Anabolic resistance: the effects of aging, sexual dimorphism, and immobilization on human muscle protein turnover.
Level 5 - mechanism / opinion, no new human data
Narrative review synthesizing mechanistic concepts without primary trial data.
PubMed 19448702 · doi:10.1139/H09-012
What was done
This narrative review summarizes physiological findings on human muscle protein turnover, evaluating how aging, sex differences, and physical inactivity or immobilization affect muscle protein synthesis and breakdown responses to essential amino acids (EAAs), insulin, and resistance exercise.
What was found
The abstract reports no numerical data, effect sizes, or confidence intervals. It describes physiological directions: - The dose-response curve between myofibrillar protein synthesis and EAA availability is shifted downward and to the right in older muscle, and providing extra amino acids does not overcome this refractory state. - Young adults show no sex differences in basal or fed muscle protein turnover, whereas postmenopausal women exhibit greater anabolic resistance than older men. - Anabolic resistance is accompanied by decreased PKB-mTOR-eIF4BP1 pathway phosphorylation following EAA administration. - In older muscle, the ability of low insulin concentrations to suppress protein breakdown is blunted. - Decreased physical activity induces anabolic resistance even in young adults, while resistance exercise may restore amino acid and insulin sensitivity in older muscle.
Why it matters
It emphasizes that age-related sarcopenia is driven by blunted anabolic sensitivity to nutrients rather than basal turnover defects, suggesting that increasing protein intake alone is ineffective without exercise to restore muscle responsiveness.
Limits
The abstract presents summarized conceptual conclusions without primary quantitative data, sample sizes, cohort details, or formal systematic review methodology.
Cited by
- supports Menopause accelerates muscle loss, leading to sarcopenia and anabolic resistance.
- supports Postmenopausal women experience anabolic resistance, where muscle tissue resists signals for growth and synthesis.