Resting metabolism refers to the minimum number of calories required to support basic physiological functions, such as circulation and breathing.
The second component of TEE, the thermic effect of food (TEF), refers to the increase in energy expenditure linked with digestion, absorption, transport, metabolism, and storage of food. The TEF generally compromises approximately 10% of TEE in an average individual.
The final component of TEE, activity energy expenditure (AEE), refers to the energy used in physical activities. The AEE can further be divided into two subcategories that include nonexercise-activity thermogenesis (or NEAT) and energy expenditure from exercise. According to data from 2015, approximately 20% of American adults fulfill federal physical-activity guidelines for both aerobic and muscle-strengthening activities, with one-third reporting no leisure-time activity. From these statistics, it is clear that, for many individuals, the energy expenditure from exercise equates to zero. Comparatively, the energy expenditure from exercise in those individuals who exercise regularly encompasses roughly 10% of TEE. The remainder of TEE is from NEAT, which represents the energy expenditure of normal daily physical activities, fidgeting, spontaneous muscle contraction, and maintaining posture when not reclining. The NEAT component of AEE varies the most among individuals.
Direct calorimetry is the gold standard measurement of RMR and BMR. Accordingly, when valid assessment of this component of TEE is required (for instance, in clinical research trials), then direct calorimetry is the desired procedure. Nevertheless, the instrumentation and cost involved with operation of a direct calorimeter can easily exceed one million dollars; clearly this factor alone makes the technique cost prohibitive for many exercise physiology laboratories. The space required to house a direct calorimeter can also be a considerable barrier. Direct calorimetry is also not ideally suited for the measurement of NEAT or exercise energy expenditure. In terms of quantifying NEAT, although it is possible for individuals to remain within a direct calorimeter for several days to acquire measurements for fidgeting and other spontaneous activity, other facets of NEAT (for instance, work-related walking) will be artificially restricted within the direct calorimeter environment. Efforts to quantify exercise-related energy expenditure while utilizing direct calorimetry can also be hindered by several factors:
Both direct calorimetry and doubly-labeled-water techniques are expensive and also require a considerable time investment to obtain the TEE measurement, factors that make them impractical for use in most clinical settings. Alternatively, dieticians and others who are in the business of designing nutritional programs frequently rely on calculation of energy expenditure requirements from prediction equations that estimate RMR, which typically equates to ± 10% of BMR, with RMR/BMR representing the largest component of TEE (Figure 1). The RMR is an excellent predictor of the 24-hr TEE. To arrive at an estimate of daily energy expenditure requirements, the predicted RMR is subsequently multiplied by an activity factor. Numerous prediction equations to estimate RMR can be found in the literature. The most common equations employed in clinical settings are presented in Table 1. It should be recognized that these equations have limitations, and these limitations differ amongst prediction equations:
These issues are important when selecting a prediction equation. Energy imbalances can result in either weight loss or weight gain. Thus, accurate RMR prediction is paramount for the design of successful caloric prescriptions. The long-term successfulness of a weight loss program may be compromised if the prediction equation yields an erroneous RMR. Lastly, there is a shortcoming in the knowledge of how valid RMR prediction equations are in different ethnic populations and the elderly.
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American College of Sports Medicine (ACSM), 401 W. Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 634-7817, http://www.acsm.org .
American Council on Exercise, 4851 Paramount Dr., San Diego, CA, 92123, (858) 576-6500, (888) 825-3636, ext. 782, Fax: (858) 576-6564, https://www.acefitness.org .
American Heart Association (AHA), 7272 Greenville Ave., Dallas, TX, 75231, (800) 242-8721, http://www.heart.org .
Lance C. Dalleck, PhD