Every year, millions of older people gradually lose muscle. This silent decline – sarcopenia – compromises their autonomy. Exercise and protein remain the classic remedies. But these solutions reach their limits in very fragile people. Spanish and Dutch researchers have just published a striking discovery in the journal Gut : a specific intestinal bacteria, Rosemary incliningwould be linked to significantly higher muscular strength.
This study is co-authored by Borja Martinez-Tellez and colleagues from the University of Granada, the University of Almeria and the Leiden University Medical Center in the Netherlands. It opens up a concrete avenue: acting on the microbiota to preserve physical strength over the years. This link between intestine and muscle is no longer a vague hypothesis – it is starting to have a name.
How researchers linked microbiota and muscle strength in humans
Before testing anything in animals, the researchers first looked for a clear signal in humans: are certain intestinal bacteria more frequent in stronger people? To seriously answer this question, the team built a precise protocol, without limiting itself to a simple food comparison or statistical intuition.
The study is based on 123 Spanish adults, divided into two age groups: 90 young people aged 18 to 25 and 33 older adults aged 65 to 71. The objective was twofold: to identify links between microbiota and physical performance. Then check if these links were maintained despite aging. This point is crucial, because muscle does not age in the same way depending on the individual.
Participants took several standardized assessments. The best known is grip strength, measured with a dynamometer. This test seems simple, but it is widely used in geriatric medicine. Indeed, it reflects the general functional state quite well. The scientists also measured lower limb strength with a leg press. And the upper body with a bench press. In some, they added a measurement of cardio-respiratory capacity.
Then, the researchers analyzed the fecal microbiota with metagenomic sequencing tools, capable of identifying the bacteria present with great finesse. This approach allows us to go beyond simple general categories such as “good” or “bad” bacteria. Here, the authors wanted to go down to the precise species.
This is how Roseburia inulinivorans appeared. Its presence was associated with better strength levels, especially for the grip, but also for other muscular parameters. According to Borja Martinez-Tellez, cited in The Conversationthe issue consisted of “ identify specific intestinal bacteria linked to fitness and muscle strength ».
Why this bacteria is linked to the Mediterranean diet and fiber
The link with the Mediterranean diet is based on the very biology of the bacteria studied. Roseburia inulinivorans is one of those gut microbes that thrives when provided with certain fermentable fibers. In short: this microbe does not develop because it is “good” in itself. But because it finds its nourishment in a varied plant diet.
The key word here is inulin. This fiber is present in several everyday foods, including certain vegetables, roots and edible plants. Once ingested, it escapes traditional digestion in the small intestine. It therefore arrives intact in the colon. Certain specialized bacteria can then use it as fuel. R. inulinivorans is precisely one of these species.
The Mediterranean diet, rich in legumes, fruits, vegetables, whole grains, olive oil, nuts and minimally processed foods, promotes high microbial diversity. It feeds bacteria capable of producing compounds useful to human metabolism. The subject is therefore not “eat Mediterranean to have more muscle”. It is more subtle: certain dietary habits shape a microbiota that could better support muscle function.
Concretely, fibers do not act as a doping substance. They modify an intestinal ecosystem, which can then influence other organs, including muscles.
As Matt Cooke, Australian researcher in sports nutrition and exercise physiology, recalls in Live Science« there are many things that affect the gut microbiota HAS”. This remark is essential. Age, physical activity, medications, sleep and even infections also modify this flora. Thus, nutrition could act on muscle strength not only through proteins or calories, but also through microbes capable of transforming fibers into useful biological signals.
Why the mouse experiment really changes the scope of discovery
In science, a correlation is impressive, but it is never enough. If a bacteria is more common in stronger people, it could mean several things. It helps muscles, or healthier people simply have it more often. To decide, it was necessary to move on to a controlled experiment.
The researchers therefore worked on mice whose microbiota had previously been disrupted by antibiotics. This choice made it possible to start from a simplified intestinal environment, then to introduce a targeted bacteria to observe its own effect. The animals then received different species of the Roseburia genus, in order to compare their respective effects.
The clearest result concerns Roseburia inulinivorans. After several weeks of colonization, the supplemented mice showed an increase of approximately 30% in their grip strength. This was a specific effect, not a simple general “boost” of the microbiota.
In other words, not all favorable intestinal bacteria are equal. Some seem linked to body mass, others to inflammation, still others to energy metabolism. Here, the observed effect relates to muscle function, and not to a vague indicator of good health.
This distinction has very concrete consequences. As we age, we can lose strength without losing a lot of muscle volume. This is what makes sarcopenia so difficult to spot at first. A muscle may seem “there” but function less well. However, it is precisely these losses of function that increase the risks of falls, dependence or hospitalization. In ScienceAlertthe authors emphasize that they were able to “ demonstrate for the first time a direct link between a specific bacteria and muscle strength ».
What this bacteria seems to do in muscle
Perhaps the most fascinating thing about this study isn’t that a bacteria improves strength. This is the way she seems to achieve it. Indeed, their analyzes suggest that Rosemary inclining acts on muscle quality. In the treated mice, scientists observed changes in the structure of muscle fibers. With in particular an orientation towards fibers more adapted to the production of rapid force. This is an important point, because not all muscles are created equal. Some are optimized for endurance, others for power.
With age, the most explosive and powerful fibers are often the first to decline. However, they are the ones that allow you to get up quickly, to recover from an imbalance or to react to a fall. If a bacteria helps preserve this functional profile, it could become a valuable tool against loss of autonomy.
The study also suggests finer metabolic changes, particularly in biochemical pathways. This bacteria could influence not only the structure of the muscle, but also the way in which it produces and uses its energy.
This is precisely what is of interest to aging medicine today. Because sarcopenia affects performance, coordination, contraction speed and the ability to recover. A microbial intervention could therefore, ultimately, aim for more than just a number on a scale.
But researchers remain cautious. Borja Martinez-Tellez essentially reminds us that it will still be necessary to demonstrate whether this approach can become a real therapeutic tool in humans. The avenue is promising, but it still needs to go through robust clinical trials.
Source: Martinez-Tellez B, Schönke M, Kovynev Aet al. “Roseburia
inulinivorans increases muscle strength†.
Gut Published Online First: 10 March
2026. doi: 10.1136/gutjnl-2025-336980
