A patient has a resting metabolic rate measured by indirect calorimetry. The respiratory quotient (RQ) is found to be 0.7. This value indicates predominant oxidation of which substrate?

• A Fat (fatty acids) ✓
• B Carbohydrate (glucose)
• C Mixed diet (protein + fat + carbohydrate)
• D Protein (amino acids)

Explanation

The respiratory quotient (RQ) = VCO2 produced / VO2 consumed. For pure fat oxidation, complete combustion of a typical fatty acid (e.g., palmitate: C16H32O2 + 23O2 → 16CO2 + 16H2O) gives RQ = 16/23 ≈ 0.70. For pure carbohydrate (glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O), RQ = 1.0. For protein, RQ ≈ 0.82. A mixed diet gives RQ ≈ 0.85. An RQ of 0.7 indicates predominantly fatty acid oxidation, as seen during prolonged fasting or ketogenic states.

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

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