Scientists have linked a specific gut bacterium, Roseburia inulinivorans, to stronger muscles in both humans and mice. Older adults who carried it had 29% greater handgrip strength, while treated mice gained about 30% more grip strength and developed larger, more fast-twitch muscle fibers. The bacterium was also less common in older people. Researchers say it could eventually become a probiotic candidate for helping combat age-related muscle loss.
A species of gut bacteria called Roseburia inulinivorans is associated with greater muscle strength in people and improved muscle performance in mice, according to research published online in the journal Gut.
The findings suggest that R inulinivorans can alter metabolic activity inside muscle and increase the proportion of fast-twitch (type II) fibers, which are important for short bursts of intense activity such as sprinting and weightlifting.
Researchers from the Netherlands and Spain say the bacterium could potentially be developed into a nutraceutical probiotic aimed at age-related muscle wasting.
The Gut Microbiome and Muscle Strength
Loss of muscle mass and strength can contribute substantially to frailty, declining mobility and physical function, and poorer health. These problems become particularly important with advancing age and in people living with long-term conditions.
Gut bacteria have already been connected with numerous aspects of health, including metabolic, neurodegenerative, and cardiovascular diseases. Growing evidence also points to a possible role for the gut microbiome in controlling muscle mass and function.
The researchers therefore set out to determine whether particular species of gut microbes were linked to muscle strength and, if so, to investigate how those bacteria might influence muscle tissue.
They analyzed stool samples from 90 healthy young adults (18-25 years old) and 33 older adults (65+) to identify the bacteria living in their guts.
Physical fitness was measured using hand grip strength, leg press and bench press performance, along with VO2 max (maximal oxygen consumption during exertion), a measure of cardiorespiratory fitness.
One Group of Bacteria Stood Out
Of all the bacteria detected in the stool samples, Roseburia was the only group (genus) that showed a positive association with both muscle mass and strength.
However, individual Roseburia species did not all show the same relationships. R faecis and R intestinalis were not significantly associated with hand grip strength or VO2 max in either age group.
Among the older participants, those with detectable levels of R inulinivorans had 29% greater handgrip strength than those in whom the bacterium could not be detected. This difference occurred without a corresponding increase in peak oxygen uptake, indicating better fitness.
In younger adults, higher levels of R inulinivorans were associated with both stronger handgrip and higher VO2 max. Greater relative abundance of both R inulinivorans and R intestinalis was also associated with better performance on the leg press and bench press.
By contrast, levels of R faecis and R hominis were not associated with any of the measures of muscle strength. The researchers say this suggests that different Roseburia species may affect different components of muscular performance.
Roseburia Levels Decline With Age
Roseburia bacteria were generally more abundant among younger participants. In this group, the proportion of R faecis ranged from 0% to 3.3%, R intestinalis from 0% to 5.5%, and R inulinivorans from 0% to 6.6%.
Levels were lower among the older adults. The proportion of R faecis ranged from 0% to 2.2%, R intestinalis from 0%-0.7%, and R inulinivorans from 0% to 1.3%.
To investigate whether Roseburia might directly influence muscle strength rather than simply being associated with it, the researchers conducted an experiment involving 32 mice.
Before the experiment, the mice received a cocktail of antibiotics for two weeks to deplete their gut microbiome. They were then given Roseburia species once a week for 8 weeks.
The animals were randomly divided into four groups. Three groups received different Roseburia strains, while the fourth received no Roseburia and served as the control group.
Muscle Strength Increased in Mice
None of the Roseburia species increased how long the mice could run before reaching exhaustion.
Muscle strength told a different story. Compared with the control animals, mice receiving R inulinivorans showed an increase of around 30% in forelimb grip strength, which the researchers used as a measure of muscle function. The improvement was observed after 4, 6, and 8 weeks of treatment.
Mice treated with R inulinivorans also developed larger muscle fibers and had a significantly greater proportion of type II (âfast twitchâ) fibers in the soleus muscle of the calf compared with the other groups. However, this difference was not significant when the R inulinivorans mice were compared specifically with those treated with R intestinalis.
A closer look at muscle fiber sizes showed a relatively even distribution among the control mice. Animals treated with R inulinivorans, meanwhile, had a greater proportion of larger fibers than mice given the other Roseburia species or no Roseburia.
The physical changes in the muscles were accompanied by alterations in proteins and enzymes involved in the metabolic processes that produce energy for muscle activity.
Important Questions Remain
The researchers caution that the study has several limitations. In the mouse experiment, none of the human Roseburia species permanently colonized the animalsâ guts.
The researchers also did not directly examine certain pathways involving inflammation or neuromuscular signaling, either of which could have contributed to the observed effects.
Long-term research will also be necessary to establish whether changing levels of R inulinivorans actually cause changes in muscle function or whether the abundance of the bacterium changes as a consequence of differences in muscle function.
Despite those uncertainties, the researchers suggest: âCollectively, our findings provide robust evidence supporting a gut-muscle axis in which R inulinivorans positively modulates muscle metabolism and muscle strength.â
They conclude: âAdditionally, 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.â
Paywalled Source:
Roseburia inulinivorans increases muscle strength . Gut , 2026; gutjnl-2025-336980 DOI: 10.1136/gutjnl-2025-336980