Which statement correctly describes the respiratory quotient (RQ) and its typical values for carbohydrate and fat oxidation?

• A. RQ equals the ratio of heat produced to oxygen consumption; about 1.5 for carbohydrate and 0.5 for fat
• B. RQ is the ratio of oxygen consumption to carbon dioxide production
• C. RQ equals CO2 produced divided by O2 consumed; about 1.0 for carbohydrate and about 0.7 for fat
• D. RQ is independent of substrate type

Answer: (C)

The respiratory quotient is defined as the ratio of carbon dioxide produced to oxygen consumed during substrate oxidation. This ratio tells you which fuel is being burned. Carbohydrates, like glucose, are metabolized with roughly equal amounts of CO2 produced and O2 used, giving an RQ around 1.0. For example, glucose oxidation follows the equation C6H12O6 + 6 O2 → 6 CO2 + 6 H2O, so CO2 produced per O2 consumed is 6/6 = 1.0. Fats require more oxygen relative to the CO2 they generate, so the RQ is lower, about 0.7; a representative fat oxidation example is palmitate: C16H32O2 + 23 O2 → 16 CO2 + 16 H2O, giving 16/23 ≈ 0.70. This shows that RQ depends on the substrate being oxidized and is not a heat-to-oxygen ratio, nor is it independent of fuel type.