One gut bacterium boosts grip strength 30% in mice and tracks with muscle power in humans

Before this study, you might have guessed that gut bacteria influence digestion, immunity, perhaps even mood. Muscle strength would not have been high on the list. But a team of Dutch and Spanish researchers has identified a single bacterial species that appears to do exactly that, and the contrast with its close relatives makes the finding even more striking.

Roseburia inulinivorans, a species within the Roseburia genus, is positively associated with grip strength, leg press, bench press, and peak oxygen uptake in healthy young adults. In older adults, those with detectable R. inulinivorans in their stool had 29% higher handgrip strength than those without. When researchers gave the bacterium to mice, forelimb grip strength increased by approximately 30% within four weeks.

Its close relatives, R. faecis and R. hominis, showed no such association. R. intestinalis had a modest link to some strength measures. The specificity is unusual. Different species within the same genus appear to influence different aspects of muscle function, or none at all.

From stool samples to the weight room

The researchers analyzed stool samples from 90 healthy young adults (ages 18 to 25) and 33 older adults (ages 65 and above). Physical fitness was assessed through handgrip strength, leg press, bench press, and VO2 max (maximal oxygen consumption during exercise). Among all bacteria found in the samples, only the Roseburia group was positively associated with muscle mass and strength.

The age comparison was telling. R. inulinivorans was more abundant in younger adults, with proportions reaching up to 6.6%, compared with a maximum of 1.3% in older adults. This age-related decline tracks with the period of life when sarcopenia, or progressive muscle loss, becomes increasingly common.

Fast-twitch conversion in the calf

To test for a causal role, the team depleted the gut microbiome of 32 mice with antibiotics, then fed different groups one of three Roseburia species or no bacteria at all. After eight weeks, none of the species improved running endurance. But R. inulinivorans produced a noticeable and sustained increase in forelimb grip strength starting at week four.

The muscle changes went deeper than strength alone. Mice treated with R. inulinivorans showed larger muscle fiber sizes and a significantly higher proportion of type II (fast-twitch) fibers in the soleus muscle, the calf muscle typically dominated by slow-twitch fibers. Fast-twitch fibers are designed for short, intense movements like sprinting and weightlifting. The shift was accompanied by metabolic changes in proteins and enzymes key to muscle energy production.

The other Roseburia species did not produce the same fiber-type conversion, reinforcing that the effect is species-specific.

A gut-muscle axis

The findings add to an emerging body of evidence for a gut-muscle axis, a connection between intestinal bacteria and skeletal muscle function. The mechanism remains unclear. The human Roseburia species did not actually colonize the mouse gut, suggesting the bacteria may exert their effect through metabolites or signaling molecules rather than through sustained colonization. Specific pathways related to inflammation or neuromuscular signaling were not directly assessed.

The researchers suggest R. inulinivorans holds promise as a nutraceutical probiotic for treating age-related muscle wasting. If the bacterium's effects on muscle fiber type and strength translate to humans, it could offer a complementary approach to exercise and nutrition interventions for sarcopenia.

What the study cannot tell us

The human component is observational. Association does not mean causation. People who are stronger may have different diets or exercise habits that independently foster R. inulinivorans abundance. Long-term studies are needed to determine whether changes in the bacterium's abundance are a cause or a consequence of muscle function.

The mouse experiments provide stronger evidence for causation but involve only 32 animals, and the human bacterial species did not colonize the mouse gut. How the bacterium influences muscle from the intestine without establishing residence there is an open question.

Clinical trials in humans, testing whether supplementation with R. inulinivorans actually improves muscle strength in aging or sarcopenic populations, have not been conducted. The path from interesting mouse data to validated probiotic therapy is neither short nor certain.