Roseburia inulinivorans, the Gut Microbiome and Muscle Strength

People who want a powerful grip usually think of training, protein shakes and sufficient sleep. A European research team now suggests there is another player involved: the gut microbiome. One particular bacterial species in the gut appears to be closely linked to muscle strength and physical performance, both in younger people and in older adults.

How researchers identified the ‘muscle bacterium’

The research began with a straightforward question: why do some people remain remarkably strong into old age, even though they are not necessarily more active than others? To investigate, the researchers analysed the gut microbiomes of 90 adults aged 18 to 25 and 33 people aged over 65.

Every participant completed standardised fitness assessments:

• hand-grip strength measured with a dynamometer
• a leg press to assess lower-body strength
• a bench press to measure upper-body strength
• a maximal oxygen uptake (VO₂max) test for endurance

At the same time, the team examined the composition of the gut microbiome using stool samples. Among the countless microorganisms present, one bacterial genus stood out: Roseburia. In particular, one species, Roseburia inulinivorans, showed a clear association with muscle strength.

Older adults in whom Roseburia inulinivorans was detected had, on average, around 29 per cent greater hand-grip strength than people of the same age without this bacterium.

The difference was substantial and could not readily be dismissed as chance. For VO₂max, a measure of endurance capacity, no distinct benefit emerged in the older group. Among younger participants, however, the pattern was different.

Younger gut microbiomes, stronger muscles: age-related differences

The researchers found considerably larger proportions of Roseburia inulinivorans in the guts of the 18-to-25-year-olds. In some samples, the species accounted for as much as 6.6 per cent of the whole bacterial community. Among older participants, the average proportion was only about 1.3 per cent.

In the younger group, higher levels of this bacterial species were associated with better performance in two areas:

• stronger hand-grip strength
• improved cardiorespiratory fitness

This indicates that the bacterium may be related not only to muscular strength, but also to the ability to cope with intense physical exertion. It was not yet direct proof, but it raised the strong possibility that the finding represented more than a statistical coincidence.

What happens in the body: the communication route between gut and muscle

To explore the cause more closely, the scientists moved into the animal laboratory. First, they treated mice with broad-spectrum antibiotics to clear out almost all of their gut bacteria. Some animals were then given different Roseburia species, while others formed the control group.

After eight weeks, the result was striking: mice given Roseburia inulinivorans increased the strength of their front paws by around 30 per cent compared with the control group. The researchers then examined the muscles in more detail.

They found:

• larger muscle fibres
• a higher proportion of fast-twitch type II fibres
• changes in proteins and enzymes involved in energy production

The muscles of the treated mice appeared to be “programmed” for short, explosive exertion such as sprinting or heavy lifting.

Fast-twitch type II fibres are particularly important for movements requiring high levels of force. Having a greater proportion of them can help in daily life with climbing stairs and carrying shopping bags, as well as in sport during sprinting or strength training.

How bacteria may affect muscles

How can a microorganism in the gut alter muscles in the arms and legs? A key part is played by metabolic products that bacteria create from dietary fibre. Many species in the Roseburia genus produce short-chain fatty acids such as butyrate, which can influence metabolism in the liver, fatty tissue and muscles.

Several mechanisms are possible:

• more efficient use of energy in muscle cells
• changes to gene activity, or gene expression, within muscle fibres
• dampening effects on low-level inflammatory processes that can weaken muscles
• effects on hormones and signalling molecules that regulate muscle growth

The study provides signs of such adaptations, but does not measure anywhere near all of the processes involved. In particular, the role of inflammation and nerve signalling between the gut and muscles remains unresolved.

A new perspective in the fight against age-related muscle loss

As people get older, many gradually lose muscle mass and strength. This process is called sarcopenia. It can lead to falls, frailty and, in severe cases, loss of independence. Until now, clinicians have mainly relied on two approaches: strength training and a protein-rich diet.

The data on the frequency of Roseburia inulinivorans now suggest that the gut may also have a role. The bacterial species appears to be less common during the period of life when sarcopenia becomes much more prevalent. This raises pressing questions:

• Is the decline of this bacterium a cause of muscle loss, or a consequence of it?
• Can the species be supplied deliberately through a probiotic?
• If so, does it remain in the gut over the long term, or disappear again?

In the mouse model, the researchers were not yet able to show that the bacterium establishes itself permanently in the gut. There are currently no data on this in humans either. What is clear is that the link between the microbiome and muscle strength appears consistent across different measurements.

The findings point to a type of “gut–muscle axis” that affects muscle metabolism and performance.

What people can do now - and what they cannot yet do

Anyone immediately searching for capsules containing Roseburia inulinivorans will be disappointed. A fully researched probiotic containing this species is not available on pharmacy shelves. The findings come from a limited group of people and from animal studies, so they do not yet amount to a treatment recommendation.

Even so, some practical steps may increase the likelihood of supporting a ‘friendly’ microbiome:

• A high-fibre diet: wholegrains, pulses, vegetables and nuts provide food for many beneficial gut bacteria.
• Regular physical activity: exercise has a proven effect on the gut microbiome while also strengthening muscles directly.
• Cautious use of antibiotics: they are essential when medically needed, but should not be used casually for every cold.
• Fermented foods: yoghurt, kefir, sauerkraut and similar foods contain living microorganisms that can influence the microbiome.

These measures do not guarantee an increase in this particular bacterial species. However, they generally support a diverse, stable gut microbiome, which may form the basis for muscles that age healthily.

What terms such as VO₂max and sarcopenia mean

Several technical terms from the study frequently appear in sports medicine and ageing research. Understanding them makes the results easier to interpret.

VO₂max: the maximum amount of oxygen the body uses

VO₂max refers to the maximum volume of oxygen the body can take up each minute. It is considered an important indicator of endurance and cardiovascular fitness. A higher value shows that the heart, lungs and muscles work efficiently together to use oxygen. The bacterium offered no apparent advantage here for older participants, whereas it did for younger people.

Sarcopenia: when muscle quietly disappears

Sarcopenia is the age-related loss of muscle mass and strength. It develops gradually, often over many years. People affected may first notice it when carrying heavy objects or rising from an armchair. Later, their risk of falling increases significantly. Sarcopenia is receiving increasing medical attention because it is a key factor in maintaining an independent life in older age.

If targeted changes to the microbiome could slow even part of this muscle decline, the consequences for geriatric medicine - and probably for ambitious recreational athletes too - would be enormous.

Looking ahead: probiotics for strength and performance?

The prospect is appealing: a tailored probiotic that introduces bacteria such as Roseburia inulinivorans into the gut, combined with dietary fibre as ‘food’ and an adjusted training plan. People at risk of sarcopenia, as well as athletes, could potentially fine-tune their muscle metabolism in this way.

However, many obstacles remain before that point is reached: questions of safety, long-term data, dosage and interactions with medicines. There is also the need to establish how substantial the effect is in real life, beyond laboratory settings and animal studies.

One thing is nevertheless becoming clear: muscles are not built only in the gym, but in the gut too. Anyone thinking about their strength in the decades ahead should therefore consider not just weights and protein, but also the trillions of fellow inhabitants of their digestive tract.