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Can a gut bacterium make you stronger?
Every few months a study arrives that sounds like a shortcut through the gym. This summer’s version came from the gut: a single species of bacterium, carried by some people and not others, linked to a stronger grip in older adults and to about thirty per cent more grip strength in mice that were given it.
It is a good study. It is also a study of 33 older people and 32 mice, and the headline it produced — a gut microbe that may keep you strong as you age — ran several steps ahead of what either group can show.
That gap is worth walking slowly, because the gut–muscle story is going to be sold to you soon, and it helps to know which parts are measured and which parts are still hope.
What the researchers actually found
The paper, led by Borja Martinez-Tellez with colleagues in the Netherlands and Spain, was published in Gut in March and picked up by the science press again in late August. It has two halves.
In the human half, the team analysed stool samples from 90 healthy young adults aged 18 to 25 and 33 older adults aged 65 to 75, and set the bacteria found there against strength and fitness tests: handgrip, leg press, bench press, and VO₂max. Among several Roseburia species, one stood out. Older adults in whom Roseburia inulinivorans could be detected had roughly 29% stronger grips than those in whom it could not. In the young group, more of it went with better grip, more leg and bench strength, and a higher VO₂max. It was also less common in the older group.
In the mouse half, the team first cleared out the animals’ gut bacteria with antibiotics, then gave them one of three Roseburia species, or nothing, repeatedly over eight weeks. Only R. inulinivorans changed anything. Those mice gained about 30% more forelimb grip strength, and their muscle fibres were larger, with a bigger share of the fast-twitch type used for short, forceful efforts.
Where the chain of evidence breaks
Read it as a chain, and count the links that are actually there.
Evidence chain
From “this bacterium goes with strength” to “take this to get stronger”
Node colours mark the kind of evidence, not its size. Sources: Martinez-Tellez et al., Gut 2026; Ni Lochlainn et al., Nature Communications 2024.
The first link is a correlation in a small group. People who carry a particular microbe differ from people who don’t in many ways that also shape grip: what they eat, how much they move, how much fibre and protein they get, their sex and body size, their general health. With 33 older people, one or two of those differences can produce a 29% gap on their own. The arrow could even point the other way. Exercise changes the gut microbiome, so stronger, more active people may simply grow more of this bacterium.
The second link is real but it is a mouse, and a particular kind of mouse: one whose own gut bacteria had been wiped out first. That makes the experiment clean. It also makes it unlike any human gut, which is crowded, competitive, and hard for a newcomer to join.
The third link is the most interesting, and it is where the story becomes less tidy. By the authors’ own account the bacteria did not settle in the mouse gut, and the short-chain fatty acids usually offered as the explanation — the butyrate story you may have read about — did not change in the gut. What did change was the muscle’s own chemistry, with the pathways that make the cell’s building blocks more active. So something happened, but not in the way the popular version of the gut–muscle axis suggests.
The fourth link — giving this bacterium to people and measuring their strength — has not been done. The authors say so clearly: human trials are the next step.
The trial that already tried the easy version
There is one human trial that comes close, and it is the part of this story that rarely makes the headlines.
In 2024, a team at King’s College London ran a placebo-controlled trial in 36 pairs of older twins. Every participant did resistance exercise and took a branched-chain amino acid supplement. In each pair, one twin also took a daily prebiotic powder — about 7.5 grams of inulin and fructo-oligosaccharides, the kinds of fibre that gut bacteria ferment — and the other took a look-alike placebo. After twelve weeks the prebiotic had clearly changed the gut, with more Bifidobacterium. Muscle strength and physical function did not differ between the twins. (Scores on one memory test did improve, which is its own interesting and separate question.)
Twins are the strongest design available for this kind of question, because they share genes and most of their upbringing. Feeding the gut did something measurable to the gut. It did not reach the muscle within three months. That doesn’t close the door on the gut–muscle axis. It does suggest the path from bacteria to strength, if it exists in people, is narrower than “eat fibre, get stronger.”
Why the idea is still worth taking seriously
None of this makes the field wishful thinking. The foundation is solid in animals. In 2019, a study in Science Translational Medicine showed that mice raised with no gut bacteria at all had less muscle and less well-formed connections between nerve and muscle. Giving them the gut bacteria of normal mice restored much of that. The gut clearly talks to muscle in some way.
What we don’t know is how much that conversation matters in a human who already has a full microbiome, trains, and eats a normal diet. That is a genuine open question, and the Roseburia paper is a useful, specific lead for anyone who wants to answer it. Specific leads are how science usually moves forward. But a lead is not a treatment, and it is certainly not a product.
Before the probiotic arrives
Here is the prediction: within a year or two, you will see “gut–muscle” or “strength” probiotics on a shelf. Some will name Roseburia. So a few things are worth keeping in mind.
There is no human evidence that swallowing this bacterium, or any other, makes you stronger. Roseburia is an oxygen-sensitive gut species, which makes it hard to keep alive in a capsule and hard to establish in a gut that already has tenants — and the mouse gut was emptied first. A product that says a microbe “builds muscle” is making a claim the science has not reached. As with most supplements marketed on a single exciting paper, the confidence of the label is the warning sign.
The honest use of this study is not a purchase. It is a better understanding of why people who train and eat well tend to have different, and probably more varied, gut communities — and why that may be one more piece of the same picture rather than a separate lever.
What actually keeps muscle strong
The things with human trial evidence behind them are less exciting and more reliable.
Resistance training is the main one. Two or more sessions a week, working large muscle groups with real effort, builds and keeps strength at any age; our guide to strength training after 40 covers the practical version. Enough protein, spread across the day, supports it — see who extra protein actually helps. For adults over 55, creatine is one of the few supplements with an authorised claim here: 3 g of creatine a day can enhance the effect of resistance training on muscle strength. That is why it sits in our creatine powder and not in a gut product.
Then eat a varied, plant-rich diet with plenty of fibre, because it is good for the gut in its own right, as we covered in fibre and the gut microbiome. Just don’t expect it to show up in your grip.
And if you want a number to watch, grip strength is a good one. It is cheap to measure, it tracks overall strength, and it is linked with healthy ageing in large studies — see what grip strength says about ageing. Test it every few months, with the same device and the same hand. Watch the trend, not the single reading.
The bottom line
A specific gut bacterium goes with stronger grips in a small group of people, and makes mice stronger when they receive it. That is a good reason for a human trial, not a reason to change what you do this week.
The one human trial that fed the gut found a changed microbiome and unchanged muscle. Until that changes, the most reliable route to strong muscle still runs through the muscle: load it, feed it, repeat. Let your gut bacteria enjoy the vegetables for their own sake.
