Gut microbes linked to fatigue through immune and metabolic pathways

Fatigue is one of the most common yet least understood symptoms in medicine, and a newly published review argues that a surprising part of the answer may lie in the gut. Writing in Food Science and Biotechnology, researchers from Sejong University, Dong-A University and Pohang University of Science and Technology in South Korea systematically assemble clinical and mechanistic evidence linking alterations in the gut microbiome to fatigue-related conditions, from myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) to post-acute COVID-19 syndrome, everyday fatigue in healthy adults, and exercise-induced exhaustion. Their synthesis points to a coherent biological circuit—an immune-metabolic highway running between gut microbes, the immune system and host energy metabolism—that may help explain why so many people feel persistently drained.

The review, led by Joonbeom Kim and corresponding authors Woongjae Yoo and Hakdong Shin, organizes the literature according to fatigue phenotype, microbiome function, microbial metabolites, gut barrier markers, inflammatory responses and food-based interventions. This phenotype-by-phenotype approach matters because fatigue is not a single entity. Clinicians distinguish pathological fatigue, such as the post-exertional malaise that defines ME/CFS, from the ordinary tiredness that follows intense exercise or a poor night’s sleep. By sorting the evidence in this way, the authors show that while the specific microbial signatures differ across conditions, the underlying mechanisms converge on a handful of recurring themes.

The first and most heavily supported theme involves short-chain fatty acids, particularly butyrate. These molecules are produced by gut bacteria as they ferment dietary fiber, and they do far more than nourish colon cells. Butyrate strengthens the intestinal barrier, promotes the differentiation of regulatory T cells that keep inflammation in check, and influences energy metabolism throughout the body. Multiple studies of ME/CFS patients have documented reduced microbial diversity and a deficient capacity to produce butyrate. One prominent study cited in the review found that this functional deficit was associated with disturbances in bacterial network structure and with the severity of fatigue symptoms, suggesting that the loss of butyrate-producing bacteria may be a driver rather than a bystander.

A second recurring mechanism is gut barrier dysfunction and the microbial translocation that follows it. When the single-cell lining of the intestine is compromised, bacterial products such as lipopolysaccharide can cross into the bloodstream. In ME/CFS patients, researchers have detected elevated serum antibodies against enterobacterial LPS, a finding consistent with increased intestinal permeability. Similar disruptions have been reported in post-COVID syndrome: a 2025 study found gastrointestinal barrier disruption in patients experiencing long COVID fatigue. The consequence is a state of low-grade, systemic inflammation—sustained immune activation that the brain interprets, in part, as fatigue. This framing aligns with the broader biology of sickness behavior, in which inflammatory cytokines act on the central nervous system to promote the desire to rest and withdraw.

The review also highlights altered tryptophan metabolism as a critical link between microbes and the brain. Tryptophan is the dietary precursor of serotonin and a substrate for the kynurenine pathway, and gut bacteria exert substantial control over how much tryptophan crosses the intestinal wall and which metabolic fate it follows. Microbial shifts that favor the kynurenine route over serotonin synthesis can influence mood, sleep and central fatigue signaling, providing a plausible biochemical route by which gut dysbiosis translates into the profound exhaustion reported by patients. Metabolomic studies of ME/CFS have repeatedly cataloged disturbances in these and related pathways, reinforcing the picture of a systemic metabolic imbalance rather than a purely psychological phenomenon.

The gut-muscle axis emerges as a third mechanistic pillar, particularly relevant to exercise-induced fatigue and athletic performance. Skeletal muscle and gut microbes communicate bidirectionally: microbial metabolites such as short-chain fatty acids influence muscle oxidative capacity and inflammation, while exercise reshapes the microbiome. One striking example cited in the review is the identification, through meta-omics analysis of elite athletes, of a bacterial strain associated with enhanced performance via lactate metabolism—the microbe appears to help clear lactate produced during intense exertion. Studies in athletes and physically active adults have also reported associations between specific gut bacterial profiles and perceived energy and fatigue, and probiotic supplementation trials in sports populations have explored whether modulating the microbiome can mitigate gastrointestinal inflammation and the perception of fatigue during training.

When it comes to interventions, the review evaluates a broad portfolio of food-based approaches. Dietary fiber and prebiotics supply the fermentable substrates that butyrate-producing bacteria need, and fiber intake is consistently associated with microbiome-mediated health benefits. Fermented foods deliver live microbes together with bioactive metabolites. Probiotics—live microorganisms administered in adequate amounts—have shown modest promise in pilot trials, including a double-blind, placebo-controlled study of a probiotic in the emotional symptoms of chronic fatigue syndrome, and a notable randomized trial in Hong Kong that tested a synbiotic preparation called SIM01 in patients with post-acute COVID-19 syndrome, reporting improvements relative to placebo. Postbiotics, defined by an international consensus as inanimate microorganisms or their components that confer health benefits, offer an alternative for people who cannot tolerate live microbes. Polyphenols and bioactive polysaccharides round out the list; the latter have been proposed to exert anti-fatigue effects specifically through the gut-muscle axis, and heat-killed strains of Lactiplantibacillus plantarum TWK10 have been examined for effects on exercise performance and body composition.

The authors are careful, however, to emphasize how much uncertainty remains. Many of the human studies to date are small, cross-sectional, or limited to stool-based 16S rRNA sequencing, which catalogs microbial species but says little about function. Causality is difficult to establish: fatigue can alter diet, sleep and physical activity, all of which in turn reshape the microbiome, so the observed associations could run in either direction or reflect shared underlying causes. Heterogeneity in case definitions—particularly for ME/CFS, for which multiple diagnostic criteria coexist—complicates comparisons across studies. The review therefore calls for controlled, phenotype-specific intervention trials that deploy multi-omics approaches, integrating metagenomics, metabolomics, immune profiling and host physiology to establish which microbial functions actually drive fatigue and which merely accompany it.

The implications, if the mechanistic picture holds up, are considerable. Fatigue underpins an enormous burden of disease worldwide, appearing in chronic infections, cancer, autoimmune conditions, depression and everyday life, yet treatment options remain limited largely to graded activity and cognitive-behavioral strategies, neither of which works for everyone. A microbiome-targeted framework would open the door to nutritional interventions that are comparatively safe, accessible and scalable: engineered prebiotic regimens to restore butyrate production, rationally selected probiotic or postbiotic formulations to reinforce barrier integrity and dampen inflammation, and dietary patterns designed to sustain a resilient gut ecosystem. The review’s authors, supported by Korea’s National Research Foundation and National Institute of Health, position food science as a central player in this emerging therapeutic landscape.

What makes the work timely is the post-pandemic context. Post-acute COVID-19 syndrome brought fatigue into global focus, with millions of patients experiencing persistent exhaustion months after infection, and studies documenting gut microbiota disturbances in prospective cohorts of these patients have fueled interest in microbial mechanisms of post-infectious fatigue. By drawing lines of continuity between ME/CFS—long dismissed by skeptics as a psychological illness—and long COVID, the review adds weight to the argument that fatigue syndromes have tangible, measurable biological underpinnings. The convergence of microbiome science, immunology and metabolomics is gradually replacing that skepticism with testable hypotheses about barrier failure, microbial metabolite deficits and chronic immune activation.

The Korean team’s synthesis does not claim to have solved fatigue. Rather, it maps the terrain: reduced short-chain fatty acid production, impaired butyrate capacity, leaky gut, microbial translocation, low-grade inflammation, disordered tryptophan metabolism, energy imbalance and gut-brain and gut-muscle signaling together form a plausible, evidence-backed architecture of fatigue-related vulnerability. What comes next—rigorous interventional trials, standardized phenotyping, and mechanistic studies that can distinguish cause from consequence—will determine whether the trillions of microbes in our intestines can be harnessed to restore what fatigue takes away.