A clinical researcher measures resting metabolic rate (RMR) by indirect calorimetry. The respiratory quotient (RQ) of a patient on a fat-predominant diet is 0.72. This indicates that per mole of substrate oxidised, the substrate is generating:

• A Equal moles of CO2 and O2 consumed
• B More moles of CO2 than O2, indicating de novo lipogenesis
• C RQ of 0.72 indicates mixed carbohydrate-protein substrate utilisation
• D Fewer moles of CO2 produced than O2 consumed, consistent with fat oxidation ✓

Explanation

Respiratory quotient (RQ) = VCO2/VO2. For palmitate oxidation: C16H32O2 + 23O2 → 16CO2 + 16H2O, giving RQ = 16/23 ≈ 0.70. For pure fat oxidation RQ is ~0.7, for carbohydrates it is 1.0, and for proteins ~0.82. Mixed diet yields ~0.85. An RQ of 0.72 indicates predominantly fat oxidation where fewer CO2 molecules are produced per O2 consumed because fatty acids are more reduced (hydrogen-rich) than carbohydrates. An RQ >1.0 indicates net lipogenesis (carbohydrate converted to fat).

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

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