*Geek Box: Indirect Calorimetry

If you read metabolic ward studies, you may come across the term ‘respiratory chamber’ and see that energy expenditure was measured by what is known as ‘indirect calorimetry’. A respiratory chamber is exactly what it sounds like; a room in which the participant can breathe, and that air can be measured within the chamber. All macronutrients – proteins, fats, carbohydrates – contain the element, carbon. Indirect calorimetry is a measurement where all oxygen consumption and carbon dioxide exhalation is continually measured, with constant values for oxygen being input, the rate of production and amount of carbon dioxide produced provides data to calculate energy expenditure through the use of equations. In addition, the values of protein, carbohydrate, and fat oxidation can also be calculated from this data, indicating what substrate is being utilised, and at what rate. Indirect calorimetry can also be measured by breathing under Perspex hoods which control the flow of oxygen, and certain technology even allows for mouthpieces to be worn to assess energy expenditure during, for example, exercise. However, because indirect calorimetry requires sophisticated equipment, it is not always feasible in research. Where available, indirect calorimetry provides a reliable estimate of energy expenditure in humans.