Measurement Methods for Physical Activity and Energy Expenditure: a Review

Department of Food and Nutrition, Gangneung-Wonju National University, Gangneung 25457, Korea.

Eun-Kyung Kim

Department of Food and Nutrition, Gangneung-Wonju National University, Gangneung 25457, Korea.

Correspondence to Eun-Kyung Kim. Department of Food and Nutrition, Gangneung-Wonju National University, 7 Jukheon-gil, Gangneung 25457, Korea. Tel: +82-33-640-2336, Fax: +82-33-640-2330, rk.ca.unwg@mikke

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Physical activity is defined as any bodily movement produced by skeletal muscles that results in energy expenditure. The benefits of physical activity for health maintenance have been well documented, especially in the prevention and management of chronic diseases. Therefore, accurate measurement of physical activity and energy expenditure is essential both for epidemiological studies and in the clinical context. Given the large number of available methods, it is important to have an understanding of each, especially when one needs to choose a technique to use. The purpose of this review was to discuss the components of total energy expenditure and present advantage and limitations of different methods of physical activity and energy expenditure assessment.

INTRODUCTION

The benefits of physical activity for health maintenance have been well documented, especially in the prevention and management of chronic diseases such as some cancers, type 2 diabetes, and cardiovascular disease [1,2,3,4,5]. In this context, accurate measurement of physical activity and energy expenditure is essential both in epidemiological studies and in assessment of intervention programs' efficacy [6]. In clinical setting, assessment of energy expenditure allows to estimate nutrient requirements for patients during nutrition support [7].

Physical activity is defined as any bodily movement produced by skeletal muscles that results in energy expenditure [8]. It is important to emphasize that the physical activity and energy expenditure are 2 different concepts. Simply stated, physical activity is a behavior that results in an elevation of energy expenditure above resting levels [9]. Total energy expenditure (TEE) refers to the total amount of energy expended during a 24-hour period, and it contains 3 main components: resting energy expenditure (REE), thermic effect of food (TEF), and activity energy expenditure (AEE) [10].

Various methods exist for assessing physical activity and energy expenditure, and each of them has advantages and limitations as summarized in Table 1 [6]. Understanding of these methods is important to decide which method to use for the specific study context. The purpose of this review was to discuss the components of TEE and present different methods of physical activity and energy expenditure assessment, with emphasis on each method's advantages and limitations.

Table 1

Advantages and limitations of different methods for physical activity and energy expenditure measurement

DLW, doubly labeled water; TEE, total energy expenditure.

COMPONENTS OF TEE

REE

The REE, the largest portion of TEE, is the energy required to maintain the basic metabolic activities including maintaining the body temperature and keeping the functioning of vital organs such as the brain, the kidneys, the heart, and the lungs. REE is defined as the energy expended by a fasting person at rest, in a thermo-neutral environment. Factors most significantly affecting the REE include body composition, gender, body temperature, age, energy restriction, and genetics and endocrine system [10]. A brief description of selected factors is presented here: (1) Body composition: fat-free mass (also called lean body mass) is the primary determinant of REE, meaning that high fat-free mass individual having higher REE [11]; (2) Gender: REE tends to be higher in males than in females [12,13], and this may be due to the higher percentage of lean body mass in males compared to females [10,14]; (3) Age: older individuals have lower REE compared to younger people [15]. The age-related decline in REE has been shown to be independent from changes in body composition, suggesting that additional metabolic changes may also be involved [15,16]; and (4) Energy restriction: efforts to lose weight by restricting energy intake lead to a decrease in REE [17]. This phenomenon may explain the difficulty to maintain weight loss by low calorie diets, which is associated with the biological response to energy restriction [18].

The REE is measured when the fasting person is resting in a comfortable environment. The fasting time is usually about 2 to 4 hours [19]. REE is slightly higher (about 10%) than the basal energy expenditure (BEE), which is the lowest energy expenditure of person and measurement of BEE requires more stringent conditions. A person's BEE is determined when individual is in a post absorptive state (i.e., no food intake for at least 12 hours), is lying down (supine), and is completely relaxed (motionless)—preferably very shortly after awakening from sleep in the morning [19].

AEE

The AEE is the most variable among the components of TEE, both at the intrapersonal and interpersonal level. In sedentary people, it can account for less than half of BEE while it can be as high as 1 to 2 times the BEE or more in case of very active people such as some athletes or heavy laborers [20]. Factors that influence AEE include intensity, duration, and frequency of activity [6].

TEF

The TEF, also referred to as diet-induced thermogenesis (DIT), is the energy required for the food digestion, absorption, transport and metabolism, storage of nutrients, and elimination of wastes. It represents increase in energy expenditure above the REE, which can be measured for several hours after a meal. The TEF is estimated as about 10% of the daily TEE [10].

DOUBLY LABELED WATER (DLW) METHOD

Principle of the method

The DLW method uses stable isotopes of oxygen ( 18 O) and hydrogen ( 2 H) for the measurement of TEE. The DLW method is widely recognized as the gold standard for the measurement of TEE [21], and has been used in various studies to validate other methods [22,23]. In addition to its high accuracy [24], the DLW method presents the advantage of its noninvasive nature and possibility for the subjects to continue their normal activities during the measurement period. The method also has a limited burden on subjects [25]. However, limitation of the method is its high cost due to the high price of DLW, the expensive equipment and expertise required for analysis [21]. Another limitation of the DLW technique is that it provides the overall measure of averaged daily TEE over the measurement period, but it does not provide any specific details on physical activity. Currently, this method has been used in a wide range of population categories including infants [26,27], pregnant and lactating women [28,29], and the elderly [23,30].

The method is based on the following principle: after the subject ingests a dose of 2 H₂ 18 O, there is an equilibration of the 2 isotopes with total body water (TBW) followed by their elimination from the body, which occurs at different rates. Deuterium ( 2 H) is lost from the body via only water (H₂O) while 18 O is lost both via water and carbon dioxide (CO₂). The rate of CO₂ production (rCO₂) is calculated as the difference between the elimination rates of 2 H and 18 O, using the following formula [31]:

where k_o and k_h (day -1 ) are the elimination rates of 2 H and 18 O, respectively.

The TEE is calculated by using the modified version of Weir's formula based on rCO₂ and food quotient (FQ) [31]:

Protocol of the DLW method

There are 2 basic protocols for the DLW method: the 2 point and the multi-point approaches. The 2-point protocol as the minimal form requires 3 specimens including a pre-dose baseline, a post-dose specimen taken on the day of dosing after the isotopes have equilibrated throughout the body, and a final specimen taken at the end of the study (that is, at day 10–14). The multi-point protocol as the most extreme form generally involves the steps taking a pre-dose baseline specimen and specimens every day after intake of the dose until the end of the sampling period. In practice, the 2 approaches have been modifications in the 2 approaches and they are quite similar [25].

Concerning the 2-point protocol, the most commonly used form is the modified approach in which a total of 5 samples are collected [13,32,33]. Subjects are requested to come to the clinical site or urine sample collection center in the morning after an overnight fast, and the protocol begins by collecting the baseline urine sample. A short time after, the participant drinks the DLW prepared based on the subject's TBW [13,32,33]. In some studies, the amount of isotope dose in DLW was determined by the participant's body weight [31,34,35]. One hour after drinking the DLW, subjects should void to empty the bladder and the time must be recorded. However, this urine is not collected since the isotope equilibrium with the body water is not yet established at this time. Three and 4 hours after drinking the DLW, 2 more urine samples should be collected. The subject should not drink and eat between the 3 to 4 hours during urine sample collections to minimize any short-term effect of water intake on urine enrichment. On the final day of experimental period, 2 more urine samples should be collected at about the same time of the day before. Typical intervals between the initial and the final urine collections are 7, 10, or 14 days [25]. In the multi-point approach, more urine samples are collected after DLW administration than those in the 2-point protocol [34,35,36]. During the study period, all samples must be collect at similar time to the previous days.

The number of urine samples collected is not a critical consideration with regard to the validity of the DLW technique. Rather, the choice of sampling frequency depends on the investigator's preference for precision of the method [25]. The 2-point protocol presents an advantage of using fewer samples, and provides the more exact estimate of TEE under conditions in which there is day to day variation in energy expenditure or water turnover. On the other hand, the multi-point protocol has the advantage of data averaging and thus minimizes the analytical error. In addition, it allows the investigator to assess the differences in energy expenditure for sub-periods within the metabolic period [25]. After urine samples collection and storage, analysis is performed by the isotope-ratio mass spectrometry method [25].

The dose of DLW

The dose of DLW is based on the body size of subject in order to match the body water enrichments to the isotope-ratio mass spectrometry precision. Considering the difficulty of measuring TBW, it must be estimated. In most DLW studies, investigators have assumed TBW as 60% of body weight. The 99 atom % deuterium ( 2 H) and 10 atom % 18 O are the most commonly used for enrichments of the labeled water available on the market. The International Atomic Energy Agency (IAEA) recommends doses of 0.12 g·kg −1 body water of 99 atom % deuterium labeled water and 1.80 g·kg −1 body water of 10 atom % 18 O. When the more highly enriched 18 O water is used, the dose should be reduced [25]. Prior to the administration, the DLW can be sterilized by pushing it through a 0.22 μm filter.

Calculation of AEE and physical activity level (PAL)

After its measurement by the DLW method, TEE can be used for the calculation of AEE and PAL. The calculations involve REE, which is measured by indirect calorimetry [37] or estimated by using predictive equations [38]. With the TEF assumed as 10% of TEE, the AEE is calculated as follow [6]:

The following equation is used for the calculation of PAL: