5:2 Fasting Protects Both Liver and Muscle in Mice With Fatty Liver Disease

Intermittent fasting has become one of the most popular dietary strategies in the world, but a nagging question has shadowed its rise: what does repeated fasting actually do to skeletal muscle? A new mouse study published in the Journal of Cachexia, Sarcopenia and Muscle offers an encouraging answer. Researchers report that a 5:2 intermittent fasting regimen—two nonconsecutive days of fasting per week with unrestricted eating on the other five days—simultaneously improved fatty liver disease and protected skeletal muscle from deterioration in mice fed a high-fat diet. The work, led by investigators at Harbin Medical University, also sketches a plausible mechanism running through the gut microbiota, bile acid signalling and fatty acid metabolism, connecting three organs in a single regulatory network.

The disease at the centre of the study, metabolic dysfunction–associated steatotic liver disease (MASLD), is now the most prevalent chronic liver condition worldwide. It is closely tied to high-fat diets, gut microbial imbalance and disrupted metabolism, and it carries consequences well beyond the liver. Sustained hepatic metabolic disturbance can spill over into skeletal muscle through inflammation-mediated pathways and abnormal metabolite production, and a growing body of evidence shows that MASLD is accompanied by reduced muscle mass. Because skeletal muscle is the body’s principal site of glucose disposal, its loss promotes insulin resistance, which in turn worsens the liver disease—a vicious cycle that clinicians have few tools to break. Non-pharmacological interventions that could protect both organs at once would therefore be of considerable clinical value.

Yet the fasting literature has been contradictory on precisely this point. Time-restricted feeding in rodents has shown direct benefits for muscle remodelling, but its pattern differs from 5:2 fasting. A murine study using three nonconsecutive 24-hour fasts per week found that intermittent fasting enhanced autophagy in the liver but failed to activate it in skeletal muscle, hinting that muscle might benefit less than the liver. Other work has suggested that combining minimal food intake with prolonged fasting could be unfavourable for muscle protein balance, and some clinical studies report reductions in lean body mass alongside weight loss, even if the decline falls short of true atrophy. Most human trials, moreover, lack comprehensive assessments of skeletal muscle. The new study was designed to address this gap directly by examining liver and muscle outcomes together in the same animals.

The experimental design was straightforward but rigorous. Seven-week-old male C57BL/6J mice were fed either normal chow or a 45% high-fat diet for 14 weeks to induce MASLD, which was confirmed by liver histopathology. The remaining high-fat-fed mice were then randomised by body weight into two groups for a 12-week intervention: one continued the high-fat diet ad libitum, while the other followed the 5:2 regimen, fasting on two nonconsecutive days each week with free access to water and eating the high-fat diet freely on the remaining five days. Crucially, all mice were sacrificed in a fed state—the fasting group 48 hours after its last fast—so that comparisons were not confounded by acute fasting physiology. Grip strength, serum biochemistry, histology, quantitative PCR, targeted fatty acid profiling by gas chromatography–mass spectrometry and faecal metagenomic sequencing rounded out the analytical toolkit.

On the liver side, the results were unambiguous. After 12 weeks, body weight was significantly lower in the fasting group than in the continuously high-fat-fed controls, and serum triglycerides, total cholesterol and the liver enzymes ALT and AST all fell markedly. The liver-to-body-weight ratio declined, gross liver appearance shifted from yellow and steatotic toward healthy, and histopathology confirmed reduced lipid accumulation and inflammation. White adipose tissue depots shrank across omental, subcutaneous, perirenal and epididymal sites. Interestingly, fasting mice showed pronounced compensatory hyperphagia during refeeding, eating more per day than the continuous-diet group, but cumulative weekly food mass and energy intake ended up similar between the two groups—the feasting apparently offset the fasting. Blood glucose did not differ significantly between the groups, a finding the authors attribute to that same compensatory intake during the refeeding phase.

The muscle findings are what set this study apart. Long-term high-fat feeding significantly shrank the cross-sectional area of quadriceps muscle fibres and reduced grip strength normalised to body weight, confirming that MASLD is accompanied by genuine muscle impairment rather than mere weight gain masking muscle loss. After the fasting intervention, absolute quadriceps mass was not significantly higher than in the high-fat group, but the muscle-to-body-weight ratio and body-weight-normalised grip strength rose markedly and returned to levels comparable with normal-chow mice. Linear regression revealed a significant positive correlation between body weight and muscle mass only in the high-fat group, indicating that weight gain there far outpaced any muscle growth. Expression of the inflammatory cytokine genes Il6 and Tnf in muscle also dropped significantly, and muscle fibre cross-sectional area was significantly larger after fasting than under the continuous high-fat diet.

To explain how a dietary schedule could protect two distant organs, the researchers turned to the gut. Histology showed that the high-fat diet thinned the intestinal wall and drove inflammatory cell infiltration, while fasting restored wall thickness and calmed inflammation. Genes encoding the barrier components Muc2 and the tight junction proteins Tjp1, Cldn4 and Ocln were downregulated by the high-fat diet and upregulated by fasting, and serum lipopolysaccharide—a marker of microbial endotoxin leaking through a damaged barrier—was elevated in high-fat mice but normalised by the intervention. Metagenomic sequencing of faecal samples then revealed that fasting reshaped the microbial community structure toward that of healthy chow-fed mice, without changing overall diversity. The pro-inflammatory species Mucispirillum schaedleri dominated in high-fat mice, whereas the fasting group was enriched in beneficial taxa including Faecalibaculum rodentium and Limosilactobacillus reuteri, and functional analysis pointed to enhanced bacterial capacity for secondary bile acid biosynthesis and bile salt hydrolase activity, with F. rodentium the top contributor.

Bile acids provided the mechanistic thread. Bacteria with bile salt hydrolase activity deconjugate primary bile acids, generating substrates for secondary bile acids that act as signalling molecules on receptors in the intestine, liver and muscle. Consistent with this, fasting upregulated the canonical bile acid receptor Fxr in intestine and liver, and hepatic Fxr suppressed its downstream target Cyp7a1. Hepatic expression of Ppara and its targets Cpt1a and Acox1—drivers of fatty acid beta-oxidation—rose significantly, as did the FGF15 receptor pair Fgfr4 and Klb in both liver and muscle. In skeletal muscle, the bile acid-responsive receptor Tgr5 was upregulated along with Pkaca, while Foxo3 and the atrophy genes Trim63 and Fbxo32—markers of muscle protein breakdown—were significantly repressed. Targeted fatty acid profiling reinforced the picture: total free fatty acid content fell in both liver and muscle to levels indistinguishable from healthy controls, with palmitic acid (C16:0) among the shared fatty acids reduced by fasting. Correlation analyses tied these threads together, showing bile acid receptor genes generally negatively correlated with differential fatty acids, and Tgr5 most strongly negatively correlated with palmitic acid, a saturated fatty acid known to induce tissue inflammation.

The authors are careful about causality. The mechanistic evidence remains largely correlative, and they note that direct proof of bile salt hydrolase activity in F. rodentium is lacking, even though the species has been implicated in converting primary bile acids to secondary forms. Species differences loom large: rodents and humans differ substantially in bile acid composition, microbiota structure and bile salt hydrolase enzymology, and the net effect of elevated bile salt hydrolase activity in human MASLD may be protective or harmful depending on metabolic context. The study also used only male mice, leaving open whether females respond similarly, and the diet-induced mouse model cannot fully recapitulate the heterogeneity of human disease. The authors call for faecal microbiota transplantation, microbiota depletion and mono-colonisation experiments, together with FXR- or TGR5-deficient mice and receptor-specific agonists, to establish whether the microbial changes are necessary and sufficient for the metabolic benefits.

Even with those caveats, the study lands at a moment of intense clinical interest. Recent human trials have found that 5:2 fasting achieves greater reductions in fat mass and glycated haemoglobin in MASLD patients than continuous energy restriction, without compromising lean body mass, and rodent work has shown it attenuates Western diet-induced steatohepatitis. The new findings extend that picture by demonstrating, in the same animals, coordinated protection of liver and skeletal muscle and a plausible gut–liver–muscle axis linking barrier integrity, microbial bile acid metabolism, receptor signalling and lipid handling. The authors emphasise that clinical translation will require trials in MASLD patients that measure not only hepatic outcomes but also muscle quality, strength, dietary intake, microbiota composition and bile acid profiles—particularly in elderly, diabetic or sarcopenic patients at risk of insufficient protein intake, who may respond differently to fasting. If those trials succeed, the humble schedule of two fasting days a week could emerge as a lifestyle intervention that defends the liver and the muscle at the same time.

Subject of Research: Effects of 5:2 intermittent fasting on MASLD and skeletal muscle impairment in mice via gut microbiota and bile acid signalling

Article Title: A 5:2 Intermittent Fasting Regimen Ameliorates High‐Fat Diet‐Induced MASLD‐Associated Skeletal Muscle Impairment in Mice

Article References: Wang, R., Yang, R., Hu, D., Xin, T., Yang, Z., Guan, Y., & Niu, Y. (2026). A 5:2 Intermittent Fasting Regimen Ameliorates High‐Fat Diet‐Induced MASLD‐Associated Skeletal Muscle Impairment in Mice. Journal of Cachexia, Sarcopenia and Muscle, 17(5), Article e70398. https://doi.org/10.1002/jcsm.70398

Image Credits: AI Generated

DOI: 10.1002/jcsm.70398

Keywords: intermittent fasting, 5:2 diet, MASLD, fatty liver disease, skeletal muscle, gut microbiota, bile acids, high-fat diet, intestinal barrier, muscle atrophy, mice study, metabolic health