Metabolic equivalent

The Metabolic Equivalent of Task (MET), or simply Metabolic Equivalent, is a physiological concept expressing the energy cost of physical activities ^[1] as multiples of Resting Metabolic Rate (RMR) and is defined as the ratio of metabolic rate (and therefore the rate of energy consumption) during a specific physical activity to a reference rate of metabolic rate at rest, set by convention to 3.5 ml O₂ ^. kg^{-1 .} min^-1 or equivalently 1 kcal (or 4.184 kJ) ^. kg^{-1 .} hr^-1. By convention 1 MET is considered as the resting metabolic rate obtained during quiet sitting. ^[2][3] MET values of physical activities range from 0.9 (sleeping) to 18 (running at 17.5 km/h).

Although the RMR of any specific person may deviate significantly from the above mentioned conventional reference value ^[4][5], MET values of physical activities provide a rough indication of the intensity of physical activities such as manual labour or exercise. MET is actually an index number and not an energy unit: a physical activity with a MET value of 2, such as walking at a slow pace (e.g., 3 km/h) would require for a specific person twice the energy that person consumes at rest (e.g., sitting quietly).

More specifically, MET is used as a practical means of expressing the intensity and energy expenditure of physical activities in a way comparable among persons of different weight. Actual energy expenditure (e.g., in Kcal or Joules) during a physical activity depends on the person's body mass, therefore the energy cost of the same physical activity will be different for persons of different weight. However, since the RMR is also dependent on body mass in a similar way, it is assumed that the ratio of this energy cost to the RMR of each person will remain more or less stable for the specific physical activity and thus independent of each person's weight.

The 1-MET reference value of 1 kcal (or 4.184 kJ) ^. kg^{-1 .} hr^-1, is used by convention and refers to a typical metabolism at rest of an "average" individual. Even so, it must not be confused or misused as an approximation of Basal Metabolic Rate (BMR), which is the minimum metabolic rate obtained under specified conditions. This is illustrated by the fact that sleeping for instance has a MET of 0.9, while normal sleeping metabolism may be greater than the BMR.

Scope of Usage of the MET Concept

Epidemiology and Public Health

The MET concept is implicitly based on a statistical approach and has been primarily designed to be used in epidemiological surveys, where survey respondents answer the amount of time they spend for specific physical activities. ^[6]

Moreover MET is used to provide general medical thresholds and guidelines to a population. ^[7][8] Since MET is a measure of intensity and rate, the concept of MET-minute can be used to quantify the total amount of physical activity in a way comparable across different persons and types of activities. Thus brisk walking at 5 km/h for half an hour (a moderate intensity activity of 3.3 MET) accounts for about 100 MET-min and is in this aspect equivalent to running at 10 km/h for ten minutes (a vigorous intensity activity of 10 MET). This way the total effort expended in different activities over a period of time can be accumulated: a common guideline is that total regular physical activity must lie in the range of 500 to 1,000 MET-minutes per week to produce substantial health benefits for adults. Note that there is a statistically observed dose-response relationship between MET-min and health benefits.

Comparison and Classification of Physical Activities

MET is also used as a method to indicate and compare the absolute aerobic intensity and energy expenditures of different physical activities. In this context the concept of MET is also used to prescribe exercise in clinical settings, such as in rehabilitation of patients with cardiovascular diseases. Tables of experimentally estimated typical MET values for exercise and other physical activities are published.

A common classification for public health purposes is: ^[9][10]

• Light-intensity activities are defined as 1.1 MET to 2.9 MET;
• Moderate-intensity activities are defined as 3.0 to 5.9 METs;
• Vigorous-intensity activities are defined as 6.0 METs or more.

Consequently the recommendation for 500 to 1,000 MET-minutes per week can be translated to at least 2.5 hours (150 minutes) of moderate-intensity activity per week, or at least 1.5 hours (90 min) of vigorous intensity activity etc.

Indicative examples of nominal MET values for different activities are provided in the following table: ^[11]

Individual Functional Capacity

MET can also be used to provide an indication of an individual's functional capacity, more formally measured by VO_{2 max} (i.e. maximal oxygen uptake, or the maximum rate of oxygen consumption that a person can achieve), thus providing an indicative translation of VO_{2 max} measurements to equivalent maximum activity intensities and vice versa. ^[12]

For example a person with a measured maximum oxygen uptake of 60ml ^. kg ^{-1 .} min ^-1 would have an indicative functional capacity of approximately 17 MET (since 1 MET is by convention 3.5 ml ^. kg ^{-1 .} min ^-1). Practically this indicates that this person can achieve to execute a physical activity of 17 MET intensity. According to typical surveys, an average healthy young male adult has a functional capacity of about 13 MET (45 ml ^. kg ^{-1 .} min ^-1 VO₂max), an average healthy middle-aged male adult one of 10 MET, while a typical patient with severe pulmonary disease a functional capacity of only 4 MET. ^[13]

Limitations in the usage of MET to calculate actual energy expenditure

It must be noted that published MET values (or exercise calorie calculators on web sites, which are based on such values) for specific activities are experimentally and statistically derived from a sample of persons and are in fact indicative averages. Obviously the level of intensity at which a specific person performs a specific physical activity (e.g., the pace of walking, the speed of running, etc.) will deviate from the representative experimental conditions used for the calculation of the standard MET values, but moreover, as is explained in the following, the actual energy expenditure and the RMR will differ according to the person's overall fitness level and other factors.

The same holds for MET (or kcal) values indicated in modern fitness exercise equipment, which are based on statistical models and are of indicative value only. In this case, even if the MET value indicated is a better statistical prediction than published tables, there is no way to account for the person's actual RMR and thus energy expenditure (e.g., Kcal). In short, a person can use the MET concept to plan or monitor physical activity levels or get an indication of the aerobic intensity and order of magnitude of energy expenditure for a specific activity, but not use the MET concept to calculate actual energy expenditure or a daily energy input-output balance.

More specifically, from a strictly scientific point of view, statistically estimated predictions, such as MET or BMI, are inaccurate when used for specific persons, and MET values must be treated as indicative only, taking into account that both RMR and actual energy consumption are highly dependent on physical and environmental factors such as adiposity, physical fitness level, cardiovascular health, or even ambient temperature.

Moreover, even the definition of MET is problematic when used for specific persons^[14][15]. By convention, 1 MET is considered equivalent to the consumption of 3.5 ml O² kg^-1 min^-1 (or 3.5 ml of oxygen per kg of body mass per minute) and is roughly equivalent to the expenditure of 1 kcal per kg of body weight per hour. This value was first experimentally derived from the resting oxygen consumption of a particular subject (a healthy 40-year-old, 70-kg man) and must therefore be treated as a convention. Since the RMR of a person depends mainly on lean body mass (and not total weight) and other physiological factors such as health status, age, etc., actual RMR (and thus 1-MET energy equivalents) may vary significantly from the kcal-per-kg-per-hour rule of thumb. RMR measurements by calorimetry in medical surveys have shown that the conventional 1-MET value overestimates the actual resting O² consumption and energy expenditures by about 20% to 30% on the average, whereas body composition (ratio of body fat to lean body mass) accounted for most of the variance.

Notes and References

Sources

• Ainsworth BE et al., Compendium of physical activities: classification of energy costs of human physical activities. Med Sci Sports Exerc. 1993 Jan;25(1):71-80.
• Ainsworth BE et al., Compendium of physical activities: an update of activity codes and MET intensities. Med Sci Sports Exerc. 2000 Sep;32(9 Suppl):S498-504.
• Byrne, N. et al., Metabolic equivalent: one size does not fit all, Journal of Applied Physiology 99: 1112-1119, 2005.([1])
• Department of Health and Human Services - Physical Activity Guidelines Advisory Committee, 2008 Physical Activity Guidelines for Americans - Appendix 1: Translating Scientific Evidence About Total Amount and Intensity of Physical Activity Into Guidelines ([2])
• Manore M., Thompson, J., Sport Nutrition for Health and Performance, Human Kinetics, 2000, ISBN 0873229398, 9780873229395
• Savage P., Ades P., A re-examination of the metabolic equivalent (MET) concept in individuals with coronary heart disease, Journal of Cardiopulmonary Rehabilitation & Prevention, September/October 2007, Volume 27 Number 5, Pages 321 - 321 ([3]).
• Sotile, Wayne M., Cantor-Cooke, R. Thriving with heart disease: a unique program for you and your family. Pages 161-162. ISBN: 0-7432-4364-1
• WHO - World Health Organisation, Global Strategy on Diet, Physical Activity & Health: Recommended Amount of Physical Activity ([4])

See also

• Anthropogenic metabolism
• Basal metabolic rate
• Calorimetry

External links

• The Compendium of Physical Activities, University of South Carolina: [5]
• Better weight management through science [6]