The clinical backdrop is significant. Sarcopenia, the progressive loss of muscle mass, strength and function that accompanies ageing, is now formally recognized by the World Health Organization as a disease in its own right. Its prevalence climbs steeply with age, reaching up to 29 percent of older adults in community settings and between 11 and 50 percent of people aged 80 and above. Beyond ageing itself, risk factors include low protein intake, malabsorption, physical inactivity, metabolic and endocrine disorders, and hospitalization. Crucially, no drug has been approved to treat the condition. Current management relies on exercise, caloric control and nutritional supplementation, while experimental pharmacological approaches such as selective androgen receptor modulators and myostatin inhibitors have focused mainly on increasing muscle mass rather than restoring strength. That gap has pushed researchers toward dietary compounds that might support muscle function at the cellular level, and urolithins have emerged as leading candidates.

Urolithin A already has a track record in muscle research. Previous preclinical and clinical studies have shown that it promotes mitophagy, the cellular housekeeping process that clears damaged mitochondria, thereby improving mitochondrial quality and muscle function. Urolithin B has attracted less attention, though recent work identified it as a regulator of skeletal muscle mass that promotes the differentiation and hypertrophy of myotubes, the multinucleated fibers that form in culture when muscle stem cells mature. Some studies also suggest urolithin B exerts testosterone-like effects that tip muscle protein balance toward growth. What remained unknown was how the two molecules compare when applied to genuine human muscle cells under identical conditions, and whether their effects converge on shared pathways or diverge into entirely different biology.

To find out, the researchers isolated satellite cells, the resident stem cells of skeletal muscle, from biopsies of the vastus lateralis muscle taken post-mortem from nine donors aged between 55 and 96, with ethical approval from the University of Liège. The cells were purified using immunomagnetic sorting for the satellite cell marker CD56, expanded in culture, and then differentiated for 48 hours into myotubes, a transition confirmed by measuring myogenic markers such as myosin. The mature myotubes were treated for 24 hours with either urolithin A or urolithin B at a concentration of 5 micromolar, a dose chosen to balance biological activity with physiological plausibility, since pharmacokinetic studies indicate that urolithin A can reach low micromolar levels in human plasma after supplementation. The team then sequenced the entire transcriptome of each sample, generating roughly 20 million reads per sample and analyzing differential gene expression with a paired statistical design that accounted for donor-to-donor baseline differences.

The scale of the response differed dramatically between the two compounds. Urolithin A significantly altered the expression of 1,918 genes, whereas urolithin B affected only 339. Despite this imbalance, 180 genes were regulated by both molecules, with 162 commonly down-regulated and 18 commonly up-regulated, pointing to shared effects that likely stem from their well-documented antioxidant and anti-inflammatory properties. More intriguing were the nine genes that moved in opposite directions. Eight genes, including EPB41L1, JPH1, GOT1, IL-17B, ESYT1, BEAN1, PCSK9 and SLCO4C1, were pushed up by urolithin B and pulled down by urolithin A, while a single gene, NQO1, was up-regulated by urolithin A and down-regulated by urolithin B. This pattern of discordant regulation suggests the two metabolites are not interchangeable, and that the net effect of a polyphenol-rich diet could depend on the relative abundance of each compound in an individual’s circulation.

Two of the oppositely regulated genes carry particular weight. IL-17B, an inflammatory signaling molecule, was decreased by roughly 49 percent under urolithin A but increased by about 45 percent under urolithin B. Recent work on dystrophinopathies has identified IL-17B as a biomarker of active muscle regeneration, enriched in regenerating fibers rather than in quiescent tissue. The authors interpret the opposing regulation not as a simple pro- versus anti-inflammatory split, but as evidence that the two compounds tune different inflammatory and regenerative programs: urolithin A may dampen the chronic low-grade inflammation implicated in sarcopenia, while urolithin B may nudge cells toward a remodeling state in which a controlled inflammatory component is part of repair. NQO1, meanwhile, encodes an enzyme that defends cells against oxidative stress by reducing quinones and preventing reactive oxygen species from forming. Its induction by urolithin A fits neatly with that compound’s established role in mitochondrial maintenance, whereas its suppression by urolithin B raises open questions about how anabolic signaling and oxidative defense are balanced under the second metabolite.

Pathway analysis sharpened the contrast. In urolithin A-treated cells, the most affected canonical pathways included inositol phosphate metabolism, calcium signaling and protein glycosylation routes, with most showing predicted inhibition. The authors propose that moderate inhibition of the inositol phosphate pathway, which is tightly linked to mitochondrial function through signaling molecules such as IP3, could trigger a compensatory boost in mitophagy and mitochondrial biogenesis, consistent with urolithin A’s known benefits. The glycosylation findings matter too, because protein glycosylation is now recognized as essential for maintaining normal muscle structure, and glycosylation defects are linked to muscle disease. Urolithin B told a different story: it strongly activated cholesterol biosynthesis and the mevalonate pathway, moderately activated adipogenesis, suppressed eicosanoid signaling in a way that could reduce inflammatory lipid mediators, and up-regulated CXCR4 signaling, a pathway recently shown to be critical for satellite cell activation, proliferation and self-renewal during muscle regeneration.

Validation experiments in cells from a subset of four donors confirmed several of these signals. Quantitative PCR showed that urolithin A increased expression of NOTCH1, a master regulator of muscle cell fate, as well as MYMX, a muscle-specific membrane protein involved in myogenic fusion, and PANX1, a newly identified regulator of myoblast proliferation and differentiation, while dose-dependently reducing FGF9 and ICAM5. Urolithin B was ineffective on these particular genes. Both compounds, however, dose-dependently reduced production of TGFBI, a protein associated with skeletal muscle remodeling, measured by ELISA in the culture supernatant. Live-cell imaging with label-free microscopy added a phenotypic dimension: fusion-like events appeared as early as six hours after treatment with either compound, and after 24 hours both treated conditions showed fewer isolated mononucleated cells than controls, consistent with enhanced incorporation of myoblasts into multinucleated myotubes.

The authors are careful to frame the work as mechanistic and exploratory rather than definitive. The myotubes were derived from post-mortem tissue, which may not fully mirror living muscle, although the use of expanded CD56-positive satellite cells, excellent RNA integrity and prior evidence that such cells retain functional competence mitigate that concern. The anonymized tissue source also meant detailed clinical phenotyping was unavailable, and the small, sex-unbalanced sample of six men and three women precluded formal analysis of age- or sex-specific responses. The short-term transcriptional data likewise cannot predict whether urolithin B’s activation of lipid-related pathways would be beneficial or, if chronically sustained in muscle, would contribute to the fatty infiltration associated with declining muscle quality. Still, the study sketches a compelling division of labor: urolithin A as a candidate for combating mitochondrial dysfunction and chronic inflammation in ageing muscle, and urolithin B as a potential ally for regeneration and repair after injury or surgery. The distinct yet complementary profiles raise the possibility of combination strategies targeting both the degenerative and regenerative faces of muscle decline, and they add a new layer to the growing science of the gut-muscle axis, in which the microbes of the colon may quietly shape how our muscles age.

Subject of Research: Transcriptomic effects of the gut microbial metabolites urolithin A and urolithin B on human skeletal muscle cells in relation to sarcopenia

Article Title: Exploring the Impact of Urolithins A and B on Muscle Health: A Transcriptomic Analysis in Human Myotubes

Article References: Henrotin, Y., Florin, A., Sanchez, C., Centonze, P., Coelho, T. P., Bekisz, S., Dubuc, J.-E., Galand, O., & Lambert, C. (2026). Exploring the Impact of Urolithins A and B on Muscle Health: A Transcriptomic Analysis in Human Myotubes. Journal of Cachexia, Sarcopenia and Muscle, 17(5), Article e70385. https://doi.org/10.1002/jcsm.70385

Image Credits: AI Generated

DOI: 10.1002/jcsm.70385

Keywords: urolithin A, urolithin B, sarcopenia, skeletal muscle, gut microbiota, ellagitannins, mitophagy, transcriptomics, myotubes, muscle regeneration, pomegranate polyphenols, inflammation