The respiratory quotient (RQ) provides information about the substrate being oxidized. Which metabolic state is associated with RQ < 0.7?

• A Pure fat oxidation with superimposed ketogenesis ✓
• B Pure carbohydrate oxidation
• C Mixed diet oxidation
• D Active lipogenesis (fat synthesis from glucose)

Explanation

RQ = VCO2 / VO2. For pure carbohydrate oxidation, RQ = 1.0; for pure fat oxidation, RQ ≈ 0.7; for protein, RQ ≈ 0.8; mixed diet RQ ≈ 0.85. When lipogenesis occurs (synthesizing fat from glucose), extra CO2 is released per O2 consumed, pushing RQ above 1.0. An RQ < 0.7 indicates active ketogenesis alongside fat oxidation — during ketogenesis, carbon is retained as ketone bodies (not released as CO2), so net CO2 production decreases relative to O2 consumption, dropping RQ below 0.7. This is seen in prolonged fasting and diabetic ketoacidosis.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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