A respiratory quotient (RQ) of 0.87 most likely suggests

• A. Primarily fat oxidation
• B. Mixed substrate utilization
• C. Primary carbohydrate oxidation
• D. Primary protein oxidation

Answer: (B)

Respiratory quotient reflects which substrates are being oxidized by the body. Carbohydrates give about 1.0, fats about 0.7, and proteins around 0.8. A value of 0.87 sits between fat and carbohydrate, and near the protein range, indicating that more than one substrate is being used rather than a single fuel source. This pattern is best described as mixed substrate utilization, common when both fats and carbohydrates contribute to energy, with some protein oxidation also possible contributing to the intermediate RQ.