Use of the metabolic equivalents system in older adults: an epidemiological perspective
Year: 2024
Type: dissertation
Abstract: The metabolic equivalents (METs) are a physiological concept widely used in epidemiology on which several physical activity quantification methods (interviews, questionnaires, diaries/logs, or accelerometer devices) are based. The METs express the number of multiples of a standardised resting metabolic rate (RMR) of 3.5 mL O2·kg-1·min-1 (standard 1 MET) and are usually categorised, using conventional METs intensity thresholds (CTs) as light-intensity (LPA), moderate-intensity (MPA) or vigorous-intensity (VPA) physical activity. Both the standard 1 MET and the CTs constitute an absolute system for physical activity intensity assessment that has been criticised for reporting unreliable measures in populations with non- standard RMR or cardiorespiratory fitness. In this regard, older adults are especially prone to age-related changes in some of the factors identified as affecting both the RMR and the cardiorespiratory fitness, including a decline in fat-free mass and aerobic capacity, an increased fat mass, and risk of morbidity. However, although the literature reports RMR values in older adults generally lower than 3.5 mL O2·kg-1·min-1, there is a notable variation between studies. Therefore, summarising the RMR (mL O2·kg-1·min-1) determinations in the literature can provide an estimate of the 1 MET value in older adults that can be compared with the standard 1 MET (Studies 1a and 1b). In addition, the RMR may also be influenced by methodological factors, like the measurement protocol and the use of a valid indirect calorimeter. In this sense, the Cosmed K5 is a widely used exercise testing metabolic device whose use for measuring VO2 at rest has been questioned. Therefore, quantifying such discrepancies and creating fitting equations that minimise them, may be valuable (Study 2). Nonetheless, since cardiorespiratory fitness decreases with age, the use of the METs system CTs could lead to an inaccurate physical intensity classification in older adults. As a result, efforts in deriving population-based METs intensity thresholds (PBTs) should be made (Study 3), and subsequently used to derive alternative METs system strategies for older adults (Studies 3 and 4). Then, the differences between the use of an adapted METs system or the "conventional" METs system in the older population can be quantified (Study 4). Finally, despite the strong acceptance of the METs system, comprehensive information on its origin, experimental basis, and use can hardly be found. Therefore, it is necessary to compile all available information on the origin, experimental basis, and uses of the standard 1 MET value and the CTs (Study 5). Methods The present PhD thesis comprises five studies, four of which include voluntary participants. The first study is a systematic review in which the subjects of the 23 selected scientific articles were included (n = 1091, = 60 years) (Study 1.a). The remaining 3 are experimental studies and included individuals from the MOBEMENT study (n = 48, 59.7 ± 23.5 years) (Studies 2 and 4), the FenotipAGING study (42; 69.5±3.6 years) (Study 3) and the PRO-Training study (n = 51; 68.6 ± 4.2 years) (Study 3). The systematic review was conducted and reported in accordance with the PRISMA statement (Study 1.a). All experimental studies included basic anthropometric data (body weight and height) (Studies 2, 3 and 4) and physical function was assessed using the Short Physical Performance Battery (Studies 3 and 4). Furthermore, RMR was assessed by indirect calorimetry using both a portable metabolic device (Cosmed K5) (Study 2) and a stationary metabolic cart (Oxycon Pro) (Studies 2 and 4), but also using RMR estimative equations (Studies 3 and 4). To measure cardiorespiratory fitness, graded exercise tests were performed using an electromagnetically braked cycle-ergometer and two different metabolic carts (Oxycon Pro and Cosmed Quark RMR) (Study 3). A portable metabolic device (Cosmed K5) was also used to assess the exercise intensity of older adults during two mutually exclusive physical activity protocols (Study 4). Briefly, data processing for the experimental studies followed standardised data cleaning and averaging procedures. Four different METs derivation approaches (standard METs: VO2/3.5 mL O2·kg-1·min-1, measured METs: VO2/measured RMR, predicted METs: VO2/predicted RMR, and population-based METs: VO2/2.7 mL O2·kg-1·min-1) were calculated and categorised for their physical intensity using the CTs and PBTs. PBTs for older adults were derived using both standard and population-based METs, according to the %VO2max and the ventilatory thresholds intensity domains and both for “very poor/fair” and “good/superior” older individuals (Study 3). These PBTs were used with the standard METs, measured METs, predicted METs and population- based METs to construct 7 alternative METs system strategies (Study 4). In addition, all statistical analyses were performed according to standard procedures, including analysis of differences (Student's t-tests), correlation (Pearson's Correlation Coefficients), agreement (Intraclass Correlation Coefficient and Bland Altman Plots), association (multiple linear regression models) and setting a significance level at a = 0.05. Finally, a comprehensive search was conducted in databases, books, grey literature, and other bibliographic sources to identify relevant documents reporting on the origin, scientific bases, and use of the METs system (Study 5). Results The overall weighted average RMR (mL O2·kg-1·min-1) value equivalent to 1 MET derived from 1091 older adults = 60 years old was 2.7 ± 0.6 mL O2·kg-1·min-1, being 11% lower (2.4 ± 0.3 mL O2·kg-1·min-1) when only best practice studies were considered (Study 1.a). On the other hand, when comparing the Cosmed K5 portable metabolic device with the Oxycon Pro stationary metabolic cart, significant differences in most metabolic and ventilatory parameters were found, including the primary outcomes of VO2 and VCO2. These differences showed an overestimation of the Cosmed K5 in all metabolic outcomes, except fat. However, when derived fitting equations were applied, differences were minimized, and agreement was maximized (Study 2). Regarding the PBTs derived in older adults following the %VO2max intensity domains, around 23% lower thresholds were obtained when using standard METs compared to using population-based METs. Furthermore, higher PBTs for MPA (%VO2max domains) and HPA (VTs domains) were consistently observed compared to the MPA threshold of the CTs (3 METs). Nonetheless, lower, similar, or even higher PBTs for VPA (%VO2max domains) and SPA (VTs domains) were obtained compared to the conventional VPA threshold (6 METs). In addition, higher PBTs were obtained in the "good/superior" cardiorespiratory fitness subgroup and following the VTs intensity domains, compared to the "very poor/fair" fitness group, and the %VO2max domains, respectively. (Study 3). Regarding the different METs derivation approaches, significantly lower total METs·min-1 were derived when using standard METs and higher when using predicted and population-based METs, compared to measured METs. Moreover, compared with the reference METs system strategy (measured METs and 2.7 or 3.5 PBTs), the strategies that included the CTs overestimated MPA and MVPA and underestimated LPA for all targeted outcomes regardless of the METs derivation approach. In addition, VPA was also consistently underestimated when the METs system (standard 1 MET value and CTs) was used. In contrast, fewer and smaller difference were observed in strategies where PBTs were used (Study 4). Regarding the origin and experimental basis of the METs system, the evidence found showed the studies conducted by Bruno Balke and Erik Hohwü- Christensen seem to have respectively played a key role in defining the standard 1 MET value and in influencing the currently used CTs. Finally, the American College of Sports Medicine appeared to be a primary driver as a disseminator of the METs concept as a measure of physical activity intensity (Study 5). Conclusions This PhD thesis showed a markedly lower weighted average RMR value (2.7 mL O2·kg-1·min-1) and reported clearly different PBTs in older adults when compared to the standard 1 MET (3.5 mL O2·kg-1·min-1) and the CTs from the METs system, respectively. In addition, when comparing alternative METs system strategies, large differences were evident when using the CTs compared to the PBTs derived from older adults. Therefore, using PBTs adapted to the METs derivation approach is suggested and the indiscriminate use of the CTs should be generally avoided in adults over 60. On the other hand, although these alternative METs strategies provide useful resources for adapting the METs system to the older population, future studies should explore the unweighted effect on the scientific conclusions drawn from their use. Finally, this PhD thesis also provided additional insight into the experimental origin of the METs system. Its authorship has been attributed to a group of leading researchers and experts in their field who, in an intergenerational way, have been shaping the current customary practice.
Author: Leal Martín, Javier
Language: English
Topic: Physical Activity and Health
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W7200715568