17/08/2025
Evofitlab Clinical-Performance Document
Muscle as an Endocrine Organ – Exercise, Myokines, and Metabolic Health
1. Introduction
Skeletal muscle, once considered solely a contractile tissue responsible for movement, is now recognized as the body’s largest endocrine organ. Through the secretion of myokines—hormone-like signalling proteins released during muscle contraction—skeletal muscle influences distant organs and regulates systemic metabolism, inflammation, and overall health.
This discovery redefines the role of exercise: beyond strength and movement, muscle functions as a metabolic regulator and disease-preventive organ.
2. Muscle as an Endocrine Organ
Traditional View:
• Mechanical function: locomotion, posture, force production.
Modern View:
• Endocrine function: skeletal muscle secretes myokines that act in:
o Autocrine signalling – affecting the same muscle cell.
o Paracrine signalling – affecting nearby cells (e.g., satellite cells, capillaries).
o Endocrine signalling – circulating to distant organs (brain, adipose, liver, pancreas, immune system).
This systemic communication is termed “muscle-organ crosstalk” and is central to understanding exercise as medicine.
3. Key Myokines and Their Functions
Interleukin-6 (IL-6)
• Released in large amounts during acute exercise.
• Improves glucose uptake and fat oxidation.
• Exhibits anti-inflammatory effects by stimulating IL-10 and inhibiting TNF-α.
Irisin
• Cleaved from FNDC5 protein.
• Stimulates the “browning” of white adipose tissue, increasing thermogenesis and energy expenditure.
Myostatin
• Negative regulator of muscle growth.
• Elevated in muscle wasting conditions; exercise suppresses myostatin.
Brain-Derived Neurotrophic Factor (BDNF)
• Enhances neuroplasticity, brain health, and cognition.
• Also promotes fat oxidation in skeletal muscle.
Fibroblast Growth Factor-21 (FGF-21)
• Improves insulin sensitivity, regulates lipid metabolism, and reduces obesity-related metabolic dysfunction.
Interleukin-15 (IL-15)
• Involved in muscle hypertrophy and immune regulation.
• Plays a role in reducing visceral fat mass.
Decorin
• Antagonizes myostatin → promotes muscle growth.
(Over 600 myokines have been identified; above are clinically relevant highlights.)
4. Exercise, Myokines, and Obesity
• Muscle contraction during aerobic and resistance training stimulates myokine release.
• Myokines act on adipose tissue to improve lipolysis, reduce fat accumulation, and fight low-grade inflammation seen in obesity.
• Exercise-induced IL-6 enhances hepatic glucose production during exercise and improves whole-body insulin sensitivity afterward.
• Muscle as an endocrine organ explains why muscle mass is protective against obesity, type 2 diabetes, cardiovascular disease, and cognitive decline.
5. Muscle Loss and Hormonal Health
In Women
• Symptoms often attributed to perimenopause (fatigue, mood changes, fat gain, poor sleep) may actually stem from loss of skeletal muscle mass and reduced myokine signalling.
• Muscle decline reduces insulin sensitivity, increases visceral fat, and lowers protective myokine output—mimicking hormonal imbalance.
• Maintaining lean mass via progressive strength training + protein-rich diet can delay or alleviate many “perimenopausal” symptoms.
In Men
• Muscle loss with age leads to:
o Decline in testosterone and growth hormone feedback.
o Increase in visceral adiposity.
o Reduced energy and metabolic resilience.
• Resistance training and maintaining muscle reverses these declines, improving hormonal balance, reducing sarcopenia, and enhancing quality of life.
6. Clinical & Performance Implications
• Exercise is medicine: prescription should include both resistance training and aerobic activity to maximize myokine release.
• Strength preservation = metabolic preservation: muscle acts as a buffer against obesity, diabetes, cardiovascular disease, and cognitive decline.
• Intervention strategy:
o 2–3x/week resistance training (compound lifts, progressive overload).
o 150 min/week aerobic training (moderate to vigorous).
o High-protein nutrition (1.6–2.2 g/kg bodyweight/day).
o Lifestyle: sleep, stress management, recovery protocols.
7. Conclusion
Skeletal muscle is far more than a system for movement—it is a dynamic endocrine organ central to human health. By secreting myokines, muscle regulates immunity, metabolism, fat balance, brain health, and hormonal systems. Loss of muscle mass with age or inactivity accelerates metabolic dysfunction, obesity, and chronic disease.
The clinical and performance takeaway is clear: exercise preserves endocrine-muscle function, prevents disease, and sustains longevity.
Evofitlab Practitioner’s Statement
Prepared by:
Gerard Nicholas, CSCS, BSc Sports Science, Physiotherapist
Evofitlab
Lp14 Pentecostal Road, Tunapuna
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📧 Email: info@evofitlab.com
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