Specific gut bacteria species (R inulinivorans) linked to muscle strength

(Press-News.org) A species of gut bacteria called Roseburia inulinivorans is specifically associated with human muscle strength and improved muscular performance in mice, finds research published online in the journal Gut. 

R inulinivorans changes certain metabolic processes in muscle, and converts muscle fibres to ‘fast-twitch (type II)---designed for short intensive movement, such as sprinting and weights—the findings show. 

R inulinivorans holds promise as a nutraceutical probiotic for treating age-related muscle wasting disease, suggest the Dutch and Spanish researchers. 

Dwindling muscle mass and strength are major contributors to frailty, reduced mobility and functionality, and poor health, particularly as people age and among those with long term conditions, they explain. 

Gut bacteria have been implicated in a wide range of health conditions, including metabolic, neurodegenerative, and cardiovascular diseases, and an emerging body of evidence suggests that they may also influence muscle mass and function, add the researchers. 

They therefore wanted to find out whether specific gut microbial species might be associated with muscle strength and, if so, what the underlying mechanisms might be. 

To do this, they analysed stool samples from 90 healthy young (18-25 year olds) and 33 older (65+) adults for the presence of bacteria. 

 Their hand grip strength, leg press and bench press performance, and VO2 max (maximal oxygen consumption during exertion) which is an indicator of cardiorespiratory fitness, were used to assess physical fitness. 

Among all the bacteria found in the stool samples, the Roseburia group (genus) was the only one that was positively associated with muscle mass and strength.  

R faecis and R intestinalis species weren’t significantly associated with hand grip strength or VO₂ max in either age group. 

But the older adults with detectable R inulinivorans in their stool sample had 29% higher handgrip strength than those without, and without a concomitant increase in peak oxygen uptake, indicating better fitness.  

In young adults, greater abundance of R inulinivorans was positively associated with both handgrip strength and VO₂ max. And the relative abundance of both R inulinivorans and R intestinalis was positively associated with leg press and bench press strength as well. 

The relative abundance of R faecis and R hominis wasn’t associated with any of the indicators of muscle strength, which suggests that different Roseburia species may influence different aspects of muscle strength, say the researchers. 

Roseburia was also more abundant in the young adults, among whom the proportion of R faecis ranged from 0% to 3.3%; that of R intestinalis ranged from 0% to 5.5%; while that of R inulinivorans ranged from 0% to 6.6%. 

Among the older adults, the proportion of R faecis ranged from 0% to 2.2%; that of R intestinalis ranged from 0%-0.7%, while that of R inulinivorans ranged from 0% to 1.3%.  

In a bid to identify a causal role for Roseburia in muscle strength, the researchers fed Roseburia species once a week for 8 weeks to 32 mice that had previously been given a cocktail of antibiotics for a fortnight to deplete their gut microbiome.  

The mice were randomly assigned to one of 4 groups, 3 of which received a different strain of Roseburia, and one of which received no Roseburia (control group). 

None of the Roseburia species enhanced running time to exhaustion. But compared with the control group, R inulinivorans induced a noticeable increase of around 30% in forelimb grip strength—a proxy for muscle function—after 4, 6, and 8 weeks of treatment. 

What’s more, mice treated with R inulinivorans had larger muscle fibre size and a significantly higher proportion of type II (‘fast twitch’) fibres in the soleus muscle located in the calf, compared with the other groups, although not when compared with the mice treated with R intestinalis. 

Further analysis of fibre size distribution revealed that this was relatively evenly distributed in the control group, whereas mice treated with R inulinivorans had higher levels of larger fibres than those treated with the other Roseburia species or mice in the control group.  

These changes were also accompanied by metabolic changes in the proteins and enzymes key to muscle energy production. 

The researchers acknowledge various limitations to their findings. In the animal study, none of the human Roseburia species colonised the mouse gut. And specific pathways related to inflammation or neuromuscular signalling weren’t directly assessed and these might have been influential. 

Long term studies are needed to determine whether changes in the abundance of R inulinivorans are a cause or consequence of muscle function change, they emphasise.  

Nevertheless, they suggest: “Collectively, our findings provide robust evidence supporting a gut-muscle axis in which R inulinivorans positively modulates muscle metabolism and muscle strength.”  

And they conclude: “Additionally, we observed that the relative abundance of R inulinivorans is lower in older adults than in young adults. Its abundance appears to decline with advancing age, a period during which the prevalence of sarcopenia [muscle loss] increases, suggesting a potential role for R inulinivorans as a probiotic candidate for preserving muscle strength.”  

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