Gut Microbe Molecules Shield Aging Muscles From Wasting, With Surprising Sex Differences

When an older adult is hospitalized or confined to bed, the consequences for skeletal muscle can be swift and devastating. Research in young adults shows that just two weeks of unloading can slash muscle strength by roughly 23 percent, with cross-sectional area shrinking by about 0.6 percent per day. In older people the picture is far worse: as few as four days of disuse are enough to cause substantial declines in lower-body strength and functional capacity. One landmark study found that ten days of bed rest in healthy older adults produced a 13.2 percent drop in knee extensor strength, a 14 percent reduction in stair-climbing power, and a 12 percent decrease in aerobic capacity. These losses feed a vicious cycle in which aging raises fall risk, falls lead to fractures and immobilization, and immobilization accelerates muscle loss and frailty, eroding independence and longevity.

Exercise would seem the obvious countermeasure, but for many elderly people it is simply impractical. Fear of injury and falling, preexisting health conditions, lack of motivation, and financial costs all act as potent deterrents. Even when resistance exercise is possible, an age-related phenomenon called anabolic resistance blunts the muscle’s response to both exercise and nutrition, limiting the efficacy of conventional interventions. Disuse itself worsens the problem, since even acute immobilization suppresses the protein synthesis machinery needed for recovery once movement resumes. Existing countermeasures, including neuromuscular electrical stimulation, protein and amino acid supplementation, beta-hydroxy-beta-methylbutyrate, and omega-3 fatty acids, have largely failed to preserve both muscle strength and physical function in older adults during immobilization.

Against this discouraging backdrop, a new study published in Physiological Reports offers a strikingly different approach: harnessing molecules produced by the gut microbiome during exercise. Researchers previously identified pipecolic acid and succinate, two so-called microbial-derived exerkines, as metabolites whose oral co-administration prevented disuse-induced muscle atrophy and preserved muscle function in adult mice. Exerkines are signaling molecules released in response to exercise, and the discovery that some of them originate from gut microbes has opened an entirely new frontier in muscle biology. The question was whether these compounds could also protect the far more vulnerable muscles of aged animals.

To find out, a team at the University of Kentucky used 22- to 25-month-old male and female C57BL/6JN mice from the National Institute on Aging aged rodent colony, animals roughly equivalent in biological terms to elderly humans. Each mouse underwent ten days of unilateral hindlimb casting, with one limb enclosed in a 3D-printed plastic cast in gentle flexion at the knee and dorsiflexion at the ankle while the contralateral limb remained free as an internal control. Throughout the immobilization period, the mice received either a vehicle solution of 2 percent sucrose or a combination of pipecolic acid and succinate, delivered at approximately 190 and 95 milligrams per kilogram per day respectively, via their drinking water.

After the casting period, the researchers evaluated the soleus, plantaris, and gastrocnemius muscles for mass, histological fiber morphology, ex vivo contractile function, and mitochondrial content. The results revealed a nuanced, partial protection. Whole-muscle mass was largely unchanged in the soleus and plantaris, but PAS-treated mice retained a significantly greater proportion of gastrocnemius mass in the casted limb, 90.0 percent versus 85.2 percent of the contralateral control limb, irrespective of sex. The gastrocnemius is the largest of the three muscles and sustains the greatest absolute mass loss during casting, which may explain why the protective effect was most detectable there. The failure to protect soleus and plantaris mass mirrors the broader literature, in which even the most intensively studied protein supplementation strategies have failed to prevent muscle loss during bed rest or limb immobilization in older adults.

At the level of individual muscle fibers, however, the story was considerably more encouraging. Mean fiber cross-sectional area was preserved in both the soleus and the plantaris of PAS-treated mice. In the soleus, the effect was sex-specific: PAS-treated females retained larger mean fiber areas than vehicle-treated females, with the same pattern evident in Type I fibers, whereas males showed no treatment difference. For Type IIa fibers there was a main effect of treatment favoring PAS across both sexes, though a trending interaction suggested females drove much of the effect. Notably, the protection extended to the plantaris, a predominantly fast, glycolytic muscle, even though succinate is known to promote an oxidative, slow-fiber program via SUCNR1 signaling. This suggests the fiber-level benefit is not confined to oxidative fibers and cannot be explained solely by succinate-driven oxidative remodeling, pointing to a possible contribution from pipecolic acid, whose actions in skeletal muscle remain largely uncharacterized.

The functional data were the most striking of all, and they diverged sharply by sex. In ex vivo tests of the casted soleus, PAS-treated females produced greater absolute force across the entire stimulation frequency range from 10 to 120 hertz, and developed force more rapidly, than vehicle-treated females. Males showed no such improvements in force or rate of force development. Instead, PAS-treated males demonstrated markedly higher fatigue resistance across 150 repeated contractions, sustaining a greater proportion of their initial force, while females showed no treatment effect on this measure. This outcome-dependent, sexually dimorphic pattern suggests that the two metabolites may influence different aspects of muscle physiology in males and females, a coherence that extends across morphological and functional outcomes alike.

One clue to the mechanism came from measurements of citrate synthase activity, a standard index of mitochondrial content, in the casted gastrocnemius. PAS-treated females exhibited significantly greater citrate synthase activity than vehicle-treated females, with no corresponding difference in males, paralleling the female-specific preservation of fiber size and force. Whether greater mitochondrial content actually causes the preservation of fiber size and force in females, or merely accompanies it, cannot be resolved from this study, but oxidative capacity now stands out as a prime target for mechanistic investigation. In males, the improved fatigue resistance without increased citrate synthase activity hints that succinate, a well-studied mitochondrial substrate and signaling metabolite, may have acted through mitochondrial function or substrate supply rather than by expanding mitochondrial content. Fatigue resistance is closely linked to oxidative capacity, mitochondrial function, and calcium handling within the muscle fiber, making succinate a compelling candidate for the male-specific benefit.

The clinical significance of these findings is hard to overstate. Neuromuscular electrical stimulation, though widely applied, produced no improvement in maximal strength, fiber cross-sectional area, or stair-climbing power in elderly postoperative patients over twelve weeks in one trial. Even when electrical stimulation is combined with protein supplementation during bed rest, functional deficits persist in older adults despite maintained muscle mass. Against this record of failure, a simple oral supplement of two microbial metabolites that preserves fiber size and contractile function in aged muscle represents a genuinely novel therapeutic avenue. It is particularly notable that females, who appear more susceptible to disuse atrophy and greater strength losses during immobilization, showed the strongest morphological and force-related responses to treatment.

Important caveats remain. The study was largely confined to phenotypic and functional outcomes, and the single index of mitochondrial content cannot resolve the molecular pathways through which pipecolic acid and succinate act. The effects on food intake and body composition were not assessed, only the casted soleus was tested ex vivo, and disuse itself is known to alter the gut microbiome, raising the possibility that age and disuse interact to lower endogenous availability of these very metabolites. Future work must also test whether the compounds enhance recovery during post-immobilization rehabilitation, and whether doses can be optimized across ages and sexes. Still, the message is clear: exercise’s chemical conversation with our gut microbes may hold the key to protecting aging muscles when movement itself is impossible, and any such therapy must be designed with sex as a primary biological variable from the outset.

Subject of Research: Microbial-derived exerkines and disuse-induced muscle atrophy in aged mice

Article Title: Microbial‐derived Exerkines preserve muscle size and function during disuse in aged mice in a sex‐dependent manner

Article References: Microbial‐derived Exerkines preserve muscle size and function during disuse in aged mice in a sex‐dependent manner. (n.d.). https://doi.org/10.14814/phy2.71137

Image Credits: AI Generated

DOI: 10.14814/phy2.71137

Keywords: exerkines, gut microbiome, pipecolic acid, succinate, sarcopenia, disuse atrophy, skeletal muscle, aging, sex differences, mitochondria, muscle function, immobilization