What does an RQ of 0.85 indicate?

• A. Lipid oxidation predominant
• B. Carbohydrate oxidation predominant
• C. Mixed substrate utilization or appropriate nutrient delivery
• D. Protein oxidation predominant

Answer: (C)

An RQ around 0.85 reflects mixed substrate use for energy. The respiratory quotient is the CO2 produced divided by O2 consumed, and it varies with the fuel being oxidized: fat oxidation is about 0.70, carbohydrate oxidation is about 1.00, and protein oxidation falls in a range around the mid-0.8s. So 0.85 sits between fat and carbohydrate oxidation, indicating the body is using a combination of fats and carbohydrates (mixed substrate utilization). In clinical nutrition, this suggests energy is being delivered in a way that matches substrate needs—not predominantly fat or carbohydrate alone—consistent with appropriate nutrient delivery. If the value were closer to 0.70, fat would predominate; closer to 1.00 would indicate carbohydrate predominance; the 0.85 value best describes a balanced mix.