Why Your Microbiome May Be a Missing Variable in Muscle Growth

By Dr. Eugene Capitano, DC, MSc

[Download the full scientific PDF with references below.]

"You are not just what you eat. You are also what your gut lets you absorb, and how well your body is set up to use it."

After a year spent reviewing hundreds of peer-reviewed studies on the gut-muscle axis, one theme kept surfacing: we tend to optimize one obvious variable , grams of protein, while under-weighting another that may shape how well that protein actually works: the gut environment.

For years, athletes, clinicians, and supplement users have focused on protein dose, timing, amino acid profile, bioavailability, and leucine. Those still matter. They are the foundation of muscle growth. But there is a second layer that may influence how much of that work pays off: the state of your gut microbiome, intestinal barrier, inflammation, and insulin sensitivity.

This does not mean the microbiome magically builds muscle. It does not. Muscle protein synthesis happens inside skeletal muscle. But the gut may help determine whether your body is in a low-friction, anabolic environment - or a chronically inflamed one that responds less efficiently to the same protein and training.

To be precise about the biology, muscle protein synthesis is triggered by resistance exercise, essential amino acids, and leucine acting through mTOR-related signaling inside skeletal muscle. The microbiome is not the switch that turns muscle protein synthesis on.

What the gut does is help shape the terrain in which that switch operates. It influences digestion, amino acid availability, immune signaling, gut barrier integrity, microbial metabolites, and inflammatory tone.

A balanced microbiome ferments fibers and resistant starches into short-chain fatty acids, especially butyrate. Butyrate helps fuel the cells lining the colon, supports tight junction integrity, and contributes to a healthier inflammatory environment.

• A healthier intestinal barrier helps keep bacterial products where they belong - inside the gut.
• A better microbial environment may support insulin sensitivity and lower inflammatory signaling.
• Lower chronic inflammation may allow muscle to respond more efficiently to protein and training.

When the gut barrier is compromised, bacterial fragments such as lipopolysaccharide, or LPS, can enter circulation and activate inflammatory pathways. Those pathways can interfere with insulin and mTOR-related signaling, nudging the body toward what researchers call anabolic resistance which is a blunted response to the same protein and exercise stimulus.

The honest framing is this: the microbiome does not determine your gains, but it may shape how well you respond to the protein and training you already do. That may matter most in older adults and in people with chronic inflammation, insulin resistance, digestive dysfunction, or low-fiber dietary patterns. In young, healthy, well-fed athletes, the human evidence is still limited and mixed.

Two people can consume the same amount of protein and perform the same training program, yet experience different results. Part of that difference comes from age, training history, total energy intake, sleep, hormones, and genetics. But the scientific literature also points to another variable: the internal environment in which muscle is trying to respond.

When the gut barrier is strong, microbial metabolites are favorable, and chronic inflammation is low, dietary amino acids arrive in a body that is more prepared to use them. When the gut barrier is disrupted and inflammatory signaling is elevated, the same protein dose may produce a smaller anabolic response.

That is the practical value of the gut-muscle axis. It does not replace the protein conversation. It makes the protein conversation more complete.

None of this replaces the basics. If you want to build or preserve muscle, the first pillars remain resistance training, adequate total protein, enough essential amino acids, sufficient leucine per meal, sleep, recovery, and enough calories to support adaptation.

The microbiome conversation adds another layer. It asks whether the body is in the right condition to use those inputs well.

This is especially important with aging. Older adults often experience anabolic resistance, meaning muscle becomes less responsive to protein and exercise. The solution is not simply to chase more protein forever. The better approach is to combine high-quality protein with progressive resistance training and a gut-supportive diet that helps reduce unnecessary inflammatory friction.

• Choose a protein source that delivers enough essential amino acids and leucine.
• Pair protein with resistance training rather than expecting supplementation to do the job alone.
• Support the gut with fiber-rich whole foods, resistant starches, polyphenols, and tolerated fermented foods.
• Choose products and meals that are easy to tolerate and fit within an overall gut-supportive dietary pattern.
• Do not treat protein grams as the only variable that matters.
• If you have IBS, inflammatory bowel disease, food intolerances, kidney disease, or metabolic disease, personalize the strategy with a qualified clinician.

Muscle growth does not begin in the gut. It begins inside skeletal muscle when training, amino acids, leucine, insulin, and cellular energy signals converge.

But the gut may influence how well that process works. A healthier microbiome and intestinal barrier can help create a more favorable physiological environment for protein to do what you are taking it to do.

That is why the next evolution in sports nutrition will not be only about more protein. It will be about better protein, better tolerance, better digestive ecology, and a body that is better prepared to use the nutrients it receives.

Dr. Eugene Capitano, DC, MSc, is a chiropractor, rehabilitation clinician, and researcher with more than 25 years of clinical experience. He earned an MSc in Psychology & Neuroscience of Mental Health from King’s College London and holds the ACSM Exercise is Medicine® (EIM) Credential and the ACSM Certified Personal Trainer® (ACSM-CPT®) certification. His research interests include the gut–brain–muscle axis, microbiome-targeted nutrition, resistance training, mitochondrial function, metabolic health, and healthy aging.

Affiliation: TLC NeuroMicrobiome Labs Inc., Winnipeg, Manitoba, Canada

This information is provided for educational purposes only and is not intended to diagnose, treat, cure, or prevent any disease. Individuals must consult a qualified healthcare provider or registered dietitian before making significant changes to their diet or exercise regimen. Important Safety Notice: Individuals with pre-existing renal impairment, diabetes, or other chronic metabolic conditions should consult a healthcare provider before significantly increasing protein intake. Higher-end protein intakes should be used only under medical supervision, with regular monitoring of renal function.