Key Concepts

• Interconnectedness of Muscle and Bone: The musculoskeletal system, composed primarily of muscle and bone, plays a crucial role in movement and metabolic health. These tissues are anatomically and physiologically connected, impacting each other's metabolism and overall health.
• Endocrine Roles: Beyond their biomechanical functions, muscle, bone, and adipose tissue (AT) secrete cytokines that act as signaling molecules, influencing the metabolism of each other and the whole body.

Effects of Aging and Inactivity:

• The article emphasizes that aging, physical inactivity, and a chronic energy surplus (consuming more calories than expended) contribute to a decline in muscle and bone mass and function, leading to conditions like:
• Osteopenia/Osteoporosis:
• Sarcopenia:
• Osteosarcopenia: The combined occurrence of osteopenia/osteoporosis and sarcopenia.

Myokines and Bone Metabolism:

• IL-6: This myokine has complex effects. Systemically, it's pro-inflammatory, but when released from muscles during exercise, it exhibits anti-inflammatory effects. However, IL-6 can promote bone resorption.
• Myostatin: A negative regulator of muscle mass, myostatin also negatively impacts bone remodeling, contributing to a catabolic, resorptive state.
• Anabolic Myokines: Growth factors like IGF-1, FGF2, and irisin have shown anabolic effects on bone, promoting bone formation.
• Irisin: This recently discovered myokine, also secreted by fat tissue, has shown potential in improving bone mass and insulin sensitivity.

Osteokines and Muscle Metabolism:

• Osteocalcin (OCN): Particularly its undercarboxylated form (ucOCN), increases insulin sensitivity and secretion. Studies suggest ucOCN promotes glucose uptake in muscles and might be involved in muscle hypertrophy.
• Sclerostin (SOST): An osteokine that inhibits bone formation, Sost's role in muscle is complex and still under investigation. While some evidence suggests a potential anabolic effect, recent studies show a negative correlation with muscle mass.

Adipokines' Impact:

• Leptin, Resistin, TNFα: These adipokines are associated with increased adiposity and LGI, negatively impacting both bone mineral density and muscle mass.
• Adiponectin: This adipokine has anti-inflammatory effects and promotes fatty acid oxidation and glucose uptake in skeletal muscle. Its expression is inversely related to fat mass.

Exercise as a Countermeasure:

• Exercise and Inflammation: Regular exercise reduces systemic LGI and the inflammatory response to stressors.
• Bone Benefits: Weight-bearing exercise promotes bone formation and strength.
• Muscle Advantages: Exercise stimulates the release of myokines that promote muscle growth and improve metabolic health.
• Adipose Tissue Modulation: Exercise promotes fat mobilization, reduces adipokine secretion (except for beneficial adiponectin), and contributes to a shift towards an anti-inflammatory state.

Suggested reading:

Muscle, Bone, and Fat Crosstalk: the Biological Role of Myokines, Osteokines, and Adipokines. https://pubmed.ncbi.nlm.nih.gov/32529456/