*Geek Box: Respiratory Quotient
To determine how much energy we use at rest, known as ‘resting metabolic rate’ [RMR], how much energy we are using after a meal, and what fuel sources we are using for this energy – carbohydrate or fat – researchers can use a technique called ‘indirect calorimetry’. It’s an indirect measure, because it measures the exchange of oxygen [O2] and carbon dioxide [CO2], to determine energy expenditure. This is possible, because all macronutrients – protein, carbohydrate, and fat – contain carbon. By measuring the ratio of CO2 being produced to O2 being consumed, either fasting or following food, it is possible to calculate the ‘respiratory quotient’, known as the ‘RQ’. Normal RQ ranges from 0.7-1.0; an RQ of 0.68-0.7 roughly indicates either a fasted state, or purely fat metabolism [such as during a ketogenic diet], while an RQ of 1.0 would reflect purely carbohydrate oxidation, which could be observed during high-intensity anaerobic exercise. For meals with mixed carbohydrate and fats, an RQ of around 0.8 is typical. Measuring RQ can be done using any apparatus that allows for collecting the breath of the person being measured, either breathing under a hood, or in modern facilities, in a whole room which allows for constant flow of O2 and CO2 to be inspired and expired, and measured.