During moderate-intensity exercise (60% VO₂max), which statement correctly describes substrate utilization and the hormonal milieu supporting it?
- A Fat oxidation and carbohydrate oxidation contribute approximately equally; glucagon/insulin ratio rises modestly, hepatic glycogenolysis provides glucose, and adipose hormone-sensitive lipase is activated by catecholamines ✓
- B Carbohydrate is the exclusive fuel; insulin rises to facilitate muscle glucose uptake via GLUT4
- C Fat oxidation is negligible; plasma glucagon is suppressed and muscle relies entirely on stored glycogen
- D Protein catabolism exceeds lipid oxidation to provide gluconeogenic substrates for sustained activity
Correct answer: A. Fat oxidation and carbohydrate oxidation contribute approximately equally; glucagon/insulin ratio rises modestly, hepatic glycogenolysis provides glucose, and adipose hormone-sensitive lipase is activated by catecholamines
Explanation
At moderate exercise intensity (~60% VO₂max), the crossover point is near; both fat and carbohydrate contribute substantially. Sympathoadrenal activation raises catecholamines, which activate adipose HSL (lipolysis) and hepatic phosphorylase (glycogenolysis). Insulin falls and glucagon rises modestly, maintaining hepatic glucose output. GLUT4 translocates to muscle membranes via contraction-mediated insulin-independent signaling (AMPK), facilitating glucose uptake. Fat oxidation is greatest at 25–65% VO₂max and diminishes at higher intensities.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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