Gut microbe *Roseburia inulinivorans* may boost muscle strength

New research suggests that a particular microbe living in the human gut may increase muscle strength.

The intestines are home to vast numbers of microscopic organisms with an enormous effect on health. Together, these organisms are known as the gut microbiome, and they are involved in many bodily functions, including digestion, metabolism, immune regulation, sleep, neurodevelopment and brain activity.

A new study points to another unexpected benefit apparently provided by at least one intestinal microbial species: enhanced muscle strength.

Gut microbiome and muscle strength

As well as revealing this notable connection between gut flora and muscle performance, the researchers identified a bacterium in the Roseburia genus as the organism behind it.

Although evidence has increasingly shown the wide-ranging influence of gut microbes, the authors say there had previously been little evidence that they affect muscle strength. No bacterial species had been identified as a direct regulator of muscle function.

The discovery strengthens the argument for a “gut-muscle axis”, comparable with the better-known gut-brain axis, according to co-author and exercise physiologist Jonatan Ruiz of the University of Grenada in Spain.

“Taken together, our findings provide solid evidence confirming the existence of a gut-muscle axis in which this identified bacterium positively modulates muscle metabolism and muscle strength,” Ruiz says.

To investigate links between particular gut flora and muscle strength, the team analysed stool samples from two groups: 90 young adults aged 18 to 25, and 33 older adults aged 65 or over.

Every participant described their lifestyle as relatively sedentary, exercising for under 20 minutes on fewer than three days each week. They had not smoked and had kept a stable body weight during the preceding three months.

The volunteers completed detailed assessments of muscle power, including tests of hand-grip, leg and upper-body strength. Their maximum oxygen consumption was also measured to assess cardiorespiratory capacity.

Roseburia inulinivorans linked with stronger muscles

The stool samples showed substantial microbial diversity. However, bacteria belonging to the genus Roseburia - previously associated with muscle strength - were notable for their positive correlations with “muscle-related outcomes”.

Some Roseburia species appeared unrelated to the measurements examined in this research, while others were linked to at least some muscle tests to varying degrees. Roseburia intestinalis, for instance, seems to be associated with leg and upper-body strength among young adults.

One species particularly attracted the researchers’ interest. In people, the relative abundance of Roseburia inulinivorans was positively linked with several strength measures, including hand grip, leg press and bench press.

The study found that older adults with this microbe in their stool had hand-grip strength almost 30 per cent greater than comparable participants in whom it was absent.

Young adults also appear to benefit: a greater prevalence of R. inulinivorans was associated both with stronger grip and increased cardiorespiratory capacity.

The research additionally included experiments in mice to test causality and examine the mechanisms behind the associations found in humans.

Researchers first used antibiotics to reduce the mice’s existing gut flora. They then replenished it with bacteria from human intestines, administered weekly over eight weeks.

They report that R. inulinivorans produced a “remarkable increase” in the rodents’ forelimb grip strength, raising this indicator of muscle function by around 30 per cent compared with the control group.

Effects on muscle fibres and metabolism

Beyond improvements in muscle performance, mice that received the bacterium developed more fast-twitch muscle and larger muscle fibres in the soleus, a key muscle at the rear of the lower leg.

The results indicate that R. inulinivorans may produce these effects by changing amino-acid metabolism, triggering the purine and pentose phosphate pathway in muscle, and encouraging muscle-fibre hypertrophy alongside a move towards fast-twitch fibres.

Further studies are required, but the researchers say this work could support the future development of probiotics to help people retain strength and physical fitness as the advantages of youth diminish.

“This opens up the possibility that the bacterium under investigation could be used as a probiotic to help preserve muscle strength during aging,” says co-author and endocrinologist Borja Martínez Téllez from the University of Almería in Spain.

The study was published in Gut.