People with stronger muscles are more likely to harbor a particular species of bacteria in their guts, and when this bacterial species was fed to mice, they became stronger, a new study finds.
The study authors say the microbe has the potential to be part of a probiotic supplement, potentially boosting muscle strength. However, this would require the researchers to find a way to preserve it in a pill. What’s more, this microbe could serve as a drug to treat frailty in the elderly, assuming future clinical trials in humans reveal the microbe safely improves muscle strength, said study lead author Borja Martinez-Tellez, a sports scientist at Leiden University in the Netherlands.
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In the new study, which was published March 10 in the journal Gut, researchers searched for gut bacteria correlated with muscle strength. They took fecal samples from 90 young participants, ages 18 to 25, and 33 older participants, ages 65 to 71, and asked them to perform strength-training exercises.
Both groups were told to squeeze a handheld device to test their handgrip strength. The researchers found that the participants from both age groups with firmer grips tended to have higher counts of the bacterium Roseburia inulinivorans in their feces. This species helps to digest fibers and typically colonizes the guts of people who consume a Mediterranean diet, which typically includes lots of fruits, vegetables, omega-3-rich fish and extra virgin olive oil.
We are able to demonstrate for the first time that there is a direct link between one bacterium and muscle strength
Borja Martinez-Tellez, sports scientist at Leiden University
The young participants were also asked to perform leg-press and bench-press exercises. Those who could lift heavier loads for 10 reps without failing had a greater abundance of gut-dwelling R inulinivorans in addition to another related bacterial species called Roseburia intestinalis, compared with the young participants who had lower strength scores.
However, the link between higher amounts of these microbes and higher load-bearing ability could be explained by other factors. For example, people who strength-train might have an overall healthier lifestyle that involves eating foods that influence their gut microbiome, thereby acquiring more of these bacteria. To determine if R. inulinivorans could actually improve muscle strength, the researchers used a feeding tube to administer various Roseburia species to mice.
Grip strength in mice was assessed using a miniature pull-up bar. Those given R. inulinivorans for four to eight weeks had 30% higher grip strength than other mice. Although this bacterial species enhanced strength, it didn’t correlate with muscle growth. Mice given another related species called Roseburia faecis, on the other hand, developed more muscle muscle mass.
Next, the team aimed to figure out how these bacteria were linked to mightier mice. They found that R. inulinivorans caused muscle architecture to change: Muscles that were previously richer in type-I (“slow-twitch”) fibers, which contribute to endurance exercises like scurrying in mice or long-distance running in humans, had remolded into type-II (“fast-twitch”) fibers, which are pivotal to strength.
“We are able to demonstrate for the first time that there is a direct link between one bacterium and muscle strength,” Martinez-Tellez said.
More research is needed in humans to confirm these animal findings
Matt Cooke, nutrition researcher at La Trobe University
Although these findings are promising, Cooke noted that the mice’s gut microbiomes were cleared out with antibiotics before they were given the Roseburia species, so it’s not clear whether this depletion affected muscle strength.
The study researchers have filed a patent for using R. inulinivorans to boost muscle strength. They are hoping to develop a probiotic that could boost performance in the gym or, if encouraging clinical trial data on its safety and efficacy emerges, use it as a drug to prevent the onset of frailty in elderly people, Martinez-Tellez told Live Science. “This is not going to be a substitute for exercise training,” he said. “It’s more of a complement.” More research is needed to confirm that such pill-form bacteria could remain in the human gut for long enough to sustain muscular improvements.
It will still be many years before this bacterium could be used by the public. “The problem that we have with this bacterium is that it is strictly anaerobic, which means that if they are exposed for five seconds to oxygen, they die,” Martinez-Tellez said. He and his colleagues will need to develop an air-tight pill that preserves the shelf life of this species.
“More research is needed in humans to confirm these animal findings,” Cooke said, adding that further studies should involve supplementing people with these bacteria and observing the effects on their muscles and how safe they are to take.
Down the line, Martinez-Tellez wants to explore whether other gut residents could have a similar effect on physical performance, broadening the possibilities of boosting brawn through the gut.
This article is for informational purposes only and is not meant to offer medical advice.
Martinez-Tellez, B., Schönke, M., Kovynev, A., Garcia-Dominguez, E., Ortiz-Alvarez, L., Verhoeven, A., Gacesa, R., Vila, A. V., Ducarmon, Q. R., Jimenez-Pavon, D., Del Carmen Gomez-Cabrera, M., Weersma, R. K., Smits, W., Giera, M., Ruiz, J. R., & Rensen, P. C. (2026). Roseburia inulinivorans increases muscle strength. Gut, gutjnl-2025. https://doi.org/10.1136/gutjnl-2025-336980
