Your Muscle Is a Metabolic Organ — and You've Been Treating It Like Decoration

By Dr. Eugene Capitano, DC, MSc

Ask most people what muscle is for and you'll hear about lifting, walking, posture, and maybe aesthetics. All true, and all beside the biggest point. Skeletal muscle is the body's largest insulin-responsive organ, a major glycogen and amino-acid reservoir, a site of oxidative metabolism, and a source of signaling molecules that communicate with your liver, fat tissue, bone, and immune system. Treating it as decoration is a metabolic mistake.

The glucose sponge

Under insulin-stimulated conditions after a meal, skeletal muscle accounts for the majority of glucose uptake from circulation, commonly estimated at 70 to 80 percent in metabolically healthy people. Glucose taken up by muscle is stored as glycogen or burned for energy, and because muscle is so much larger than the liver, it confers the body's biggest total glycogen-storage capacity. When you eat carbohydrate, muscle is where most of it is supposed to go.

This is why muscle loss is clinically relevant long before a person looks frail. Cohort studies associate lower muscle indices and muscle loss with insulin resistance, metabolically unhealthy phenotypes, and future type 2 diabetes, although body fat, activity levels, and reverse causality complicate interpretation, so these links should be read as associations rather than proof.

The part the fitness industry gets wrong

Here is where accuracy matters because the phrase 'more muscle means better glucose control' is incomplete. Muscle glucose uptake depends on the amount of tissue, yes, but also on insulin signaling, GLUT4 transport, capillary perfusion, glycogen status, recent contraction, mitochondrial flexibility, and fat infiltration. A large muscle compartment that is inactive, marbled with lipid, and insulin resistant can dispose of glucose less effectively than a smaller but active, insulin-sensitive one.

The flip side is genuinely good news. Muscle contraction stimulates glucose uptake through pathways partly independent of insulin. That is why resistance and aerobic exercise can improve glycemic control before any measurable muscle growth occurs. The metabolic benefits of training are not waiting on your physique to change, they begin with the activity itself. The accurate model is that whole-body glucose disposal reflects muscle quantity, tissue quality, perfusion, insulin sensitivity, and how recently the muscle contracted.

A signaling organ — described carefully

Contracting muscle releases peptides and metabolites such as myokines and exerkines, including contraction-associated interleukin-6, interleukin-15, apelin, and signals involved in tissue remodeling and immune-metabolic communication. Muscle can confidently be called a signaling organ. But this field demands careful language because the same molecule can act differently depending on source, concentration, and context. The transient IL-6 pulse from exercise is not the same chronically elevated inflammatory IL-6 of disease. Brain-derived neurotrophic factor (BDNF) is one of the most extensively studied neurotrophins in the exercise-cognition literature. It is produced primarily in the central nervous system (brain), but BDNF expression has also been reported in skeletal muscle and other peripheral tissues. Muscle contraction is the major stimulus for this signaling. Preserving muscle without using it is not biologically equivalent to maintaining an active muscle organ. The tissue and its use are a package.

Mitochondria: aging is not the whole story

Muscle houses an extensive mitochondrial network, and earlier literature blamed aging directly for its decline. More recent work complicates that: in active adults, mitochondrial respiration can be preserved across much of the lifespan, while inactive adults show poorer energetics. In a 2025 study of 139 men aged 20 to 93, physical activity, not age itself, was associated with better mitochondrial energetics, although mitochondrial calcium-retention capacity declined with age regardless of activity. The fair summary: some mitochondrial aging is real, but habitual inactivity masquerades as aging far more than we assumed.

What this changes in practice

If muscle is a metabolic organ, then training is organ maintenance, not vanity. The research supports a combined approach, because the two modalities do different jobs. Resistance training preserves and builds the amount and force-generating capacity of tissue, while aerobic exercise improves its oxidative quality and vascular support. The strongest healthy-aging strategy in the literature is explicitly not 'cardio versus weights', it is both, with balance and functional work added as age or fall risk warrant.

Muscle you have is potential. Muscle you use is metabolism.

DOWNLOAD THE FULL SCIENTIFIC PDF - Skeletal Muscle as a Metabolic, Endocrine, and Functional Organ

---

DISCLOSURE

Dr. Eugene Capitano is Co-Founder and Scientific Lead of TLC PureOrigin™ and Founder of TLC NeuroMicrobiome Labs Inc. This commercial relationship should be considered when evaluating commentary about protein products, ingredients and formulation.

Medical and Nutritional Disclaimer

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.