A specific microbe found in the human gut appears to be able to boost muscle strength, new research suggests.
Your intestines teem with tiny creatures that wield outsized influence on your health. Collectively called the gut microbiome, they play key roles in a variety of bodily processes, from digestion and metabolism to immune modulation, sleep, neurodevelopment, and brain function.
In a new study, researchers highlight another surprising service that seems to be offered by at least one species of intestinal microbe: improving your muscle strength.
In addition to demonstrating this intriguing relationship between gut flora and muscle function, the researchers also identified a specific bacterium from the genus Roseburia as the microbe responsible.
Until now, despite mounting evidence of gut microbes' broad reach, there were few indications of their influence on muscle strength, the authors note, and no bacterial species had been identified as direct modulators of muscle function.
By identifying this link, the researchers bolster the case for the existence of a "gut-muscle axis," akin to the more famous gut-brain axis, explains co-author and exercise physiologist Jonatan Ruiz from 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 search for associations between specific gut flora and muscle strength, the researchers examined stool samples from two groups of human subjects: 90 young adults (18 to 25 years old) and 33 older adults (65 or above).
All participants reported fairly sedentary lifestyles, with less than 20 minutes of exercise on fewer than three days per week. They also maintained a stable body weight over the prior three months, and didn't smoke.
Study subjects underwent extensive measurements of muscle power, including tests designed to assess hand grip, leg, and upper body strength. Researchers also tested participants' maximum oxygen consumption as a measure of cardiorespiratory capacity.
Stool samples contained rich microbial biodiversity, but bacteria from the genus Roseburia – which has previously been linked to muscle strength – stood out due to positive correlations with "muscle-related outcomes."
While some Roseburia species seem unconnected to the metrics covered in this study, others showed varying associations with at least some of the muscle tests. Roseburia intestinalis, for example, is evidently linked to leg and upper body strength in young adults.
But one species in particular drew the researchers' attention. The relative abundance of Roseburia inulinivorans was positively associated with multiple strength metrics in humans, including hand grip, leg press, and bench press.
Older adults who have this microbe in their stool also have a nearly 30 percent stronger hand grip than comparable subjects with no sign of it, the study found.
It appears to benefit young adults, too, in whom a higher prevalence of R. inulinivorans was associated with stronger grip as well as higher cardiorespiratory capacity.
The study also featured additional experiments with mice, designed to assess causality and explore mechanisms of the associations observed in humans.
The researchers used antibiotics to deplete existing gut flora in mice, then restocked the mice with bacteria from human intestines in weekly installments over an eight-week period.
R. inulinivorans induced a "remarkable increase" in the rodents' forelimb grip strength, boosting this muscle-function proxy by roughly 30 percent above the control group, they report.
Beneath the effects on muscle performance, the study found that mice given this bacterium also grew more fast-twitch muscle and larger muscle fibers in the soleus, an important muscle located at the back of the lower leg.
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The findings suggest R. inulinivorans achieves this by altering amino acid metabolism, activating the purine and pentose phosphate pathway in muscle, and promoting muscle-fiber hypertrophy, with a shift toward fast-twitch fibers.
More research will be needed, but this research could pave the way for future development of probiotics, the researchers note, to help people maintain strength and physical fitness as the buffer of youth fades.
"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.