Does Your Gut Bacteria Decide How Strong You Are?

Key takeaways

• Researchers identified a specific gut bacterium, Roseburia inulinivorans, that's linked to greater muscle strength in both humans and mice.
• Older adults who carried detectable levels of this bacterium had 29% greater handgrip strength than those without it, and the effect held even without a matching boost in cardiovascular fitness.
• In mice, adding the bacterium increased grip strength by about 30% and produced larger, more powerful muscle fibers, hinting at a genuine gut-muscle connection rather than a coincidence.

A microbe that stood out from the crowd

Your gut is home to hundreds of bacterial species, and most research on the gut-muscle connection has stayed pretty general — more diversity tends to look better, less diversity tends to look worse. A new study out of the Netherlands and Spain took a more targeted approach, looking at whether specific bacterial species, not just overall gut diversity, tracked with muscle strength and performance.

Researchers analyzed stool samples and fitness measures, including handgrip strength, leg press, bench press, and VO2 max, in 123 people split between a younger group (18 to 25) and an older group (65 and up). Out of everything they measured, one genus, Roseburia, stood out as the only one positively associated with both muscle mass and muscle strength. But it didn't stop there. Within that genus, one specific species did the heavy lifting.

The species that actually mattered

Not every Roseburia species behaved the same way, which turned out to be the most interesting part of the story. Roseburia faecis and Roseburia intestinalis weren't meaningfully tied to handgrip strength or VO2 max in either age group. Roseburia inulinivorans was different. Among older adults, people with detectable levels of it had 29% greater handgrip strength than those without it, and this showed up without any accompanying increase in peak oxygen uptake — meaning it looked like a strength-specific effect, not just a general fitness effect.

Among younger adults, higher levels of R. inulinivorans tracked with stronger handgrip and higher VO2 max, and higher relative abundance of both R. inulinivorans and R. intestinalis lined up with better leg press and bench press performance. The researchers also noticed that this specific bacterium was less abundant in older participants than in younger ones, raising the question of whether its decline over time might be part of why muscle strength tends to slip with age.

Testing it directly in mice

Correlation in humans is one thing; the researchers wanted to see if giving the bacterium directly would actually change muscle function. They depleted the gut bacteria of 32 mice with antibiotics, then gave three groups different Roseburia species once a week for eight weeks, while a fourth group got none.

Mice given R. inulinivorans showed roughly a 30% increase in forelimb grip strength compared to the control group, an improvement that showed up at 4, 6, and 8 weeks of treatment. Their muscle fibers were also larger, and they had a notably higher proportion of type II, or fast-twitch, fibers — the type used for short bursts of power like sprinting or lifting. Interestingly, none of the Roseburia species improved how long the mice could run before tiring out, which suggests this bacterium's effect is specifically about strength and power, not general endurance.

What might be happening under the hood

The researchers found changes in proteins and enzymes tied to the metabolic processes that generate energy for muscle activity, suggesting R. inulinivorans is doing something at the level of muscle metabolism itself, not just correlating with strength by coincidence. They're calling this a gut-muscle axis: a two-way communication line between gut bacteria and muscle tissue that could influence how muscle fibers are built and maintained.

There are still open questions. The researchers didn't directly test whether inflammation or nerve-signaling pathways played a role, and it's not yet clear whether having more of this bacterium causes stronger muscles, or whether people with stronger muscles simply create better conditions for it to thrive. Long-term studies will need to sort out that direction.

The takeaway

Muscle strength is one of the most consistently useful markers for tracking healthy aging, and this study adds an unexpected variable to the list of things that might influence it: your specific gut bacterial makeup, not just what you eat or how much you exercise. R. inulinivorans is now being discussed as a potential future probiotic candidate for supporting strength as we age, though it's still early days. For now, it's a good reminder that gut health and muscle health may be more connected than most people realize, and a space worth watching as this research develops.

References:

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, Patrick CN Rensen. Roseburia inulinivorans increases muscle strength. Gut, 2026. DOI: 10.1136/gutjnl-2025-336980