Handgrip strength is one of the simplest tests a doctor can run, and it keeps proving to be one of the most telling.
Now a study published in the journal Gut in March 2026 has tied grip strength in older adults to something you cannot feel or squeeze at all, a single species of gut bacterium called Roseburia inulinivorans.
Older adults who carried detectable levels of the microbe had 29% greater handgrip strength than peers of the same age in whom it could not be detected, according to the research from a team led by the University of Almería, the University of Granada and Leiden University Medical Center.
The bacterium was also less common in the older group than in young adults, hinting at an age-related decline in the microbe alongside the well-known age-related decline in muscle.
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What the study actually did
The researchers compared two human groups, 90 healthy adults aged 18 to 25 and 33 adults aged 65 and over.
In young adults, higher levels of R. inulinivorans tracked with stronger handgrip and higher peak oxygen uptake. In the older group, the presence of the bacterium alone was enough to separate stronger from weaker.
To probe whether the link might be more than coincidence, the team then ran a mouse experiment.
Animals given the bacterium gained roughly 30% more forelimb grip strength, developed larger muscle fibers and shifted toward the type II "fast twitch" fibers used in short, intense efforts like lifting or sprinting.
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According to a statement from Leiden University Medical Center, the bacteria living in the intestines help process nutrients and regulate inflammation.
Patrick Rensen, professor of endocrinology at Leiden, described the mouse result as striking, with the treated animals becoming around 30 percent stronger.
The authors caution that the human data are observational and that the mouse work, while suggestive, does not by itself prove the microbe causes muscle gains in people.
Human strains did not permanently colonize the mice, and some of the underlying biological pathways were not measured directly.
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Why grip strength matters for longevity
Grip strength is not just a proxy for how well you can open a jar.
In a prospective study of 121,383 adults across 28 countries, published in the journal Age and Ageing in 2022, researchers found a linear inverse link between handgrip strength and all-cause mortality, with thresholds around 42 kg for men and 25 kg for women.
Older adults in the top third of grip strength had a fraction of the death rate of those in the bottom third over a median follow-up of 7.4 years.
That is why sarcopenia, the gradual loss of muscle mass and strength with age, is treated as a serious clinical concern rather than a cosmetic one.
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Weaker grip in later life is tied to more falls, slower recovery from illness and shorter life expectancy.
What feeds this microbe
The species name is a clue. R. inulinivorans is a butyrate-producing gut bacterium that thrives on inulin, a soluble fiber found in many everyday plants.
Chicory root is the richest known source, but inulin also shows up in Jerusalem artichokes, garlic, onions, leeks, asparagus and bananas.
No study has yet shown that eating more of these foods will raise your grip strength through this specific microbe.
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But a fiber-rich diet is already recommended by health authorities for gut and cardiovascular health, and this work adds muscle to the list of tissues that may benefit indirectly.
Borja Martínez-Téllez of the University of Almería, who led the work, noted in a statement from the University of Granada that the finding opens the possibility of using the bacterium as a probiotic to help preserve muscle strength during aging.
A commercial probiotic based on R. inulinivorans does not yet exist, and the authors describe it as a candidate for future development rather than a current recommendation.
For readers over 60 who want to act on evidence that already exists, the same journal has previously highlighted protein at breakfast and omega-3 intake as levers for holding onto muscle with age.
Adding more fiber-rich vegetables is a cheap, safe next step while the microbiome research matures.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.