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Fasting Substrate Cycling

Meaning

Fasting Substrate Cycling is the dynamic, metabolically adaptive process where the body shifts its primary energy source from exogenous glucose, derived from recent food intake, to endogenous reserves, primarily fatty acids and ketone bodies. This shift is a crucial hormonal response to a fasting state, governed by a sharp drop in insulin and a coordinated rise in counter-regulatory hormones like glucagon and growth hormone. Efficient cycling is a definitive clinical marker of superior metabolic flexibility and resilience.