The Open Door Web Site, a coalition of teachers, describes the respiratory quotient as a quotient of the rate of CO2 produced compared to the rate of O2 consumed. It is used to determine the rate of respiration of tissues and organisms and to measure metabolism in animals, including humans.
According to Louis J. Gross at UT Knoxville, the respiratory quotient changes depending on the fuel source consumed. For instance, metabolism of fat requires more oxygen than metabolism of carbohydrates. Using this information, scientists can determine a person or animal's metabolic rate, both at rest or while expending energy, by evaluating the respiratory quotient, notes the American Council on Exercise.