The respiratory quotient (RQ) of pure fat oxidation is approximately 0.7. The physiological basis for this low value compared with carbohydrate (RQ = 1.0) is:

• A Fat oxidation produces more CO2 per O2 consumed due to longer carbon chains
• B Fat bypasses the TCA cycle and directly generates CO2 via the pentose phosphate pathway
• C Fat oxidation requires less ATP hydrolysis than carbohydrate metabolism
• D Fat is more reduced (higher H:C ratio) than carbohydrate, requiring more O2 to oxidise but generating less CO2 per unit energy ✓

Explanation

RQ = VCO2/VO2. Carbohydrates are already partially oxidised (C(H2O)n), so each carbon already has one oxygen attached; RQ = 1.0. Fatty acids are highly reduced (low O:C ratio), requiring proportionally more molecular oxygen for complete combustion relative to CO2 produced — hence RQ ≈ 0.7. Mixed diet RQ ≈ 0.85. Protein RQ ≈ 0.82. An RQ > 1.0 indicates net lipogenesis from carbohydrate.

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

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