Definition of 1 MET
1 MET (Metabolic Equivalent) equals 3.5 mL of oxygen per kilogram of body weight per minute (3.5 mL O₂/kg/min), which represents the amount of oxygen consumed while sitting at rest. 1
Standard Definition and Clinical Application
The American Heart Association defines 1 MET as the unit of sitting/resting oxygen uptake, standardized at 3.5 mL O₂/kg/min. 1
This value can also be expressed as approximately 1 kcal/kg/hour of energy expenditure. 2
The MET system allows clinicians to express the energy cost of physical activities as multiples of resting metabolic rate, making it a practical tool for exercise prescription and functional capacity assessment. 1, 2
Calculating METs in Practice
To convert measured oxygen consumption to METs, divide the absolute oxygen uptake (mL O₂/kg/min) by 3.5. 3
- For example: If someone consumes 17.5 mL O₂/kg/min during activity, this equals 5 METs (17.5 ÷ 3.5 = 5). 3
The European Heart Journal notes that 1 MET represents basal metabolic rate, and activities requiring 4 METs (such as climbing two flights of stairs) are clinically significant thresholds for assessing perioperative risk. 1
Important Clinical Caveats
The standard 3.5 mL O₂/kg/min value is a convention that may not accurately reflect actual resting metabolic rate for all individuals, particularly in certain populations. 3, 4
Research demonstrates that the actual measured resting oxygen consumption averages 2.6 mL O₂/kg/min in large heterogeneous samples, meaning the standard 1 MET value overestimates actual resting VO₂ by approximately 35%. 4
In older adults (≥60 years), the measured resting metabolic rate averages 2.7 mL O₂/kg/min, and may be as low as 2.4 mL O₂/kg/min in best practice studies—substantially lower than the conventional 3.5 mL O₂/kg/min. 5
Body composition (fat mass and fat-free mass) accounts for 62% of the variance in resting VO₂, while age accounts for only 14%. 4
Individuals with BMI >25 kg/m² have significantly lower resting VO₂ (3.4 mL/kg/min) compared to those with normal BMI (4.0 mL/kg/min). 6
Clinical Relevance
Despite these limitations, the 3.5 mL O₂/kg/min convention remains the standard reference value used in clinical guidelines for exercise testing, risk stratification, and activity prescription. 1
The American Heart Association uses this standard to classify sedentary behavior (<1.5 METs), moderate-intensity activity (3.0-5.9 METs), and vigorous-intensity activity (≥6.0 METs). 1, 3
When precise energy expenditure calculations are needed for individual patients—particularly in obese, elderly, or metabolically diverse populations—using measured or predicted resting metabolic rate as a correction factor provides more accurate estimates. 4, 5