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Metabolic equivalent: one size does not fit all

Nuala M. Byrne,^1,2 Andrew P. Hills,¹ Gary R. Hunter,^2,3 Roland L. Weinsier,^2, and Yves Schutz⁴

¹School of Human Movement Studies, Queensland University of Technology, Kelvin Grove, Australia; ²Department of Nutrition Sciences, Division of Physiology and Metabolism, and ³Department of Human Studies, University of Alabama at Birmingham, Birmingham, Alabama; and ⁴Institute of Physiology, Faculty of Medicine, University of Lausanne, Lausanne, Switzerland

Submitted 9 January 2004 ; accepted in final form 13 April 2005

The metabolic equivalent (MET) is a widely used physiological concept that represents a simple procedure for expressing energy cost of physical activities as multiples of resting metabolic rate (RMR). The value equating 1 MET (3.5 ml O₂·kg^–1·min^–1 or 1 kcal·kg^–1·h^–1) was first derived from the resting O₂ consumption (O₂) of one person, a 70-kg, 40-yr-old man. Given the extensive use of MET levels to quantify physical activity level or work output, we investigated the adequacy of this scientific convention. Subjects consisted of 642 women and 127 men, 18–74 yr of age, 35–186 kg in weight, who were weight stable and healthy, albeit obese in some cases. RMR was measured by indirect calorimetry using a ventilated hood system, and the energy cost of walking on a treadmill at 5.6 km/h was measured in a subsample of 49 men and 49 women (26–45 kg/m²; 29–47 yr). Average O₂ and energy cost corresponding with rest (2.6 ± 0.4 ml O₂·kg^–1·min^–1 and 0.84 ± 0.16 kcal·kg^–1·h^–1, respectively) were significantly lower than the commonly accepted 1-MET values of 3.5 ml O₂·kg^–1·min^–1 and 1 kcal·kg^–1·h^–1, respectively. Body composition (fat mass and fat-free mass) accounted for 62% of the variance in resting O₂ compared with age, which accounted for only 14%. For a large heterogeneous sample, the 1-MET value of 3.5 ml O₂·kg^–1·min^–1 overestimates the actual resting O₂ value on average by 35%, and the 1-MET of 1 kcal/h overestimates resting energy expenditure by 20%. Using measured or predicted RMR (ml O₂·kg^–1·min^–1 or kcal·kg^–1·h^–1) as a correction factor can appropriately adjust for individual differences when estimating the energy cost of moderate intensity walking (5.6 km/h).

exercise prescription; body composition; exercise intensity; energy expenditure

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