Exercise prescriptions are designed to improve fitness, reduce risk factors for chronic disease, and promote safety during exercise participation. One of the essential components of an individualized exercise prescription is intensity, which can be determined from either directly measured or estimated workloads. A mechanism used to derive the intensity of the exercise prescription is the metabolic equivalent (MET). The MET is an expression of energy cost and is commonly used by health and fitness professionals to quantify the amount of energy required to perform an activity. This value, when expressed as a resting measure, has widely been accepted to represent 3.5 ml oxygen/kg body weight/minute or 1 kcal/kg body weight/hour. A widespread acceptance has allowed the MET to become increasingly used as a common descriptor of workload across most modalities and all populations (2). In fact, to ease exercise and activity prescription, a compendium of physical activities has been described (1), in which physical activities were assigned intensities based on the ratio of work metabolic rate to a standard resting metabolic rate (MET). Although these intensity values were originally derived from the best available published and unpublished data, they were not designed to determine precise energy costs (1). This compendium, however, is frequently used by researchers, fitness professionals, and exercise physiologists to identify and prescribe physical activities for various populations of clients (1,22). There is, however, increasing evidence that the current MET value of 3.5 ml oxygen/kg body weight/minute, or 1 kcal/kg body weight/hour, significantly overestimates directly measured resting oxygen consumption values and caloric expenditure, potentially reducing the accuracy of the MET as a tool for estimating energy expenditure and caloric expenditure during physical activity (1,3,8,9,25). This is particularly true in people with increased body fat percentages or cardiovascular disease (11,24,25).
HOW THE MET ORIGINATED
The exact origin and definition of the MET are somewhat unclear. The original concept of energy expenditure was first described in an 1890 text, which described oxygen consumption data in a male using the ratio of work to resting oxygen consumption (12). Dill (5) expanded upon this concept in 1936 when he identified work capacity as the ratio of work metabolic rate to basal metabolic rate, and then again in 1941 when Gagge et al. (7) related human heat exchange with resting metabolism. This historical information helped transform the definition of the MET to the value used today, the quantity of oxygen consumed by the body from inspired air in a seated position, which is equal, on average, to 3.5 ml oxygen/kg body weight/minute (14,21) or 1.0 kcal/kg body weight/hour (1). Thus, despite the fact that historical data suggest that the value for 1 MET was originally derived from a single 70-kg man (20,26), its use and acceptance as the standard unit of measure for exercise prescription in the health and fitness industry have not been questioned.
THE INACCURACIES OF THE MET
Unfortunately, as a scientific convention becomes widely accepted, there is the risk that its underlying premise may no longer be questioned, resulting in limitations being overlooked and misuse of the original concept (3). It seems that might be the case with the MET as it has gained unquestioned acceptance of value in the health and fitness industry. Recently, studies have identified potential flaws in the use of the MET, raising questions relative to the accuracy of its use as a standard measure of resting energy expenditure across all populations. These studies, which included large samples of demographically diverse and healthy individuals, suggest that the resting MET value of 3.5 ml of oxygen/kg body weight/minute overestimates resting energy expenditure by as much as 30% to 35% when compared with directly measured energy expenditure (3,8,9). In addition, coronary artery disease patients and those who are on β-blockade therapy demonstrate a significantly lower (7%-36%) resting metabolic rate (RMR) as compared with the commonly accepted MET value (6,25). Many factors can influence RMR (Table), including fat mass, fat-free mass, and age, which account for the largest variance from the standard MET value (3).
Energy expenditure also is overestimated when compared with a MET value of 1.0 kcal/kg body weight/hour. Average measured values have been shown to be 15% to 23% below the estimated norm (3,18,19), resulting in resting values that fall below those assigned as sleeping energy costs (0.9 MET) by the currently recognized MET value system (1). Obviously, as RMR decreases, the calculated ratio of work metabolic rate to RMR increases. How does this affect what exercise professionals do?
IMPLICATIONS ON EXERCISE PRESCRIPTION
Functional capacity provides prognostic and diagnostic information and is significant to the foundation of developing an exercise prescription. Although this information can generally be determined by directly measuring oxygen consumption during an exercise stress test, this practice is not routinely done. However, oxygen consumption typically is estimated by linear regression equations (17), which unfortunately have been shown to overestimate exercise capacity (16,20) in many populations. If a client's functional capacity is measured or estimated to be 24.5 ml/kg/min, the MET value is 7.0 (using the standard 3.5 ml/kg/min). If the client's measured RMR was 2.7 ml/kg/min, the ratio would be 9.1. The important thing to note is that the measured or estimated V˙O2 is, of course, unchanged. It is that value, not some calculated ratio, that provides prognostic or diagnostic value. It must be noted that if the standard MET value is calculated, one can determine the original energy cost of an activity, simply multiply by 3.5.
The MET, expressed as a value of 3.5 ml of oxygen/kg body weight/minute, also is used when prescribing exercise via oxygen uptake reserve (22). In this equation, RMR is taken into consideration, and historically, a MET value is used to represent RMR. Research, however, has indicated that true values for RMR fall below this estimated value and are related to weight or clinical status. This lower RMR value would seem to affect the prescribed exercise intensity. However, given the relatively small difference in measured versus estimated RMR, the impact is small (see Box). So, although a slight increase in accuracy may be obtained by measuring RMR before prescribing exercise in all individuals, the feasibility of completing this task is not realistic. Thus, despite the small variation in exercise prescription found between using measured RMR and using a constant of 3.5 ml oxygen/kg body weight/minute, the exercise prescription is still a valid and safe formula for most individuals.
As has been noted, body composition and, in particular, fat-free mass play a significant role in RMR. Although diet and aerobic exercise do not seem to provide the needed stimulus to significantly impact RMR, strength training has been shown to increase RMR levels. In three recent landmark studies (4,13,23), strength training programs were shown to increase resting metabolic rate by 7% to 8%,resulting in an approximate daily increase in energy expenditure of 100 to 120 kcals/day. Although an increase in lean body weight may be partially responsible for these changes, it seems that muscle repair and remodeling processes also may be benefits associated with a strength-training program (10). These physiological changes, if implemented correctly using an individualized exercise prescription, could result in an approximate fat loss of 10 lbs or more per year (27).
As has been demonstrated, RMR is often lower than the current 3.5 ml of oxygen/kg body weight/minute often associated with this measure. However, this difference has little impact on the calculated exercise training intensity when prescribing exercise using the oxygen uptake reserve method (22). Although an individualized exercise prescription is a standard for exercise training, the inclusion of a measured RMR does not seem to provide the significant impact on the exercise prescription needed to justify the difficulty in obtaining this measure for all participants. Although regression equations have been developed (3) to address the differential between measured RMR and the 3.5 ml of oxygen/kg body weight/minute, value currently used, these equations add additional error in predicting the RMR, which ultimately underscores their use for an exercise prescription. Perhaps future research should focus on identifying a better MET value to replace the 3.5 ml of oxygen/kg body weight/minute, resulting in a more accurate, yet standardized, value for most individuals. In the meantime, although the MET seems to have some inherent shortcomings, its ability to be used as a quick and easy conversion factor remains important for the clinical and fitness industry. It may not be the perfect value, but it is the MET we have all come to know and use effectively on a daily basis.
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