A novel new study suggests that the gut microbiome has a role in mechanisms related to muscle strength in older adults.
The work, led by researchers at the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts, is available as a pre-proof in advance of print in Experimental Gerontology.
The gut-muscle axis, or the relationship between gut microbiota and muscle mass and physical function, has gained momentum as a research topic in the last few years as studies have established that gut microbiota influences many aspects of health.
While researchers have begun exploring the connection between the gut microbiome, muscle, and physical function in mice and younger adults, few studies have been conducted with older adults.
To gain insight into this population, the researchers compared bacteria from the gut microbiomes of 18 older adults with high-physical function and a favorable body composition (higher percentage of lean mass, lower percentage of fat mass) with 11 older adults with low-physical function and a less favorable body composition.
The small study identified differences in the bacterial profiles between the two groups.
Similar bacterial differences were present when mice were colonized with fecal samples from the two human groups, and grip strength was increased in mice colonized with samples from the high-functioning older adults, suggesting a role for the gut microbiome in mechanisms related to muscle strength in older adults.
Specifically, when compared to the low-functioning older adult group, the researchers found higher levels of Prevotellaceae, Prevotella, Barnesiella, and Barnesiella intestinihominis–all potentially good bacteria–in the high-functioning older adults and in the mice that were colonized with fecal samples from the high-functioning older adults.
No significant differences in body composition or endurance capacity were observed in the colonized mice; however, the researchers note that the length of the intervention period was short and these data may warrant further study.
“While we were surprised that we didn’t identify a role for the gut microbiome on the maintenance of body composition, with these results we now start to understand the role of gut bacteria in the maintenance of muscle strength in older adults,” said Michael Lustgarten, last and corresponding author on the study and a researcher in the Nutrition, Exercise Physiology & Sarcopenia (NEPS) Laboratory at the HNRCA.
“For example, if we were to conduct an intervention to increase Prevotella levels in the gut microbiome, we would expect to see an increase in muscle strength if these bacteria are involved.
Prevotella’s role in the maintenance of muscle strength in older adults is one area we expect to continue to explore.”
New study identifies differences in gut microbiome composition in physically high-functioning vs low-functioning older adults, successfully transfers some of these effects into mice.
“As we age, body composition, muscle strength, and lean mass all decrease,” said first author Roger Fielding, director of the NEPS Laboratory at the HNRCA.
“Identifying differences in bacteria present in the high-functioning and low-functioning groups in this study moves us toward a fuller understanding of both the gut microbiome and healthy aging.”
For the study, the researchers measured lower extremity function, mobility, and strength in the sedentary older adult group (ages 70-85) at the first and one-month study visits.
In the mice, they measured body composition with quantitative magnetic resonance imaging, and grip strength and treadmill endurance capacity to test physical function.
Fecal samples from the older adults were transplanted into young, gender-matched germ-free mice.
Four weeks after fecal transfer, the researchers measured body composition, physical function, and gut microbiome in the 18 mice colonized with fecal samples from the high-functioning human group and the 18 mice colonized with fecal samples from the low-functioning human group.
The authors note the small sample size and brief time period as potential study limitations.
Funding: This work was supported by awards from the National Institutes of Health’s National Institute on Aging (K01AG050700), the Boston Claude D. Pepper Older Americans Independence Center (5P30AG031679), and the U.S. Department of Agriculture’s Agricultural Research Service. The content of this announcement is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health or the U.S. Department of Agriculture.
Fielding is also a professor at the Friedman School of Nutrition Science and Policy at Tufts and Tufts University School of Medicine
Additional authors on this study are Andrew R. Reeves, Christine Liu, and Brittany B. Barrett, HNRCA; and Ravi Jasuja, Boston Claude D. Pepper Older Americans Independence Center, Brigham and Women’s Hospital, Harvard Medical School.
Skeletal muscle is the largest organ in the human body, comprising ~ 40% of total body mass .
The musculoskeletal system is best known for its role in locomotion and postural stabilization, but also serves as a large macronutrient reservoir, protects internal organs, maintains core temperature, and communicates with other bodily through the release of growth factors and cytokines .
For the past decade, the endocrine nature of skeletal muscle has been extensively examined, facilitating the characterization of a muscle secretome and providing unique insights into inter-organ communication networks [3, 4].
Equally as important as the effects of skeletal muscle secreted products on peripheral tissues are the ways in which external factors may act to modify skeletal muscle.
One system with tremendous potential to impact host physiology is that of gut microbiota, the bacteria, archaea, viruses, and eukaryotic microbes that reside in the gastrointestinal tract .
The transformative power of gut microbiota on human physiology was emphasized in a groundbreaking study by Ridaura and colleagues , who demonstrated that fecal microbiota transplant from obese humans into lean animals precipitated body composition shifts consistent with the phenotype of their donor, highlighting the transmissible effects of metabolic phenotype via microbial exchange.
The human body is composed of an estimated 30 trillion cells living in symbiosis with diverse microbial communities .
In an effort to better understand our nature as human superorganisms, a decade ago the National Institute of Health launched the human microbiome project (HMP), an international public health initiative aimed at characterizing microorganisms associated with human health and disease .
Making use of advances in the field of metagenomics, HMP-associated studies employ high-throughput 16 s rRNA gene sequencing to streamline the analysis of gut microbial communities .
Human gut microbiota are mainly composed of Bacteroidetes and Firmicutes , which together account for more than 90% of all phylogenetic types, and are further subdivided to consist of more than 100 distinct bacterial species .
Although the underlying cause(s) (host vs environment) of this apparent age-related microbial imbalance have yet to be defined, its relevance is underscored by the growing number of studies showing associations between gut ecology and cancer , cardiovascular disease , obesity , diabetes , and muscle wasting , some of the most prevalent conditions afflicting older members of society today.
Furthermore, the existence of a gut-muscle communication pathway has been posited .
The purpose of this review will be to discuss existing literature suggesting communication between gut microbiota and skeletal muscle, with emphasis on the potential significance of this axis for mediating changes in aging muscle health.
We will begin by reviewing the more well-recognized impact of gut microbiota on skeletal muscle metabolism before transitioning to the lesser acknowledged role of gut microbiota on the regulation of skeletal muscle size, composition, and function.
To conclude, we will integrate these findings and propose a “gut-muscle axis” that we believe plays a role in the age-related loss of muscle mass and function (sarcopenia) .
Gut Microbiota and Skeletal Muscle Metabolism
Gut microbiota are intricately involved in helping to perform a number of necessary functions for their host. Given their gastrointestinal habitation, perhaps the more ostensible of these roles is their influence on metabolic function (e.g., nutrient absorption and amino acid synthesis) .
The importance of gut microbial health in energy metabolism is exemplified by cases of Kwashiorkor, a form of severe acute malnutrition accompanied by perturbations in amino acid and carbohydrate metabolism that can be transiently ameliorated by dietary therapeutics that promote microbial reconfiguration (increased Bifidobacterium, and Lactobacillus reuteri and gasseri) .
Meanwhile, excess nutrient uptake and storage (i.e., obesity) is associated with low microbial diversity and changes in the relative abundance of the major bacterial phyla (Bacteroidetes and Firmicutes) [27–29].
While these findings provide support for microbial regulation of local metabolic function, evidence also suggests that gut microbiota may impact distal metabolic activity in skeletal muscle tissues [23, 30].
In a pioneering study conducted in 2004, Backhed et al. infused germ-free (GF) mice, animals born and raised in sterile conditions and lacking a microbiome, with the cecal contents of conventionally raised animals, and demonstrated a 60% increase in body fat with reciprocal reductions in insulin sensitivity and glucose tolerance .
As skeletal muscle is an integral component of glucose disposal, these findings suggest microbial-mediated regulation of muscle metabolic function. In support of this sentiment, bacterial colonization of GF mice appears to decrease skeletal muscle metabolic efficiency, as evidenced by an increase in tricarboxylic acid cycle intermediates without an appreciable increase in high-energy phosphate stores .
To understand the seemingly superior metabolic phenotype in microbiome-deficient mice, the same research group compared skeletal muscle protein activity and gene expression in GF and conventionally raised animals .
Compared to mice with gut microbiota, skeletal muscle from GF mice was characterized by markedly greater activity of AMP-activated protein kinase (AMPK) and carnitine palmitoyl transferase-1 (CPT-1), indicative of an elevated oxidative capacity. Taken together, these early studies suggest that gut microbiota can influence body composition by means of regulating skeletal muscle bioenergetic pathways.
To identify a mechanistic link between microbial dysbiosis and metabolic derangement (i.e., skeletal muscle insulin resistance), Cani et al.  subjected young mice to a high-fat diet, which is known to induce chronic low-grade chronic inflammation (e.g., IL-1, IL-6, and TNF-α elevation), a reported contributor to insulin resistance [32, 33].
As expected, high-fat fed mice exhibited an increase in body weight and inflammatory markers as well as a decrease in glucose tolerance.
In addition, circulating levels of lipopolysaccharide (LPS), a component of the outer membrane of gram negative bacteria, were 2- to-3-fold elevated in high-fat fed mice when compared with control fed animals. Pertinently, high-fat feeding is shown to compromise epithelial tight junctions and increase intestinal permeability [34, 35], evidence that suggests a role for high-fat induced leak of LPS from the intestine into the circulation.
To examine the contribution of circulating LPS (i.e., metabolic endotoxemia) to insulin insensitivity, the same research team subjected wild-type and CD-14 mutant mice, which lack an endotoxin receptor, to LPS infusion .
In support of LPS-mediated dysregulation of insulin signaling, pro-inflammatory cytokine induction and impairments in glucose tolerance were observed exclusively in the wild-type animals. Endotoxin-mediated reductions in glucose tolerance have been similarly shown in humans inoculated with LPS, as evidenced by significant increases in skeletal muscle NF-κβ binding activity and JNK phosphorylation, which synergistically inhibit insulin signaling .
These studies and others [30, 37], show that circulating LPS, a prominent component of gut microbiota, induces skeletal muscle inflammation and insulin resistance thereby contributing to the development of metabolic syndrome.
While these findings implicate LPS and inflammation as causative agents in the process, inflammation-independent alterations in skeletal muscle metabolism have also been observed.
For example, in response to the gut microbiota-specific metabolite indoxyl sulfate , C2C12 myoblasts exhibit an up-regulation of glycolysis and an increase in the activity of the pentose phosphate pathway .
Similarly, exposure of myotubes to gut microbial-derived extracellular vesicles induces insulin resistance , highlighting the multitude of ways in which gut microbiota may influence the metabolic function of skeletal muscle.
Insulin-resistant type 2 diabetes may occur at any age, but its prevalence is greater in older years (> 25% in persons ≥ 65 years) .
Older adults with diabetes are at an increased risk for cardiovascular complications, reduced functional status, loss of independence, and mortality .
With the relationship between inflammation and insulin resistance in mind, Ghosh et al. sought to assess the contribution of circulating endotoxin to this age-related inflammatory/insulin-resistant phenotype . As hypothesized, in older individuals circulating LPS and skeletal muscle TLR4 (a receptor for LPS ) gene expression and protein content were higher, and insulin sensitivity (quantified by HOMA-IR) was lower than their younger counterparts. These findings suggest that an age-related increase in LPS levels might contribute to the greater occurrence of diabetes in older persons.
Gut Microbiota and Skeletal Muscle Size, Composition, and Function
Age-related inflammation is associated with reductions in skeletal muscle size and function (sarcopenia) .
Furthermore, obesity appears to exacerbate the progression of sarcopenia by compounding inflammatory burden , resulting in a state of sarcopenic obesity, defined as inadequate muscle mass relative to total body size .
A variety of phenomena have been proposed to contribute to this age-related heightened inflammatory state including redox stress , immuno- and endocrine senescence [47, 48], DNA damage , epigenetic modification , and a distortion of gut microbial homeostasis  that is associated with increased levels of circulating endotoxin .
An extreme model of LPS-induced inflammation is the clinical condition sepsis, a life-threatening illness characterized by severe muscle wasting due to both increases in proteolytic degradation and reductions in protein synthesis .
Interpreted in unison, these findings demonstrate the ability of LPS-associated cytokines to determinately affect protein equilibrium (i.e., synthesis and breakdown), and suggest that elevated endotoxin levels with age may contribute to reductions in muscle mass.
It is interesting to note that the tryptophan derivative indoxyl sulfate, which is similarly shown to increase with age , is reported to induce inflammatory cytokine expression (IL-6 and TNF-α) and markers of muscle atrophy (myostatin and atrogin-1) similar to that of LPS in cultured myocytes .
Furthermore, in vivo administration of indoxyl sulfate is shown to increase atrogene expression in conjunction with decreased muscle mass in mice . These findings highlight the importance of comprehensively examining microbial metabolites for triggers of myocellular adaptation , rather than focusing on a single microbial byproduct.
From a feasibility standpoint, the ability to influence skeletal muscle size by altering the composition of gut microbiota is attractive, particularly in circumstances where muscle loading may be challenging (e.g., immobilization, hospitalization, spaceflight). One of the first studies to target gut microbiota as a means to affect lean tissue mass was conducted by Bindels et al. using a leukemic mouse model .
Microbial profiling in these cachectic mice revealed gut dysbiosis characterized by selective modulation of Lactobacillus spp.
To restore lactobacilli levels, the leukemic mice were given an oral probiotic containing Lactobacillus reuteri and L. gasseri that appeared to decrease serum levels of inflammatory cytokines (IL-6 and MCP-1) and atrogene expression (MuRF1 and Atrogin-1) in conjunction with an increase in mass of the tibialis anterior.
Interestingly, these effects appear to be bacterial species specific, as L. acidophilus supplementation did not appear to affect inflammatory or atrophy markers . However, L. plantarum supplementation has demonstrated to increase lean mass and function (grip strength and swim time) in healthy young mice . Taken together, these studies suggest a link between Lactobacillus species and skeletal muscle size that is worthy of further investigation in humans.
Prebiotics are fermented non-digestible compounds that support the proliferation of health-promoting bacteria  and thus may influence skeletal muscle health.
The efficacy of prebiotic supplementation to support beneficial skeletal muscle changes was demonstrated in a study by Cani et al., who showed a decrease in circulating LPS and inflammation and an increase in muscle mass in obese mice fed the prebiotic fiber oligofructose .
Follow-up analyses confirmed the favorable effect of prebiotic feeding on gut microbiota, as was evidenced by a shift in the ratio of Bacteroidetes/Firmicutes in addition to increases in the levels of Lactobacillus and Bifidobacterium spp. .
In further support of a link between prebiotics with skeletal muscle, administration of a synbiotic containing inulin-type fructans and Lactobacillus reuteri was shown to reverse increases in Escherichia and promoted proliferation of Lactobacillus and Bifidobacterium in leukaemic mice .
Coupling of these microbial alterations with restoration of intestinal homeostasis (e.g., increase tight junction proteins) and a reduction in muscle wasting suggests that Lactobacillus and Bifidobacterium may influence gut-muscle communication and regulate muscle size.
Thus, an age-related decrease in gut Bifidobacterium content may underlie increases in circulating endotoxin that are shown to induce skeletal muscle atrophy .
While we were not able to identify any studies showing Bifidobacterium supplementation to increase muscle mass, data showing butyrate (associated with Bifidobacterium ) treatment to protect against age-related muscle atrophy  supports the idea that pre- and/or probiotic supplementation, which is shown to increase the abundance of Bifidobacterium and butyrate producers in older individuals [74, 75], may prophylactically moderate aging muscle loss.
While the age-associated loss of muscle mass is purported to play a role in the loss of strength and function [76, 77], strength declines far exceed reductions in muscle mass , suggesting a change in intrinsic muscle function (i.e., muscle quality).
Although a number of factors have been implicated in aging muscle quality deficits [79, 80], adipose tissue infiltration in skeletal muscle (i.e., change in muscle composition) has been shown to increase with age  and is associated with mobility impairment .
To assess the relationship between gut microbiota, inflammation, and muscle composition, Collins et al. subjected young rats to a high-fat high-sucrose diet and found an increase in Enterobacteriaceae (gram-negative member of the Proteobacteria phylum) and a decrease in the abundance of Lactobacillus spp., changes that have been similarly reported with aging [83, 84].
Concomitant with these changes, the authors reported an increase in circulating inflammatory cytokines (IL-6, TNF-α, and MCP-1) and intramuscular fat after just 3 days , suggesting rapid dysregulation of these seemingly inter-connected systems.
Regulation of muscle composition by gut microbiota is supported by associations between gut bacteria involved in energy metabolism and porcine intramuscular fat content .
Furthermore, age-related changes in gut microbiota have also been proposed to evoke fat infiltration into bone [87, 88], culminating in a triad of aberrant muscle, bone, and adipose tissue dysregulation (osteosarcopenic obesity).
A limitation of these analyses is an inability to distinguish if changes in microbiota are a driver or a product of this compromise to bone and muscle integrity. However, probiotic rescue (Lactobacillus reuteri and gasseri) of inflammation and muscle/bone loss in mice proposes that gut microbiota initiate this relationship [19, 67, 89].
Also contributing to changes in muscle composition, aging has been associated with alterations in myosin heavy chain (MHC) distribution .
Although a topic of intense scrutiny, studies have yet to elucidate a definitive mechanism underlying this phenomenon.
It is thus interesting to note that over the course of the past year, two independent studies have observed changes in skeletal muscle MHC composition following interventions aimed at manipulating gut microbiota [63, 94].
Pertinently, in one of these studies by Yan and colleagues , transfer of gut microbiota from an obese pig to a GF mouse replicated the host myocellular phenotype, which mirrored that of aging skeletal muscle (i.e., fast fiber atrophy concomitant with a fast-to-slow fiber type shift).
These findings highlight the possibility that gut microbiota can transfer muscle fiber characteristics, and open the door for studies investigating the transplant of microbiota from young to old as a means to improve skeletal muscle size and composition.
Age-related changes in skeletal muscle size and composition summate to yield reductions in skeletal muscle function (strength and power)  that ultimately affect physical performance and the ability to live independently .
One of the first studies to link changes in gut microbiota with functional capacity was conducted by van Tongeren et al. , who studied fecal microbes in a relatively small cohort of advanced agers (n = 23; ~ 86 years).
Upon stratifying subjects by frailty score, a unique bacterial signature characterized by marked reductions in Lactobacillus (~ 8-fold less) and commensurate increases in the quantity of Enterobacteriaceae (~ 6-fold more) in frail agers emerged.
Moreover, higher functioning subjects may have been protected from dysbiotic shifts in gut microbiota by a greater abundance of butyrate-producing bacteria such as Faecalibacterium prausnitzii.
Indeed, butyrate is shown to enhance intestinal barrier function by reinforcing tight junction assembly , which, in theory, should prevent endotoxin translocation and reduce circulating inflammation .
The association between butyrate-generating bacteria and functional capacity is supported by metagenomics findings in a larger sample of older adults (n = 178; ~ 78 years)  demonstrating that community-dwelling elders had more butyrate-producing microbes than those in long-stay residence.
Additionally, institutionalized elderly had a greater abundance of Enterobacteriaceae and Escherichia/Shigella, and less gut microbial diversity when compared with the community dwellers.
Given the corresponding change in diet (i.e., low in plant-based nutrients and fiber and high in sucrose and saturated fat) and microbial composition when individuals enter long-term care facilities , geriatric clinicians are encouraged to work with older individuals undergoing residence relocation to pursue nutritional strategies that aim to prevent the loss of healthy microbes (e.g., butyrate-producing bacteria).
Relevantly, prebiotic supplementation (inulin plus fructooligosaccharides) has been shown to increase muscle strength (handgrip) and endurance (exhaustion) in frail older adults , thereby highlighting the utility of prebiotic supplementation as a treatment for age-associated deficits in muscle function.
Studies investigating gut microbiota of athletes provide unique insight into microbial characteristics associated with high levels of physical function.
Comparison of these bacterial traits to those of populations with reduced functional capacity (e.g., mobility-limited older adults) can provide data-driven targets for therapeutic intervention. One of the first investigations to assess gut microbiota in athletes was conducted by Clarke et al. who studied elite rugby players, individuals with large quantities of lean muscle tissue .
Comparison of microbial diversity among these athletes and both healthy normal weight (BMI ≤ 25) and overweight (BMI ≥ 28) age-matched controls elicited a hierarchical pattern (athletes > healthy > overweight), suggesting an association between microbial diversity, body composition, and physical function.
While these findings implicate exercise training as a possible therapy for favorably altering the gut microbiota, greater total energy and protein intake in the athlete group may have affected these results. However, correlations between microbial diversity and cardiorespiratory fitness  in concert with the exercise training mediated rescue of high-fat diet-induced microbial disturbance  support the idea that exercise training may impart beneficial effects on gut health. In particular, regular physical activity may help to combat reductions in butyrate-producing microbiota (Clostridiales, Roseburia, Lachnospiraceae, and Erysipelotrichaceae)  and Lactobacillus levels  that are commonly seen with aging [15, 84, 98].
The beneficial changes in gut health seen with exercise training implicate skeletal muscle contraction as a regulator of microbial composition, and provide evidence for bi-directional gut-muscle effects.
As a result, the superior microbial milieu in exercise trained persons may explain lower levels of plasma endotoxin and inflammatory markers (plasminogen activator inhibitor type-1) in active vs sedentary individuals .
Interpreted as a whole, these findings advocate that exercise training is an effective means to promote beneficial changes in gut microbial health that seem to reduce circulating inflammation and support the maintenance of muscle mass and function.
Establishing an Aging Gut-Muscle Communication Axis
Inspection of the existing literature suggests that gut microbiota may play an important role in influencing aging skeletal muscle health (Fig. 1).
We propose regulation of skeletal muscle by gut microbiota to occur in an endocrine manner, beginning with a disturbance in gut homeostasis and culminating with alterations in skeletal muscle characteristics.
While changes in gut microbiota can be attributed to either host (genetics, age) or environmental (activity, diet) factors, we have focused on microbial dynamics with organismal aging.
Although limited in number, investigations of age-related changes in gut microbiota describe a bacterial community that is both less diverse and lacking in microorganisms responsible for the production of butyrate (indicative of gastrointestinal health) [15, 84, 98, 99], and more abundant in pathogens belonging to the Proteobacteria phylum (e.g., Enterobacteriaceae) [15, 68, 83].
Associated with these changes is a decrease in the integrity of epithelial tight junctions and increased intestinal permeability , facilitating the translocation of microbial byproducts into circulation.
Perhaps the most widely studied of these bacterial derivatives is LPS, the major component of the outer membrane of gram-negative bacteria that is found to be elevated in the circulation with aging .
Once in the blood, circulating endotoxin promotes systemic inflammation that appears to trigger maladaptation of skeletal muscle.
As described throughout this review, these changes may manifest in older adults as decreases in muscle size and integrity that compromise physical function and ultimately detract from quality of life.
Lisa LaPoint – Tufts University
The image is in the public domain.
Original Research: Closed access
“Muscle strength is increased in mice that are colonized with microbiota from high-functioning older adults”. Fielding, R.A., Reeves, A.R., Jasuja, R., Liu, C., Barrett, B.B., Lustgarten, M.S.
Experimental Gerontology doi:10.1016/j.exger.2019.110722.