The Compendium of Physical Activities (Compendium) was developed in the late 1980s for use in epidemiologic and surveillance studies to standardize the MET intensities used in various physical activity questionnaires. The Compendium was not developed to determine the precise energy cost of activities within an individual. However, the Compendium is frequently used outside of its original scope. Therefore it is of interest to supply a method that is acceptable for adjusting MET values to fit this additional application.
The term METs is used in the Compendium to reflect the energy costs of physical activities. The MET levels in the 2011 Compendium are a direct translation of the mass-specific energy costs, computed by taking the energy costs (VO2 ml.kg-1.min-1) and dividing them by 3.5 ml.kg-1.min-1. This resulting MET level is termed a “standard MET” value. As such, the standard MET is a shorthand way of expressing the mass-specific energy costs of activities1. It should be noted that expressing the standard MET values relative to body weight is desirable because this normalizes the data and minimizes the variation between large and small individuals.
The Compendium uses 3.5 ml.kg-1.min-1 as a proxy value for the resting metabolic rate (RMR) of 1 MET. The RMR reflects about 50 to 75% of one's daily energy expenditure, with variation by age, weight, height, sex, and lean body mass. The RMR is higher in men than women and increases with height, weight, and lean mass and it decreases with age2.
Developed in 1989 and published in 19933, 20004, and 20115, the Compendium presents standard MET values. It does not correct the MET levels for age, body mass, and sex. Recently, there has been concern about the accuracy of using 1 MET = ml.kg-1.min-1 as a proxy value for the RMR because of its potential to overestimate measured RMR values that are less than 3.5 ml.kg-1.min-1 6-11. Critics argue the Compendium’s use of ml.kg-1.min-1 as the RMR referent value to compute METs underestimates the true energy cost of physical activities obtained when using a measured RMR. A recently published paper, Kozey et al.10 used the following example to demonstrate their point to compute the MET value for an activity with a measured VO2 of 20 ml.kg-1.min-1. Two values were used for the RMR: the standard value (3.5 m ml.kg-1.min-1) and a measured value (2.5 ml.kg-1.min-1) obtained from indirect calorimetry.
Example from Kozey et al.10
Standard MET: 20 ml.kg-1.min-1/3.5 ml.kg-1.min-1= 5.5 METs (moderate intensity)
Measured MET: 20 ml.kg-1.min-1/2.5 ml.kg-1.min-1= 8.0 METs (vigorous intensity)
Thus, the criticism of using the standard MET as referent value for RMR is that it underestimates the measured MET values 89% of the time10. Furthermore, Kozey et al.10 found that the standard MET misclassifies the intensity category 12.2% of the time compared to measured MET values with greater misclassification in individuals that are overweight, older, low fit, or women.
Recent efforts have been made by Byrne et al.7 and Kozey et al.10 to determine an appropriate method to correct MET values to account for personal variation in sex, body mass, height, and age to provide more accurate estimates of individual level physical activity. By dividing the standard MET (3.5 ml.kg-1.min-1) by a predicted RMR obtained from the Harris-Benedict equation2 (using age, height, body mass, and sex), the underestimation and misclassification of MET values in the Compendium were reduced significantly. The resulting MET value is referred to as a “corrected MET” value. The procedure is illustrated in Figure 1
Figure 1. Equation for Compendium of Physical Activities corrected MET values for estimated RMR
Harris Benedict equation2 for RMR (kilocalories per day):
Male = 66.4730 + 5.0033 (Height cm) + 13.7516 (Weight kg) – 6.7550 (Age yr)
Female = 655.0955 + 1.8496 (Height cm) + 9.5634 (Weight kg) – 4.6756 (Age yr)
To convert kilocalories per day obtained from the Harris Benedict equation2 to ml.kg-1.min-1, the following formula is used.
kcal.day-1/1440 = kcal.min-1; kcal.min-1/5 = L.min-1; L.min-1/(weight kg)x1000 = ml.kg-1.min-1
The corrected MET values demonstrated in Table 1 offer insight into how an individual’s variation in age, height, and body mass may influence the intensity of physical activity. This is illustrated with seven activities using the standard and the corrected MET values for a middle-aged (35 yrs) normal weight male and female along with an older (55 yrs) overweight male and female. A summary value in MET-minutes (MET x minutes an activity is performed) is computed for each column using 30 minutes of participation per activity for comparison purposes.
Table 1. Standard and corrected MET values for selected activities: 2011 Compendium
The 2011 Compendium MET value and the MET-minutes obtained using the standard MET value of 3.5 ml.kg-1.min-1 are highlighted in bold font. The data show the impact of an individual’s height, body mass, age, and sex on the estimated energy cost of an activity. In all activities in Table 1, the standard MET values from the 2011 Compendium underestimated the energy cost in MET-minutes for the various activities reported. Individuals in the normal body mass index (BMI) category and with a younger age have corrected MET levels closer to the 2011 Compendium values. However, as the BMI and age increase, the actual energy cost of the activity increases. The potential for misclassification of the energy cost of physical activity exists when comparing standard and corrected MET values. In the 2008 Physical Activity Guidelines for Americans12 (USPAG) physical activity intensities are defined as, light < 3.0 METs, moderate 3.0-5.9 METs, and vigorous ≥ 6.0 METs. For most activities, the intensity categories for standard and corrected MET values are similar. However, for activities with a standard MET level at the upper range of an intensity category (i.e., 5.8 METs, moderate intensity) or for persons with high body mass and older age, correction of the standard MET level may increase the intensity of an activity into the next higher category (e.g., 6.3, vigorous intensity).
There is some disagreement about defining a MET and correcting MET values using RMR. The use of corrected MET values has been criticized by Howley13 in an editorial written for the Journal of Physical Activity and Health in response to the Kozey et al.10 publication introducing the corrected METs procedures. He states, “The use of a ratio of the work metabolic rate to the resting metabolic rate (RMR) has been around for about 150 years. In 1861, Smith13 developed this ratio-method to describe exercise intensity and he assigned a value of 1.0 for lying posture, 3.0 for walking at 3 miles per hour (mph), and 7.0 for running at 6 mph. In the 1930s, Dill used this ratio approach to describe the intensity of physical labor during an 8-hour workday15. The use of an individual’s measured RMR value, as described above by Kozey, et al.10, is consistent with this approach; however, the resulting values are not “METs”.”13 (p. 141). In providing a historical tracking of the development of the MET, Howley concludes his editorial noting that, ”…a MET value assigned to a particular stage of a graded exercise test, listed in the Compendium of Physical Activities, or used in physical activity categories by the USPAG is nothing more than an alternate expression of the VO2 in ml.kg-1.min-1. If investigators want to use the ratio of work VO2 to each individual’s measured RMR, that is fine, but they need to assign a name to this ratio other than MET, which is, by definition, restricted to denominator of 3.5 ml.kg-1.min-1” 13 (p 142).
While the corrected MET adjustment procedure described by Kozey et al.10 provides a method to adjust a standard MET value for an individual’s estimated resting metabolic rate, the standard MET value is presented in the 2011 Compendium of Physical Activities published manuscript5 and web page (https://sites.google.com/site/compendiumofphysicalactivities).To our knowledge, it has not been shown that expressing the energy costs as corrected METs would be superior for normalizing the data. Nevertheless, the corrected MET level may be appropriate for use by clinicians and fitness professionals who use the Compendium to develop individualized exercise prescriptions and to estimate individual daily physical activity energy expenditure. The standard MET levels published in the Compendium should be used to classify the intensity of physical activities in survey research settings to ensure consistency between studies and to avoid the effects of embedding the height, weight, age, and sex in physical activity measures in statistical analyses.
In summary, the MET correction using the Harris-Benedict2 equation and defined as corrected METs is used to adjust the standard MET level (3.5 ml.kg-1.min-1) for personal characteristics that may alter an individual’s RMR (i.e., age, sex, height, and body weight). The corrected METs may be appropriate to provide personalized MET values to reflect an individual’s energy cost of physical activity and thus avoid potential underestimation and misclassification of the energy cost of PA reported when using the standard MET values. However, it must be emphasized that the corrected MET adjustment procedure should not replace the use of standard MET values when generalizing the energy cost of PA to populations, for use in surveillance and epidemiologic research activities, and in settings when data are compared between groups. Finally, neither the standard nor the corrected MET values replace the direct assessment of RMR or energy expenditure and do not remove all error and misclassification identified from direct measurement.
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