Examination of the Estimated Resting Metabolic Equivalent (MET) in Overweight and Obesity

Renee J. Rogers, PhD

Department of Health and Physical Activity, Physical Activity and Weight, Management Research Center, University of Pittsburgh, 32 Oak Hill Court, Pittsburgh, PA 15261, USA, Tel. 412-383-4015; Fax: 412-383-4045; E-mail: r.j.rogers@pitt.edu

INTRODUCTION

The metabolic equivalent task (MET) is a measure of resting oxygen consumption that has the benefit of providing a common descriptor of workload or metabolic intensity.¹ The MET is considered to be a universal measure of expressing energy expenditure as a multiple of the resting or reference level in relation to body weight.² Based on work conducted in 1941 that involved heat exchange in a neutral environment under resting conditions, Gagge at el³ are credited with coining the MET terminology, which most closely mirrors the current use of the MET with regard to energy expenditure.

The resting MET is commonly defined as 3.5 ml/kg/min or 0.250 L/min of oxygen consumption.^4,5 The origins of 3.5 ml/kg/min to represent a resting MET value of 3.5 ml/kg/min has been agreed to have resulted from the resting VO₂ data obtained from one 40 year old male subject weighing 70 kg.^6,7 Multiples of a resting MET are commonly used to estimate the energy expenditure and work performed during various activity tasks. Therefore, it is important that the estimate of the resting MET be accurate to minimize the likelihood of under- or over-estimation of energy expenditure. Moreover, given the importance of energy expenditure to the treatment of obesity, an understanding of whether the current estimates of a resting MET are accurate in individuals who are overweight or obese may be of clinical and scientific importance.

Therefore, the purpose of this investigation is to examine whether the measured resting oxygen consumption, which is used to define a resting MET, in individuals who are overweight or obese is consistent with the widely used estimation of a resting MET (3.5 ml/kg/min or 0.250 L/min of oxygen consumption). Moreover, this study examined whether this varied by gender (male or female) or by grade of overweight or obesity.

METHODS

Data were obtained from 45 overweight or obese, sedentary, but otherwise healthy adults (age: 37.2±10.5 years; body mass index [BMI]: 32.4± 3.5 kg/m²). Subjects included 34 females and 11 males, with 11 overweight (25.0 to <30.0 kg/m²), 21 with Class I obesity (30.0 to <35.0 kg/m²), and 13 with Class II obesity (35.0 to <40.0 kg/m²). Descriptive data are presented in Table 1.

Height was measured to the nearest 0.5 inch via a wall-mounted stadiometer and weight was measured to the nearest 0.5 pound on a calibrated scale with subjects wearing a cloth medical gown or light-weight clothing. BMI was calculated by dividing weight in kilograms (kg) by height in meters squared (m²).

Resting oxygen consumption (VO_2rest) was measured with a metabolic cart using the dilution technique. Measurements were obtained between 7:30 AM and 10:30 AM. Pre-test instructions included: fasting for at least 12 hours the night before testing, avoiding consumption of any over-the-counter medications, abstaining from all vigorous physical activity the day before testing, and vehicle transportation to the research center the morning of testing. Study participants were questioned to confirm adherence to these pre-testing instructions upon arrival at the research center. Subjects were placed in a supine position in a semi-darkened room for a period of 30 minutes prior to data collection. Data collection occurred for at least 15 minutes with 5 consecutive minutes representing a steady state condition, defined as the range of energy expenditure across this 5 minutes differing by <150 kcal/d.⁸ This is consistent with a technique that defined steady state of resting energy expenditure at a coefficient of variation of no more than 5% for both oxygen consumption and carbon dioxide production.⁹ The initial 5 minutes were discarded to allow for the subject to acclimate to the dilution canopy, with the average of a subsequent five consecutive data points meeting the <150 kcal/d difference criteria used to represent VO_2rest.

Statistical analyses were conducted using IBM SPSS Statistics (release version 21.0.0.0). One-sample t-tests were used to compare relative and absolute measured VO_2rest to the reference-MET (relative=3.5ml/kg/min, absolute=0.250 L/min) value in all subjects. A multivariate analysis of variance was used to examine the difference measured versus estimated VO_2rest
between genders (males versus females) and BMI categories (overweight, Class I obesity, Class II obesity). Main effects were further examined using post-hoc analysis with Bonferroni adjustment.

RESULTS

Measured relative VO_2rest was significantly less than the reference-MET value (3.0±0.6 ml/kg/min versus 3.5 ml/kg/min; p<0.001). There was no significant Gender X BMI interaction. There was also no significant main effect by BMI category for difference between measured and the reference-MET value for relative VO_2rest (Table 2). However, there was a significant difference (p=0.002) between males and females for the difference between measured and the reference-MET value for relative VO_2rest (Table 2). The reference-MET value over-estimated VO_2rest for females by 0.7±0.5 ml/kg/min and under-estimated VO_2rest for males by 0.2±0.4 ml/kg/min.

Overall, measured absolute VO_2rest (0.275±0.083 L/min) did not differ from reference-MET value (0.250 L/min) (Table 2). While there was no significant Gender X BMI interaction, there was a significant main effect for both Gender (p<0.001) and BMI category (p<0.001). The reference-MET over-estimated by 0.008±0.59 for females while it under-estimated VO_2rest by 0.125±0.066 L/min for males. The difference between the measured and reference-MET value for absolute VO_2rest was significantly less in the overweight category compared to both the Class I (p=0.010) and Class II (p=0.026) obesity categories.

CONCLUSIONS

This study examined if measured VO_2rest differed from widely accepted reference-MET values (3.5ml/kg/min, 0.250 L/min) in adults who are overweight or obese. This study found that measured VO_2rest (3.0±0.6 ml/kg/min) in a sample of adults who are overweight or obese is less than the typically used reference-MET value of 3.5 ml/kg/min (Table 2). Results from this study are similar to results from others in which measured VO_2rest was less than the reference-MET value of 3.5 ml/kg/min. For example, in a sample of 36 males (age=40.0±3.3 years; BMI=25.9±3.8 kg/m²) measured relative VO_2rest (3.0±0.3 ml/kg/min) was significantly lower than the reference-MET value (3.5 ml/kg/min).¹⁰ Gunn et al¹¹ have also reported a similar pattern in a sample of 50 males (age=60.6±3.2 years; BMI=26.7±3.2 kg/m²). In a mixed sample of males and females, Byrne et al¹² also reported that VO_2rest (2.6±0.4 ml/kg/min) was significantly lower than the reference-MET value (3.5 ml/kg/min). However, Byrne et al¹² did not examine whether there was a gender influence on the difference between the measured and reference-MET value. By comparison, the current study did report a gender difference between the measured and reference-MET value, with reference-MET value over-estimating VO_2rest for females and under-estimating VO_2rest for males.

Another unique contribution of the current study is the examination of whether the difference between measured and the reference-MET value was influence by BMI. Results showed no difference across BMI categories when examining relative VO_2rest; however, the difference between measured and the reference-MET value for absolute VO_2rest (L/min) was less in the overweight category compared to the Class I and Class II obesity categories. This may suggest that at higher levels of BMI the reference-MET for absolute VO_2rest (0.250 L/min) has less utility as an accurate measure. Others studies that have directly compared the measured and reference-MET value for VO_2rest have not examined the potential influence of BMI. This finding also has implications when using the reference-MET value of 0.250 L/min to estimate the energy costs of physical activity, as this may over-estimate the energy cost more as BMI increases.

Currently, VO_2rest is used to represent the resting MET, and multiples of this resting value are universally used to express energy expenditure of various forms of physical activity.^1,2 The origins of the MET to express energy expenditure during different forms of physical activity relative to resting energy expenditure appears to date back to approximately 1890,¹³ which was followed by similar observations made decades later.^3,14 Thus, the findings of the current study, which are similar to the finding of others,^10,12 may suggest that the energy cost of a variety of physical activities may be over-estimated when using common reference-MET values. Moreover, the data from this study may suggest that this over-estimation may be of particular concern for women and for individuals at a higher BMI. However, given the relatively small sample and the limited inclusion criteria, these findings should be interpreted with caution and warrant further replication.

In summary, this study demonstrated that the conventional estimates of VO_2rest, represented as the MET, may differ from measured values in adults who are overweight or obese. These findings may have implications on estimates of resting and physical activity energy expenditure. Thus, while additional research to confirm these findings is warranted, this may suggest the need to establish new estimates of the resting MET that can be used broadly in clinical applications aimed at prevention or treatment of obesity.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the staff and students at the Physical Activity and Weight Management Research Center at the University of Pittsburgh for their contributions to the project.

CONFLICTS OF INTEREST

The authors declared conflicts of interest.

Author Contributions

RJR: Manuscript development; Statistical analyses.

JMJ: Data collection; Statistical analyses; Manuscript revision.

CONSENT STATEMENT

As per University of Pittsburgh Instructional Review Board Guidelines, all original signed consent forms have been retained by the principal investigator.

1. Balady GJ. Survival of the fittest – more evidence. N Engl J Med. 2002; 346(11): 852-854. doi: 10.1056/NEJM200203143461111

2. Montoye HJ, Kemper HCG, Saris WHM, Washburn RA. Measuring physical activity and energy expenditure. Champaign, IL, USA: Human Kinetics; 1996: 4-5.

3. Gagge AP, Burton AC, Bazett HC. A practical system of units for heat exchange of man with his environment. Science. 1941; 94(2445): 428-430. doi: 10.1126/science.94.2445.428

4. Jette M, Sidney K, Blumchen G. Metabolic equivalents (METS) in exercise testing, exercise prescription, and evaluation of functional capacity. Clin Cardiol. 1990; 13(8): 555-565. doi: 10.1002/clc.4960130809

5. Morris C, Myers J, Froelicher V, Kawaguchi T, Ueshima K, Hideg A. Nomogram based on metabolic equivalents and age for assessing aerobic exercise capacity in men. J Am Coll Cardiol. 1993; 22(1): 175-182. doi: 10.1016/0735-1097(93)90832-L

6. Howley ET. You asked for it: question authority. ACSM Health Fitness J. 2000; 4(6): 6-8. Web site. http://journals.lww.com/acsm-healthfitness/Citation/2000/04060/You_asked_for_it_Question_Authority.4.aspx. Accessed April 25, 2016

7. Wasserman K, Hansen JE, Sue DY, Whipp BJ, Casaburi R. Measurements during integrated cardiopulmonary exercise testing. Principles of Exercise Testing and Interpretation. 2^nd ed. Philadelphia, USA: Lea & Febiger; 1994: 59-60.

8. Jakicic JM, Wing RR. Differences in resting energy expenditure and respiratory quotient in black versus white overweight females. Int J Obes. 1998; 22(3): 236-242. doi: 10.1038/sj.ijo.0800575

9. Foster GD, Wadden TA, Feurer ID, et al. Controlled trial of the metabolic effects of a very-low-calorie diet: short- and long-term effects. Am J Clin Nutr. 1990; 51(2): 167-172. Web site. http://ajcn.nutrition.org/content/51/2/167.long. Accessed April 25, 2016

10. Gunn SM, van der Ploeg GE, Withers RT, et al. Measurement and prediction of energy expenditure in males during household and garden tasks. Eur J Appl Physiol. 2004; 91(1): 61-70. doi: 10.1007/s00421-003-0932-1

11. Gunn SM, Brooks AG, Withers RT, Gore CJ, Plummer JL, Cormack J. The energy cost of household and garden activities in 55- to 65-year-old males. Eur J Appl Physiol. 2005; 94(4): 476-486. doi: 10.1007/s00421-004-1302-3

12. Byrne NM, Hills AP, Hunter GR, Weinsier RL, Schultz Y. Metabolic equivalent: one sizes does not fit all. J Appl Physiol. 2005; 99(3): 1112-1119. doi: 10.1152/japplphysiol.00023.2004

13. LaGrange F. Physiology of bodily exercise. New York, USA: D Appleton and Company; 1890.

14. Dill DB. The economy of muscular exercise. Physiol Rev. 1936; 16: 263-291. Web site. http://www.cabdirect.org/abstracts/19361402015.html. Accessed April 25, 2016