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Background Gut microbiota dysbiosis and sarcopenia commonly occur in the elderly. Although the concept of the gut–muscle axis has been raised, the casual relationship is still unclear. This systematic review analyses the current evidence of gut microbiota effects on muscle/sarcopenia. Methods A systematic review was performed in PubMed, Embase, Web of Science, and The Cochrane Library databases using the keywords (microbiota* OR microbiome*) AND (sarcopen* OR muscle). Studies reporting the alterations of gut microbiota and muscle/physical performance were analysed. Results A total of 26 pre‐clinical and 10 clinical studies were included. For animal studies, three revealed age‐related changes and relationships between gut microbiota and muscle. Three studies focused on muscle characteristics of germ‐free mice. Seventy‐five per cent of eight faecal microbiota transplantation studies showed that the recipient mice successfully replicated the muscle phenotype of donors. There were positive effects on muscle from seven probiotics, two prebiotics, and short‐chain fatty acids (SCFAs). Ten studies investigated on other dietary supplements, antibiotics, exercise, and food withdrawal that affected both muscle and gut microbiota. Twelve studies explored the potential mechanisms of the gut–muscle axis. For clinical studies, 6 studies recruited 676 elderly people (72.8 ± 5.6 years, 57.8% female), while 4 studies focused on 244 young adults (29.7 ± 7.8 years, 55.4% female). The associations of gut microbiota and muscle had been shown in four observational studies. Probiotics, prebiotics, synbiotics, fermented milk, caloric restriction, and exercise in six studies displayed inconsistent effects on muscle mass, function, and gut microbiota. Conclusions Altering the gut microbiota through bacteria depletion, faecal transplantation, and various supplements was shown to directly affect muscle phenotypes. Probiotics, prebiotics, SCFAs, and bacterial products are potential novel therapies to enhance muscle mass and physical performance. Lactobacillus and Bifidobacterium strains restored age‐related muscle loss. Potential mechanisms of microbiome modulating muscle mainly include protein, energy, lipid, and glucose metabolism, inflammation level, neuromuscular junction, and mitochondrial function. The role of the gut microbiota in the development of muscle loss during aging is a crucial area that requires further studies for translation to patients.

Excessive training may limit physiological muscle adaptation through chronic oxidative stress and inflammation. Improper diet and overtraining may also disrupt intestinal homeostasis and in consequence enhance inflammation. Altogether, these factors may lead to an imbalance in the gut ecosystem, causing dysregulation of the immune system. Therefore, it seems to be important to optimize the intestinal microbiota composition, which is able to modulate the immune system and reduce oxidative stress. Moreover, the optimal intestinal microbiota composition may have an impact on muscle protein synthesis and mitochondrial biogenesis and function, as well as muscle glycogen storage. Aproperly balanced microbiome may also reduce inflammatory markers and reactive oxygen species production, which may further attenuate macromolecules damage. Consequently, supplementation with probiotics may have some beneficial effect on aerobic and anaerobic performance. The phenomenon of gut-muscle axis should be continuously explored to function maintenance, not only in athletes.

Human gut microbiota is able to influence the host physiology by regulating multiple processes, including nutrient absorption, inflammation, oxidative stress, immune function, and anabolic balance. Aging is associated with reduced microbiota biodiversity, increased inter-individual variability, and over-representation of pathobionts, and these phenomena may have great relevance for skeletal muscle mass and function. For this reason, the presence of a gut-muscle axis regulating the onset and progression of age-related physical frailty and sarcopenia has been recently hypothesized. In this narrative review, we summarize the studies supporting a possible association between gut microbiota-related parameters with measures of muscle mass, muscle function, and physical performance in animal models and humans. Reduced muscle mass has been associated with distinct microbiota composition and reduced fermentative capacity in mice, and the administration of probiotics or butyrate to mouse models of muscle wasting has been associated with improved muscle mass. However, no studies have targeted the human microbiome associated with sarcopenia. Limited evidence from human studies shows an association between microbiota composition, involving key taxa such as Faecalibacterium and Bifidobacterium, and grip strength. Similarly, few studies conducted on patients with parkinsonism showed a trend towards a different microbiota composition in those with reduced gait speed. No studies have assessed the association of fecal microbiota with other measures of physical performance. However, several studies, mainly with a cross-sectional design, suggest an association between microbiota composition and frailty, mostly assessed according to the deficit accumulation model. Namely, frailty was associated with reduced microbiota biodiversity, and lower representation of butyrate-producing bacteria. Therefore, we conclude that the causal link between microbiota and physical fitness is still uncertain due to the lack of targeted studies and the influence of a large number of covariates, including diet, exercise, multimorbidity, and polypharmacy, on both microbiota composition and physical function in older age. However, the relationship between gut microbiota and physical function remains a very promising area of research for the future.

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: Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host (FAO/WHO). Probiotic administration has been linked to a multitude of health benefits, with gut and immune health being the most researched applications. Despite the existence of shared, core mechanisms for probiotic function, health benefits of probiotics are strain- and dose-dependent. Athletes have varying gut microbiota compositions that appear to reflect the activity level of the host in comparison to sedentary people, with the differences linked primarily to the volume of exercise and amount of protein consumption. Whether differences in gut microbiota composition affect probiotic efficacy is unknown. The main function of the gut is to digest food and absorb nutrients. In athletic populations, certain probiotics strains can increase absorption of key nutrients such as amino acids from protein, and affect the pharmacology and physiological properties of multiple food components. Immune depression in athletes worsens with excessive training load, psychological stress, disturbed sleep, and environmental extremes, all of which can contribute to an increased risk of respiratory tract infections. In certain situations, including exposure to crowds, foreign travel and poor hygiene at home, and training or competition venues, athletes’ exposure to pathogens may be elevated leading to increased rates of infections. Approximately 70% of the immune system is located in the gut and probiotic supplementation has been shown to promote a healthy immune response. In an athletic population, specific probiotic strains can reduce the number of episodes, severity and duration of upper respiratory tract infections. Intense, prolonged exercise, especially in the heat, has been shown to increase gut permeability which potentially can result in systemic toxemia. Specific probiotic strains can improve the integrity of the gut-barrier function in athletes. Administration of selected anti-inflammatory probiotic strains have been linked to improved recovery from muscle-damaging exercise. The minimal effective dose and method of administration (potency per serving, single vs. split dose, delivery form) of a specific probiotic strain depends on validation studies for this particular strain. Products that contain probiotics must include the genus, species, and strain of each live microorganism on its label as well as the total estimated quantity of each probiotic strain at the end of the product’s shelf life, as measured by colony forming units (CFU) or live cells. Preclinical and early human research has shown potential probiotic benefits relevant to an athletic population that include improved body composition and lean body mass, 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. However, these potential benefits require validation in more rigorous human studies and in an athletic population.

Background Age‐related muscle dysfunctions are common disorders resulting in poor quality of life in the elderly. Probiotic supplementation is a potential strategy for preventing age‐related sarcopenia as evidence suggests that probiotics can enhance muscle function via the gut–muscle axis. However, the effects and mechanisms of probiotics in age‐related sarcopenia are currently unknown. In this study, we examined the effects of Lactobacillus casei Shirota (LcS), a probiotic previously reported to improve muscle function in young adult mice. Methods We administered LcS (1 × 108 or 1 × 109 CFU/mouse/day) by oral gavage to senescence‐accelerated mouse prone‐8 mice for 12 weeks (16‐ to 28‐week‐old). Sixteen‐week‐old and 28‐week‐old SMAP8 mice were included as non‐aged and aged controls, respectively. Muscle condition was evaluated using dual‐energy X‐ray absorptiometry for muscle mass, holding impulse and grip strength tests for muscle strength, and oxygen consumption rate, gene expressions of mitochondrial biogenesis, and mitochondrial number assays for mitochondria function. Inflammatory cytokines were determined using enzyme‐linked immunosorbent assay. Gas chromatography–mass spectrometry was utilized to measure the short‐chain fatty acid levels. The gut microbiota was analysed based on the data of 16S rRNA gene sequencing of mouse stool. Results The LcS supplementation reduced age‐related declines in muscle mass (>94.6%, P < 0.04), strength (>66% in holding impulse and >96.3% in grip strength, P < 0.05), and mitochondrial function (P < 0.05). The concentration of short‐chain fatty acids (acetic, isobutyric, butyric, penic, and hexanoic acid) was recovered by LcS (>65.9% in the mice given high dose of LcS, P < 0.05) in the aged mice, and LcS attenuated age‐related increases in inflammation (P < 0.05) and reactive oxygen species (>89.4%, P < 0.001). The high dose of LcS supplementation was also associated with distinct microbiota composition as indicated by the separation of groups in the beta‐diversity analysis (P = 0.027). LcS supplementation altered predicted bacterial functions based on the gut microbiota. Apoptosis (P = 0.026), p53 signalling (P = 0.017), and non‐homologous end‐joining (P = 0.031) were significantly reduced, whereas DNA repair and recombination proteins (P = 0.043), RNA polymerase (P = 0.008), and aminoacyl‐tRNA biosynthesis (P = 0.003) were increased. Finally, the genera enriched by high‐dose LcS [linear discriminant analysis (LDA) score > 2.0] were positively correlated with healthy muscle and physiological condition (P < 0.05), while the genera enriched in aged control mice (LDA score > 2.0) were negatively associated with healthy muscle and physiological condition (P < 0.05). Conclusions Lactobacillus casei Shirota represents an active modulator that regulates the onset and progression of age‐related muscle impairment potentially via the gut–muscle axis.

The gut microbiota could influence the pathophysiology of age-related sarcopenia through multiple mechanisms implying modulation of chronic inflammation and anabolic resistance. The aim of this study was to compare the fecal microbiota composition and functionality, assessed by shotgun metagenomics sequencing, between two groups of elderly outpatients, differing only for the presence of primary sarcopenia. Five sarcopenic elderly subjects and twelve non-sarcopenic controls, classified according to lower limb function and bioimpedance-derived skeletal muscle index, provided a stool sample, which was analyzed with shotgun metagenomics approaches, to determine the overall microbiota composition, the representation of bacteria at the species level, and the prediction of bacterial genes involved in functional metabolic pathways. Sarcopenic subjects displayed different fecal microbiota compositions at the species level, with significant depletion of two species known for their metabolic capacity of producing short-chain fatty acids (SCFAs), Faecalibacterium prausnitzii and Roseburia inulinivorans, and of Alistipes shahii. Additionally, their fecal metagenome had different representation of genes belonging to 108 metabolic pathways, namely, depletion of genes involved in SCFA synthesis, carotenoid and isoflavone biotransformation, and amino acid interconversion. These results support the hypothesis of an association between microbiota and sarcopenia, indicating novel possible mediators, whose clinical relevance should be investigated in future studies.

Sarcopenia represents a major health burden in industrialized country by reducing substantially the quality of life. Indeed, it is characterized by a progressive and generalized loss of muscle mass and function, leading to an increased risk of adverse outcomes and hospitalizations. Several factors are involved in the pathogenesis of sarcopenia, such as aging, inflammation, mitochondrial dysfunction, and insulin resistance. Recently, it has been reported that more than one third of inflammatory bowel disease (IBD) patients suffered from sarcopenia. Notably, the role of gut microbiota (GM) in developing muscle failure in IBD patient is a matter of increasing interest. It has been hypothesized that gut dysbiosis, that typically characterizes IBD, might alter the immune response and host metabolism, promoting a low-grade inflammation status able to up-regulate several molecular pathways related to sarcopenia. Therefore, we aim to describe the basis of IBD-related sarcopenia and provide the rationale for new potential therapeutic targets that may regulate the gut-muscle axis in IBD patients.

The hypermetabolic response associated with burns is characterized by skeletal muscle atrophy and an increased incidence of disability and death. Significant remodeling of the gut microbiota occurs after severe burn trauma. However, the specific mechanisms by which gut microbiota contribute to burn‐induced muscle atrophy remain unexplored. The results showed that the disruption of the gut microbiota exacerbated skeletal muscle atrophy. Fecal metabolite analysis revealed perturbations, primarily within the tryptophan (Trp) metabolic pathway. Animal models further demonstrated that gut microbiota disorder enhanced the expression of indoleamine 2,3‐dioxygenase 1 (IDO‐1) in the colon, ultimately resulting in Trp depletion and increased kynurenine (Kyn) levels in the serum and skeletal muscle. Excessive colonic Kyn is released into circulation, transported into skeletal muscle cells, and binds to the aryl hydrocarbon receptor (AHR), consequently triggering AHR nuclear translocation and initiating the transcription of skeletal muscle atrophy‐related genes. Notably, serum samples from patients with burns exhibited Trp depletion, and Trp supplementation alleviated skeletal muscle atrophy in rats with burns. This study, for the first time, demonstrates that gut microbiota dysbiosis upregulates colonic IDO‐1, promotes Trp‐Kyn metabolism, and exacerbates burn‐induced skeletal muscle atrophy, suggesting that Trp supplementation may be a potential therapeutic strategy. This study elucidates a pivotal crosstalk between the gut microbiota and skeletal muscles atrophy induced by burn injuries, a process fueled by the overactivation of IDO‐1 in the colon. This overactivation initiates the metabolism of tryptophan into kynurenine. Subsequently, Kyn is transported into myotubes through SLC38A2 and binds to AHR, and ultimately exacerbates the skeletal muscle atrophy.

Abstract The gut microbiota (GM) is a dynamic ecosystem intricately linked to human health, including metabolic, immune, endocrine, and gastrointestinal functions. Exercise is recognized as a significant modifier of this microbial ecosystem, yet the complexities of this relationship are underexplored. Here, we delve into the multifaceted interactions between structured physical activity and the GM, emphasizing the role of exercise-induced stressors in shaping microbial composition and function. Unique to our review, we discuss the acute effects of different forms of exercise-induced stress on the GM and explore how these responses may influence long-term adaptability, stability, and resilience. Furthermore, we address critical junctures in microbial dynamics leading to shifts between different stable states. Finally, we explore the implications of host-controlled factors such as diet, exercise training, and nutritional supplementation in modulating the microbial community in the gut to optimize athletic performance. We conclude that while the potential to harness the synergistic effects of exercise-induced stressors, dietary interventions, and microbial adaptations appears promising, current evidence remains preliminary, highlighting the need for additional targeted research to guide future strategies that manipulate the GM for optimal health and athletic performance. Exploring the complex interplay between exercise and the gut microbiota, this review highlights how structured physical activity acts as a hormetic stressor, altering gut microbial diversity and resilience, and shaping systemic health and athletic performance. Image created with BioRender.com.

New Findings What is the topic of this review? The contribution of gut microbial signalling to skeletal muscle maintenance and development and identification of potential therapeutic targets in progressive muscle degenerative diseases such as Duchenne muscular dystrophy. What advances does it highlight? Gut microbe‐derived metabolites are multifaceted signalling molecules key to muscle function, modifying pathways contributing to skeletal muscle wasting, making them a plausible target for adjunctive therapy in muscular dystrophy. Skeletal muscle is the largest metabolic organ making up ∼50% of body mass. Because skeletal muscle has both metabolic and endocrine properties, it can manipulate the microbial populations within the gut. In return, microbes exert considerable influence on skeletal muscle via numerous signalling pathways. Gut bacteria produce metabolites (i.e., short chain fatty acids, secondary bile acids and neurotransmitter substrates) that act as fuel sources and modulators of inflammation, influencing host muscle development, growth and maintenance. The reciprocal interactions between microbes, metabolites and muscle establish a bidirectional gut–muscle axis. The muscular dystrophies constitute a broad range of disorders with varying disabilities. In the profoundly debilitating monogenic disorder Duchenne muscular dystrophy (DMD), skeletal muscle undergoes a reduction in muscle regenerative capacity leading to progressive muscle wasting, resulting in fibrotic remodelling and adipose infiltration. The loss of respiratory muscle in DMD culminates in respiratory insufficiency and eventually premature death. The pathways contributing to aberrant muscle remodelling are potentially modulated by gut microbial metabolites, thus making them plausible targets for pre‐ and probiotic supplementation. Prednisone, the gold standard therapy for DMD, drives gut dysbiosis, inducing a pro‐inflammatory phenotype and leaky gut barrier contributing to several of the well‐known side effects associated with chronic glucocorticoid treatment. Several studies have observed that gut microbial supplementation or transplantation exerts positive effects on muscle, including mitigating the side effects of prednisone. There is growing evidence in support of the potential for an adjunctive microbiota‐directed regimen designed to optimise gut–muscle axis signalling, which could alleviate muscle wasting in DMD. Interactions within the large intestine between the gut microbiota and host epithelium, as well as between the microbiome and substrates (digested and non‐digestible molecules) produce metabolites that affect downstream pathways influencing muscle development and maintenance. Similarly, exercise/sedentary behaviour can feed back and modulate microbial populations within the colon forming a bidirectional gut–muscle axis. Recent evidence suggests that manipulation of both the substrates and microbial populations in the colon can mitigate muscular dystrophy‐related atrophy, establishing novel adjunctive therapeutic strategies.