The Metabolic Equivalent of Task (MET) is a cornerstone in quantifying the energy expenditure of physical activities. It expresses the intensity of exercise as a multiple of resting metabolic rate (RMR). The foundational 1-MET value, defined as 3.5 ml O2 per kilogram of body weight per minute (or 1 kcal per kilogram per hour), was initially established based on the RMR of a single, average individual. However, given the pervasive use of MET values to gauge physical activity levels and work output, it is crucial to critically assess the accuracy and applicability of this long-standing convention. This investigation seeks to determine if the widely accepted 1-MET value adequately represents the actual resting metabolic rate across a diverse population.
The study encompassed a substantial cohort of 642 women and 127 men, with ages ranging from 18 to 74 years and weights varying from 35 to 186 kg. Participants were in a stable weight condition and generally healthy, although some individuals were classified as obese. Resting metabolic rate (RMR) was meticulously measured for all participants using indirect calorimetry, employing a ventilated hood system. Furthermore, in a subsample of 49 men and 49 women (selected with body mass indices between 26-47 kg/m2 and ages 29-47 years), the energy cost of walking at a consistent pace of 5.6 km/h was also determined. This comprehensive approach allowed for a robust evaluation of the 1-MET standard against real-world physiological data.
Findings: Overestimation of Resting Energy Expenditure
Analysis of the collected data revealed a significant discrepancy between the commonly cited 1-MET values and the measured resting metabolic rates. The average oxygen consumption (VO2) at rest was found to be 2.6 ± 0.4 ml O2 x kg(-1) x min(-1), and the corresponding average energy cost was 0.84 ± 0.16 kcal x kg(-1) x h(-1). Both of these measured values are notably lower than the established 1-MET benchmarks of 3.5 ml O2 x kg(-1) x min(-1) and 1 kcal x kg(-1) x h(-1), respectively. This suggests that the conventional 1-MET values tend to overestimate the actual resting metabolic rate for a significant portion of the population.
Further examination highlighted the dominant role of body composition in determining resting VO2. Fat mass and fat-free mass together accounted for an impressive 62% of the variance in resting VO2. In contrast, age, a factor often considered significant in metabolic studies, explained only 14% of the variance. This finding underscores the importance of individual body characteristics when assessing metabolic rate, suggesting that a one-size-fits-all approach based solely on age may be insufficient.
Implications for Physical Activity Measurement
For a large and heterogeneous sample, the established 1-MET value of 3.5 ml O2 x kg(-1) x min(-1) overestimates the actual resting VO2 by an average of 35%. Similarly, the 1-MET value of 1 kcal/h overestimates resting energy expenditure by approximately 20%. These overestimations have direct implications for the accuracy of physical activity assessments and exercise prescription, particularly when using METs as a primary measure. Activities that are intended to be of moderate intensity, based on standard MET charts, might actually be less metabolically demanding than originally calculated if the individual’s RMR is lower than the assumed 1-MET baseline.
Adjusting for Individual Differences
The study proposes a practical solution to address these inaccuracies: using measured or predicted RMR as a correction factor. By incorporating an individual’s actual resting metabolic rate (expressed in ml O2 x kg(-1) x min(-1) or kcal x kg(-1) x h(-1)), it is possible to appropriately adjust the estimated energy cost of physical activities. This is particularly effective for moderate-intensity exercises like walking at 5.6 km/h, where individual differences in RMR can significantly influence the overall energy expenditure. Implementing such a correction factor can lead to more precise and personalized physical activity guidelines, enhancing the effectiveness of exercise programs and public health recommendations.
