CHRISTMAS 2022: PHYSICAL (FAST FACTS) Measuring human energy expenditure: public health application to counter inactivity This article discusses the use of indirect calorimetry for the measurement of human energy expenditure. Quantification of energy expenditure during low efficiency walking (as depicted in the Monty Python sketch The Ministry of Silly Walks) is presented as an example of how physical activity recommendations could be met by exchanging ~11 minutes per day of normal walking with low efficiency walking—without requiring any further time commitment. Glenn A Gaesser, 1 David C Poole, 2 Siddhartha S Angadi 3 Energy expenditure Human energy expenditure involves the catabolism of macronutrients—mainly carbohydrates and fat—to carbon dioxide and water, which is coupled to the consumption of oxygen. Because this process produces heat, energy expenditure can be measured via direct calorimetry. This method requires a human calorimeter that is cost prohibitive and, more importantly, is limited in its capacity to measure energy expenditure from physical activity. Indirect calorimetry, which requires measurement of ventilation and gas exchange, has greater functionality with broader applications,and is the most widely used method of measuring human energy expenditure during physical activity. Historically, ventilation and gas exchange were assessed by collecting expired air in impermeable canvas (Douglas) bags or large calibrated spirometers and then measuring the concentrations of oxygen and carbon dioxide either chemically or with calibrated electronic gas analysers. 1 Technology advances led to the development of automated systems that are now the standard for assessment of energy expenditure. Automated indirect calorimetry systems consist of a flow meter to measure ventilation, gas analysers to measure concentrations of CO2 and O2 in both inspired and expired air, and computer software that calculates ventilation (V̇E), oxygen uptake (V̇ O2), and carbon dioxide production (V̇CO2). 1 V̇E, V̇O2, and V̇CO2 are usually expressed with a dot above the V to denote the time derivative of measurement (typically per min; ie, L/min for V̇E, and either L/min or mL/kg/min for V̇O2 and V̇CO2), 2 and they are expressed this way in our main article. 3 All require participants to wear a facemask or mouthpiece (with nose clip) to ensure that total ventilation is measured and samples of expired air can be directed to the gas analysers that are secured to a shoulder harness worn by the participant (fig 1). The ratio of V̇CO2:V̇O2 defines the respiratory quotient, which can be used to obtain the energy value per litre of O2 consumed (kcal/L of O2), 4 and then allows for conversion of V̇O2 to energy expenditure (kcal/min or kcal/kg/min). Automated systems have been validated against the optimal Douglas bag method, and have been shown to be accurate and reliable. 1 1 the bmj | BMJ 2022;379:o2937 | doi: 10.1136/bmj.o2937 RESEARCH 1 College of Health Solutions, Arizona State University, Phoenix, AZ, USA 2 Departments of Kinesiology and Anatomy and Physiology, Kansas State University, Manhattan, KS, USA 3 Department of Kinesiology, School of Education and Human Development, University of Virginia, Charlottesville, VA, USA Correspondence to: G A Gaesser Glenn.gaesser@asu.edu https://orcid.org/0000-0001-5127-5336 Cite this as: BMJ 2022;379:o2937 http://dx.doi.org/10.1136/bmj.o2937 Published: 21 December 2022 on 4 October 2023 by guest. Protected by copyright. http://www.bmj.com/ BMJ: first published as 10.1136/bmj.o2937 on 21 December 2022. Downloaded from