June 18, 2024

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A new study adds to existing evidence that the gut microbiome is important for skeletal muscles. Stefania Pelfini, La Waziya Photography/Getty Images
  • Various studies have suggested that gut microbes have a role in regulating muscle mass and function.
  • A new study in mice indicates that an unhealthy or disrupted gut microbiome could impair muscle growth.
  • Scientists came to this conclusion after comparing two groups of mice, one of which continuously received antibiotics.
  • The muscles of the mice that received antibiotic treatment, and thus had an unhealthy gut, did not grow as much.
  • If researchers are able to pinpoint which substances that gut bacteria produce can help muscles grow after exercise, new treatments for muscle loss conditions could be on the horizon.

The gut microbiome is a dynamic ecosystem that consists of trillions of bacteria, fungi, and other microbes that live inside the digestive system. It greatly influences overall health, and usually, we notice its effects only when there is an imbalance.

Having a disrupted gut microbiome, whether due to poor diet or antibiotic treatment, can impact the digestive, immune, and central nervous systems, as well as various bodily functions and processes.

Scientists have established one newer link between the microbiome and exercise, and more recently, between the microbiome and muscle growth.

To further explore this area, Prof. John McCarthy, Ph.D., an associate professor of physiology at the University of Kentucky in Lexington, and his colleagues decided to conduct a study involving mice in which they used antibiotics and weighted wheels.

“As exercise physiologists with a particular interest in how skeletal muscle adapts to exercise, we wondered if the gut microbiome might have a role in this process, given its central role in human health,” Prof. McCarthy told Medical News Today.

The study findings appear in The Journal of Physiology.

As part of the experiment, the researchers analyzed 42 female mice. They administered antibiotics to one group via their drinking water and kept the other group germ-free. The mice voluntarily exercised on running wheels for about 9 weeks.

The researchers then measured and compared both groups’ skeletal muscles, which include the muscles of the arms and legs.

Skeletal muscles facilitate movement, maintain posture, stabilize bones and joints, and generate body heat. A person can alter the physical characteristics of skeletal muscles through exercise.

Both groups of mice ran for a similar amount of time during the training period, and the scientists concluded that the antibiotics did not have a profound effect on running performance.

Antibiotic use also did not lead to systemic inflammation, but it did deplete gut microbiome levels. The drugs caused mice to experience dysbiosis, which is an imbalance in the gut microflora.

However, the group of mice that received antibiotics had slower and less pronounced muscle growth in response to exercise.

This suggests that for muscles to grow following exercise, an intact microbiome is necessary in mice.

Prof. McCarthy stated that the fact that both groups ran for the same amount of time was critical, because running performance is the stimulus for skeletal muscle growth.

“If antibiotic-treated mice had run less than untreated mice, then it would be more difficult to conclude a healthy gut microbiome was required for skeletal muscle to fully adapt to exercise, because it would more likely be the mice just ran less,” he said.

Prof. McCarthy noted it was significant that dysbiosis led to an impairment in the ability of skeletal muscles to adapt to exercise.

“The muscle does not grow (hypertrophy) as much in a mouse with a disrupted gut microbiome as a result of antibiotic treatment, [compared with] a mouse with a healthy gut microbiome,” he told MNT.

There have been a few studies suggesting the gut microbiome may impact muscle growth in mice and pigs. Evidence is also growing by the day.

Prof. McCarthy pointed out that the lead author, Taylor Valentino, is an ultramarathoner who had previously been involved in studies looking at gastrointestinal health in ultramarathoners.

“From an athletic standpoint, world-class runners were found to have more of a particular type of bacteria that provided an additional source of energy, which was thought to help them run faster,” he said.

A 2015 study found that mice that did not have a microbiome had reduced skeletal muscle mass. The authors of the study conclude that microbial status of the gut is crucial for exercise performance.

A 2019 study confirmed these findings. Antibiotic-treated mice had a lower muscle mass-to-body weight ratio, and their gastrocnemius and quadriceps, two of the biggest muscles in the leg, were smaller than in mice with an intact microbiome.

Collectively, these data suggest that there are factors within the gut microbiome that promote muscle mass, says Michael Lustgarten, Ph.D.

“Metabolites produced by gut bacteria can affect muscle mass. [One] group of metabolites that can do that are short-chain fatty acids, which are produced by the bacterial fermentation of dietary fiber, which have been shown to increase muscle mass,” he says.

Previous research indicates that increasing fiber intake for just 2 weeks can significantly alter a person’s microbiome and increase the species of bacteria that break down fiber.

Although it is too early to know exactly what the clinical implications of the study are, Prof. McCarthy said the findings do suggest “a healthy gut microbiome will contribute to getting the most benefit out of regular exercise in terms of skeletal muscle.”

The study also had several limitations that one needs to keep in mind when considering the implications of the findings.

As the study involved mice, there is always the concern that the results might not translate to humans, said Prof. McCarthy.

He pointed out that he and his colleagues only used female mice because they are better runners than male mice. And so, the results in male mice may be different.

Moreover, the dose of antibiotics the researchers administered was relatively low compared with the doses in previous studies.

“While both groups of mice ran the same amount, we do not know if the antibiotics themselves might have interfered with the ability of the skeletal muscle to adapt to exercise,” he added.

The researchers are now looking to identify which substances produced by gut bacteria can help muscles fully adapt to exercise and improve athletic performance.

“We are currently trying to determine how exercise changes the composition and function of the gut microbiome. This investigation, along with other studies in bacteria, will allow us to identify the substances made by the gut microbiome that help skeletal muscle to grow larger in response to exercise,“ says Prof. McCarthy.

This discovery could also pave the way for treating muscle loss conditions, such as those from cancer, and unlocking the key to aging, as well as achieving peak exercise outcomes.

“If we can identify the substances that gut bacteria are making to help muscles [grow] after exercise, we might be able to use some of those substances to promote the growth of muscles in people suffering from the loss of muscle as typically seen with aging or cancer.”

– Taylor Valentino

Age-related muscle loss, called sarcopenia, happens naturally due to loss of muscle fibers and atrophy. People in their 80s and 90s can lose as much as 50% of their muscle mass due to this condition.

More research is necessary to uncover the underlying mechanism between gut bacteria and muscle growth before it can lay the groundwork for treatment.