The Metabolic Equivalent of Task (MET) is a fundamental concept in exercise physiology, employed to express the energy expenditure of physical activities relative to an individual’s resting metabolic rate (RMR). The standard definition of 1 MET, set at 3.5 ml O2 x kg(-1) x min(-1) or 1 kcal x kg(-1) x h(-1), was originally derived from the resting oxygen consumption (VO2) of a single 70-kg, 40-year-old male. Given the widespread reliance on MET values for quantifying physical activity levels and work output, a study was conducted to assess the accuracy and adequacy of this convention.
The research involved a diverse cohort of 642 women and 127 men, ranging in age from 18 to 74 years and weighing between 35 and 186 kg. Participants were healthy and weight-stable, though some were classified as obese. Resting metabolic rate was precisely measured using indirect calorimetry within a ventilated hood system. Furthermore, the energy cost of walking at a consistent pace of 5.6 km/h on a treadmill was assessed in a subsample of 49 men and 49 women, all of whom had a body mass index (BMI) between 26-47 kg/m2 and were aged 29-47 years.
Analysis revealed that the average VO2 and energy cost associated with rest were significantly lower than the established 1-MET values. Specifically, resting VO2 averaged 2.6 +/- 0.4 ml O2 x kg(-1) x min(-1), and resting energy expenditure was 0.84 +/- 0.16 kcal x kg(-1) x h(-1). These measured values are notably lower than the commonly cited 1-MET equivalents of 3.5 ml O2 x kg(-1) x min(-1) and 1 kcal x kg(-1) x h(-1), respectively.
The study further elucidated that body composition, specifically fat mass and fat-free mass, played a more significant role in determining resting VO2 than age. Body composition accounted for 62% of the variance in resting VO2, whereas age only accounted for 14%. This highlights the substantial impact of an individual’s physical makeup on their baseline energy expenditure.
Consequently, for a broad and heterogeneous sample population, the standard 1-MET value of 3.5 ml O2 x kg(-1) x min(-1) tends to overestimate actual resting VO2 by approximately 35%. Similarly, the 1-MET value of 1 kcal/h overestimates resting energy expenditure by about 20%.
To address these discrepancies and enhance accuracy, the research suggests that utilizing measured or predicted RMR as a correction factor is crucial. This approach can effectively adjust for individual differences in baseline metabolism when estimating the energy cost of moderate-intensity activities, such as walking at 5.6 km/h. By incorporating an individual’s RMR, a more personalized and accurate assessment of energy expenditure can be achieved, moving beyond the limitations of a one-size-fits-all MET definition.