Scientists have found a positive correlation between the abundance of the bacterium Roseburia inulinivorans in the gut and muscle strength in mice and humans, although the mechanism behind it is still unclear [1].
Can bacteria mimic exercise?
As we age, we lose muscle mass and strength. This decline is a major driver of frailty, disability, and poor health outcomes in older adults [2]. Exercise and nutrition are the best-known countermeasures, but they have limits, especially in people who are too frail or ill to exercise effectively. This is why researchers are on a hunt for exercise mimetics, therapies that recapitulate some benefits of exercise without the need to actually flex muscles. The need for such therapies has only grown after the introduction of Ozempic and other GLP-1 receptor agonists, which have been shown to cause a concerning lean mass loss alongside weight loss [3].
Over the past decade, researchers have discovered that the gut microbiome does far more than help digest food. It produces a myriad of molecules that influence metabolism, inflammation, and tissue function throughout the body, including muscle. However, no specific bacterial species had been causally linked to muscle strength in humans or animals.
The one bug that could
To bridge this crucial gap, scientists from the University of Almería and the University of Granada, together with researchers from Leiden University Medical Center (LUMC, Netherlands), started by taking stool samples from two human cohorts of 33 older adults and 90 young adults. The samples were analyzed and bacterial DNA sequenced. Microbiome composition was then cross-referenced with two metrics of physical performance: handgrip strength and maximal oxygen consumption during exercise (VO₂ peak), which measures cardiorespiratory fitness.
The researchers focused on the genus Roseburia, which initially showed positive associations with muscle-related outcomes. They then drilled down to the species level, comparing three Roseburia species: R. inulinivorans, R. faecis, and R. intestinalis. In older adults, those who had detectable R. inulinivorans in their stool showed 29% higher handgrip strength compared to those without it, with no corresponding difference in VO₂ peak. The other two species showed no significant association with handgrip strength.
In young adults, higher R. inulinivorans abundance was positively associated with both handgrip strength and VO₂ peak. R. inulinivorans and R. intestinalis also correlated with leg press and bench press strength. Importantly, the authors found no significant correlation between Roseburia abundance and dietary intake (energy, carbohydrate, fat, protein, or fiber), reducing the likelihood that diet was a confounder.
To move from correlation to causation, the authors gave live Roseburia bacteria to mice and measured whether it changed muscle strength. Thirty-two male mice (6 weeks old) were first treated with a broad-spectrum antibiotic cocktail for 2 weeks to deplete their native gut bacteria. Mice were then randomized into four groups (eight mice in each): vehicle control, R. faecis, R. intestinalis, or R. inulinivorans, delivered three times per week for 8 weeks.
None of the Roseburia species improved running time to exhaustion (an endurance/cardiorespiratory measure). However, R. inulinivorans produced a remarkable 30% increase in forelimb grip strength. This effect persisted even after correcting for lean body mass, meaning it was not simply because the mice were bigger. Mice receiving R. inulinivorans also had a larger muscle fiber cross-sectional area (CSA) compared to controls.
Interestingly, R. inulinivorans treatment shifted the soleus muscle toward a higher proportion of type II (fast-twitch) fibers relative to type I (slow-twitch) fibers. Type II fibers are associated with power and strength output, while type I fibers are more endurance-oriented. This finding squared well with the results obtained in the human cohort (increased muscle strength but not endurance).
Going after the mechanism
Since Roseburia species are well-known producers of butyrate, a short-chain fatty acid (SCFA) that has anti-inflammatory and metabolic signaling roles, the obvious hypothesis was that R. inulinivorans was boosting butyrate levels. The authors measured SCFAs in the cecal content and found no significant differences across the groups; butyrate was not the answer.
They then profiled amino acids and found that mice treated with R. inulinivorans showed the most dramatic shifts: cecal levels of methionine, leucine, isoleucine, alanine, valine, and lysine were all markedly reduced compared to controls. This looked like a paradox: why would a decrease in amino acid abundance lead to an increase in muscle strength?
Further experiments, which included a broad sweep of all detectable small molecules (untargeted metabolomics) on plasma and skeletal muscle from the mice, revealed that R. inulinivorans was associated with a much more pronounced shift in metabolites than other Roseburia species, including in those related to purine metabolism. Purines form the building blocks of DNA/RNA and are crucial for energy (ATP) and metabolism.
While the team clearly documented the metabolic changes, the full mechanism connecting them remains hypothetical. Roughly, when R. inulinivorans depletes amino acids in the gut, the host may compensate by prioritizing amino acid allocation to metabolically important tissues like muscle. Meanwhile, the muscle activates purine pathways to support nucleotide production and energy supply under amino acid-limited conditions, essentially becoming more effective. More research is needed to confirm this intriguing hypothesis.
Finally, the authors compared R. inulinivorans abundance between age groups. In their own cohorts, older adults (65+) had significantly lower R. inulinivorans than young adults (18-25). To validate this, they analyzed a dataset of 3,512 fecal metagenomes from healthy individuals. In that dataset, adults (18-65) had slightly higher R. inulinivorans than older adults, with no significant differences for the other two species. On the other hand, a meta-analysis incorporating all publicly available cohorts did not reach statistical significance for any Roseburia species, though the effect size for R. inulinivorans trended negative.
“Taken together, our findings provide solid evidence confirming the existence of a gut-muscle axis in which this identified bacterium positively modulates muscle metabolism and muscle strength,” said Jonatan Ruiz, professor in the Department of Physical Education and Sport at the UGR and researcher at the Joint University Institute for Sport and Health (iMUDS).
Literature
[1] Martinez-Tellez, B., Schönke, M., Kovynev, A., Garcia-Dominguez, E., Ortiz-Alvarez, L., Verhoeven, A., … & Rensen, P. C. (2026). Roseburia inulinivorans increases muscle strength. Gut.
[2] Cruz-Jentoft, A. J., Bahat, G., Bauer, J., Boirie, Y., Bruyère, O., Cederholm, T., … & Zamboni, M. (2019). Sarcopenia: revised European consensus on definition and diagnosis. Age and ageing, 48(1), 16-31.
[3] Wilding, J. P., Batterham, R. L., Calanna, S., Davies, M., Van Gaal, L. F., Lingvay, I., … & Kushner, R. F. (2021). Once-weekly semaglutide in adults with overweight or obesity New England Journal of Medicine, 384(11), 989-1002.
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