A patient undergoing resting metabolic rate (RMR) measurement by indirect calorimetry has VO2 = 250 mL/min and VCO2 = 200 mL/min. What is the respiratory quotient (RQ) and what does it indicate about the primary fuel being oxidized?
- A RQ = 1.0; predominant carbohydrate oxidation
- B RQ = 0.80; mixed substrate oxidation with predominant fat utilization ✓
- C RQ = 0.70; predominant fatty acid oxidation
- D RQ = 0.85; predominant protein oxidation
Correct answer: B. RQ = 0.80; mixed substrate oxidation with predominant fat utilization
Explanation
RQ = VCO2/VO2 = 200/250 = 0.80. RQ values: pure fat oxidation = 0.70 (low CO2 produced per O2 consumed due to high H:O ratio in fatty acids); pure carbohydrate = 1.0 (equal CO2 and O2 for glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O); pure protein = ~0.82. An RQ of 0.80 indicates a mixed substrate utilization with fat as the predominant fuel, typical of a fasting-state measurement. Values >1.0 indicate lipogenesis (CO2 generated without proportionate O2 consumption).
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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