Measuring Energy Expenditure Independent Of The Respiratory Quotient During Rest And Exercise

Abstract

Widely used measures of energy expenditure (EE) are based on Weir’s (1949) equation that relies on oxygen utilization and carbon dioxide production to derive a respiratory quotient (RQ). However, Weir’s RQ-free equation, presented in the same article, was only recently validated to measure home-cage EE in rodents. It has not yet been used to measure EE in humans. PURPOSE: To evaluate an RQ-free method for measuring EE at rest and during submaximal and maximal intensity treadmill running. METHODS: A convenience sample of 27 physically-active college students (17 women) were recruited to perform a maximal treadmill test until volitional exhaustion (Bruce protocol). EE (kcal/min) was measured continuously at 10 Hz using an open-flow respirometry system (Sable Systems Intl.; Las Vegas, NV). EE was down-sampled and averaged at rest (EERest), during the first stage (EE1), during the last completed stage (EESubmax), and during the final stage (EEMax). The intensity (VO2; ml/kg*min) of the last completed stage and the final stage were calculated based on speed and incline using ACSM equations. Heart rate (HR) was measured at rest and every minute during the test. Participants rated their perceived exertion (RPE) at the end of each stage. VO2max was estimated based on the total time spent walking/running on the treadmill. Four multiple regression models were used to predict EE from participant traits (PT; gender, age, weight), stage intensity, HR, RPE, and estimated VO2max (EEMax only). A repeated-measures ANOVA was used to test the degree to which changes in EE were explained by changes in HR (treated as a time-varying covariate). RESULTS: EERest was predicted by PT and resting HR [p<.001, R2 = .658]. EE1 was predicted by PT and HR and RPE in Stage 1 [p<.001, R2 = .871]. EESubmax was predicted by PT, the intensity of, and HR and RPE response to, the last completed stage [p<.001, R2 = .844]. EEMax was predicted by PT, VO2max, and the intensity of, and HR and RPE response to, the final stage [p<.001, R2 = .746]. 83% of the increase in EE, from rest to the final stage, was explained by increasing HR. CONCLUSIONS: The reliability (R2) of our models for predicting EE at rest and during exercise are comparable to published RQ-dependent models, supporting the utility of an RQ-free method for measuring EE during submaximal exercise.