Scientists have identified a gut microbe that appears to be associated with stronger muscles and better fitness, with levels declining as people age.
What if some of the microbes living in your gut could help determine how strong your muscles are? New research suggests that one bacterial species, Roseburia inulinivorans, may play an important role in muscle strength, raising the possibility that the gut microbiome could one day become a target for preserving physical function as people age.
In a study published online in the journal Gut, researchers found that R. inulinivorans was associated with greater muscle strength in people and improved muscle performance in mice. The bacterium also appeared to alter muscle metabolism and promote a higher proportion of fast-twitch (type II) muscle fibers, which are specialized for short bursts of powerful movement such as sprinting, jumping, and weightlifting.
The findings are particularly significant because age-related loss of muscle mass and strength, known as sarcopenia, is a leading contributor to frailty, disability, and loss of independence in older adults. With few effective treatments currently available, the Dutch and Spanish researchers suggest that R. inulinivorans could represent a promising probiotic candidate for helping maintain muscle health.
Scientists have increasingly recognized that gut microbes influence far more than digestion, affecting processes linked to metabolism, immunity, brain health, and cardiovascular disease. Emerging evidence also points to a “gut-muscle axis,” prompting researchers to investigate whether specific bacterial species might contribute to muscle strength and performance—and how they might do so.
One bacterium tracks strength
To investigate this, they analyzed stool samples from 90 healthy young adults (18-25 year olds) and 33 older adults (65+) to identify the bacteria present.
Physical fitness was assessed using hand grip strength, leg press and bench press performance, and VO2 max (maximal oxygen consumption during exertion), which is an indicator of cardiorespiratory fitness.
Of all the bacteria detected in the stool samples, only the Roseburia group (genus) showed a positive association with muscle mass and strength.
The species R faecis and R intestinalis were not significantly linked to hand grip strength or VO₂ max in either age group.
Among older adults, however, those with detectable R inulinivorans in their stool had 29% higher handgrip strength than those without the bacterium. This increase occurred without a corresponding rise in peak oxygen uptake, suggesting better fitness.
In younger adults, higher levels of R inulinivorans were positively linked to both handgrip strength and VO₂ max. The relative abundance of both R inulinivorans and R intestinalis was also positively associated with leg press and bench press strength.
By contrast, the relative abundance of R faecis and R hominis was not associated with any muscle strength measure. The researchers say this suggests that different Roseburia species may affect different aspects of muscle strength.
Roseburia was also more common in young adults. In this group, the proportion of R faecis ranged from 0% to 3.3%; R intestinalis ranged from 0% to 5.5%; and R inulinivorans ranged from 0% to 6.6%.
Among older adults, the proportion of R faecis ranged from 0% to 2.2%; R intestinalis ranged from 0%-0.7%, while R inulinivorans ranged from 0% to 1.3%.
Mouse tests suggest causality
To explore whether Roseburia might play a causal role in muscle strength, the researchers gave Roseburia species once a week for 8 weeks to 32 mice whose gut microbiomes had first been depleted with a cocktail of antibiotics for two weeks.
The mice were randomly divided into one of 4 groups. Three groups received a different Roseburia strain, while one group received no Roseburia (control group).
None of the Roseburia species improved how long the mice could run before exhaustion. Compared with the control group, however, R inulinivorans produced a clear increase of about 30% in forelimb grip strength, a proxy for muscle function, after 4, 6, and 8 weeks of treatment.
Mice treated with R inulinivorans also had larger muscle fiber size and a significantly higher proportion of type II (‘fast twitch’) fibers in the soleus muscle located in the calf, compared with the other groups. This difference was not significant when compared with mice treated with R intestinalis.
A closer look at fiber size distribution showed that fibers were distributed relatively evenly in the control group. In mice treated with R inulinivorans, larger fibers were more common than in mice treated with the other Roseburia species or in control mice.
These muscle changes were accompanied by shifts in proteins and enzymes that are important for muscle energy production.
Probiotic promise remains early
The researchers note several limitations. In the animal study, none of the human Roseburia species colonized the mouse gut. The researchers also did not directly assess specific pathways related to inflammation or neuromuscular signaling, even though these may have played a role.
Long-term studies are needed to determine whether changes in R inulinivorans abundance cause changes in muscle function or occur as a result of them, the researchers emphasize.
Nevertheless, they suggest: “Collectively, our findings provide robust evidence supporting a gut-muscle axis in which R inulinivorans positively modulates muscle metabolism and muscle strength.”
And they conclude: “Additionally, we observed that the relative abundance of R inulinivorans is lower in older adults than in young adults. Its abundance appears to decline with advancing age, a period during which the prevalence of sarcopenia [muscle loss] increases, suggesting a potential role for R inulinivorans as a probiotic candidate for preserving muscle strength.”
Reference: “Roseburia inulinivorans increases muscle strength” by Borja Martinez-Tellez, Milena Schönke, Artemiy Kovynev, Esther Garcia-Dominguez, Lourdes Ortiz-Alvarez, Aswin Verhoeven, Ranko Gacesa, Arnau Vich Vila, Quinten Raymond Ducarmon, David Jimenez-Pavon, Maria del Carmen Gomez-Cabrera, Rinse K Weersma, Wiep-Klaas Smits, Martin Giera, Jonatan R Ruiz and Patrick CN Rensen, 10 March 2026, Gut.
DOI: 10.1136/gutjnl-2025-336980
Funding: Health Research Fund of the Carlos III Health Institute (Instituto de Salud Carlos III); Society Challenges Program (Retos de la Sociedad); European Regional Development Fund (ERDF).
Disclosure: BMT, MS, LOA, JRR and PCNR are inventors of the international patent “Improvement of Muscle Mass and Strength” (WO2024025418A1), derived from this work.
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