632 had beneficial effects on insulin sensitivity in youths with obesity. Microbiota functionality could influence metabolic answers to probiotics. Long-term studies to confirm and enrich our findings are justified. Tailored probiotic treatments could be an additional strategy for obesity. NCT03261466.

Background. Type-2 inflammation commonly marks asthma in childhood. Also, gut and lung dysbiosis is detectable in patients with asthma. Strain-related probiotic supplementation may restore a physiological immune response, dampen airway inflammation, and repair dysbiosis. Therefore, the probiotics in pediatric asthma management (PROPAM) study is aimed at demonstrating that Ligilactobacillus salivarius LS01 (DSM 22775) and Bifidobacterium breve B632 (DSM 24706) mixture could reduce asthma exacerbations in children, followed in a primary care setting. Methods. The study was randomized, placebo-controlled, and double-blind. It involved 11 Italian primary care pediatricians. The probiotic mixture (containing Ligilactobacillus salivarius LS01$1\times {10}^9$live cells and Bifidobacterium breve B632$1\times {10}^9$live cells) or placebo was taken twice daily (1 sachet in the morning and 1 in the evening) for eight weeks and subsequently once daily for a further eight weeks. Outcomes included number, severity, and duration of asthma exacerbations, intensity of maintenance and as need treatments, and safety. Results. The per-protocol population included 422 children (mean age seven years, 240 males and 182 females). The probiotic mixture significantly reduced the number of asthmatic exacerbations ($\text{OR}=3.17$). In addition, the number of children with two exacerbations was less than a third in the active group ($\text{OR}=3.65$). Conclusions. This PROPAM study demonstrated that probiotic strains Ligilactobacillus salivarius LS01 (DSM 22775) and Bifidobacterium breve B632 (DSM 24706) were safe and significantly reduced by more than a third the frequency of asthma exacerbations. At present, the first-line treatment of asthma is still drug-based, but specific strains of probiotics may be auxiliary remedies.

The number of bacterial cells is currently recognized as the most important parameter for the efficacy and quality of finished probiotic or live biotherapeutic products (LBP). Cell enumeration is generally performed by culture-dependent methodologies like plate count (PC). These techniques are able to reveal the number of viable cells able to replicate and generate a colony. However, they are limited by their dependence on the combination of culture conditions (e.g. nutrients, temperature) selected for cell recovery. Additionally, they do not provide information on the heterogeneity of a bacterial culture, namely they do not detect the cells in a viable but not cultivable (VBNC) status. Flow-cytometry (FC) is a culture-independent methodology having the potential to enumerate selectively live and damaged or dead cells. FC relies on the use of specific probes for different cell targets (e.g. membrane, enzymes) to unveil information on the cell structure and physiological statuses within a bacterial population. In this context, we monitored three batches of freeze-dried Lactobacillus rhamnosus GG (ATCC 53103) during a 3 year of storage at different conditions of temperature and relative humidity, according to ICH guidelines, by means of PC and FC. The Arrhenius model was applied to assess the suitability of the model to predict the mortality of probiotic cells in finished products. The higher destruction rate (k) obtained by PC data compared to FC data suggests a faster reduction of cultivability compared to membrane integrity, probably representing a dynamic shift of the bacterial population into a VBNC/dormant status during storage time. Interestingly, this mechanistic approach works both for PC and FC methodologies increasing the chances to monitor biological phenomenon within a mathematical modelling. The combined use of PC and FC shed lights on the true bacterial potency within a closed system like a finished product and the complexity of its heterogeneity.

Autism spectrum disorders (ASDs) represent a diagnostic challenge with a still partially uncertain etiology, in which genetic and environmental factors have now been assessed. Among the hypotheses underlying the involvement of biological and environmental factors, the gut–brain axis is of particular interest in autism spectrum disorders. Several studies have highlighted the related incidence of particular gastrointestinal symptoms (GISs) in children suffering from ASDs. Probiotics have shown success in treating several gastrointestinal dysbiotic disorders; therefore, it is plausible to investigate whether they can alleviate behavioral symptoms as well. On these bases, a randomized double-blind crossover study with a placebo was conducted, evaluating the effects of a mixture of probiotics in a group of 61 subjects aged between 24 months and 16 years old with a diagnosis of ASD. Behavioral evaluation was performed through the administration of a questionnaire including a Parenting Stress Index (PSI) test and the Vineland Adaptive Behavior Scale (VABS). The Psycho-Educational Profile and the Autism Spectrum Rating Scale (ASRS) were also evaluated. Microbial composition analyses of fecal samples of the two groups was also performed. The study showed significant improvements in GISs, communication skills, maladaptive behaviors, and perceived parental stress level after the administration of probiotics. Microbiome alpha diversity was comparable between treatment arms and no significant differences were found, although beta diversity results were significantly different in the treatment group between T0 and T1 time points. Streptococcus thermophilus, Bifidobacterium longum, Limosilactobacillus fermentum, and Ligilactobacillus salivarius species were identified as some of the most discriminant taxa positively associated with T1 samples. This preliminary study corroborates the relationship between intestinal microbiota and ASD recently described in the literature.