Energy expenditure can be defined as the collective energy cost for maintaining constant conditions in the human body plus the amount of energy required to support daily physical activities. Physical activity refers to any bodily movement produced by contraction of skeletal muscles that substantially increases energy expenditure.
Therefore, the measurement of energy expenditure using various calorimetric techniques has become an increasingly important topic area within the exercise physiology and public health fields. The science of exercise prescription also uses energy expenditure to quantify both exercise intensity and overall exercise program volume. The measurement of energy expenditure can be employed to assess metabolic needs, fuel use, and the relative thermic effect of different foods and beverages.
Total energy expenditure (TEE) is the average amount of calories that is burned by a human body in a 24-hour period. It is adjusted by activity (sedentary, moderate, or strenuous), along with further modifications based on age, gender, height, and weight. TEE is composed of three major components: basal metabolic rate, thermic effect of food, and activity energy expenditure.
The basal metabolic rate (BMR) is the energy expended when an individual is lying face up at complete rest. A valid estimate of BMR requires measurement to be in the morning after a standard night of sleep. BMR is sometimes considered the amount of energy required to keep biological processes (i.e., the regulation of organ systems and body temperature) performing efficiently. Typically, BMR comprises 60% of TEE in an average individual. The variability in BMR between sexes and across different age groups is largely explained by lean body mass. For this reason maintenance of lean body mass is often a key goal of exercise programs designed for older adults.
Resting metabolic rate (RMR) is frequently measured in place of BMR for convenience. RMR also requires measurement at complete rest. The RMR is typically within 10% of BMR under usual circumstances.
The thermic effect of food (TEF) refers to the increase in energy expenditure linked with digestion, absorption, transport, metabolism, and storage of food. TEF generally compromises approximately 10% of TEE in an average individual.
The activity energy expenditure (AEE) refers to the energy expenditure from physical activities. AEE can be divided into two subcategories: non-exercise activity thermogenesis (or NEAT) and energy expenditure from exercise. As of the 2008 release of the Physical Activity Guidelines for Americans (PAG), by the Office of Disease Prevention and Health Promotion (ODPHP; U.S. Department of Health and Human Services), over 80% of American adults did not meet the guidelines for both aerobic (cardiovascular-benefiting) and anaerobic (muscle-strengthening) activities. Likewise, more than 80% of adolescents in the United States did not perform sufficient amounts of aerobic physical activity to meet the ODPHP guidelines for youth. These statistics indicate that for many individuals the energy expenditure resulting from exercise equates to almost zero or zero. Comparatively, the energy expenditure from exercise in those individuals who exercise regularly encompasses roughly 10% of TEE. The remainder of TEE is accounted for by the energy expenditure resulting from NEAT. NEAT entails the energy expenditure of normal daily physical activities, including fidgeting, spontaneous muscle contraction, and maintaining posture when not reclining. This component of AEE varies the most among individuals.
Direct calorimetry is the gold standard measurement of BMR. When valid assessment of this component of TEE is required (i.e., in clinical research trials), then direct calorimetry is the desired procedure. However, the instrumentation and cost involved with operation of a direct calorimeter can easily exceed one million U.S. dollars, making 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 not ideally suited for the measurement of NEAT for exercise energy expenditure. In terms of quantifying NEAT, although it is possible for individuals to remain within a direct calorimeter for several days with measurements acquired for fidgeting and other spontaneous activity, other facets of NEAT (e.g., work-related walking) are artificially restricted within the direct calorimeter environment.
Another form of indirect calorimetry is the doubly labeled water method. Though not ideally suited for assessment of activity energy expenditure, it is commonly considered the gold standard for estimation of total energy expenditure. In the doubly labeled water technique, baseline samples of blood, saliva, and urine are collected. The individual then ingests non-radioactive isotopes. The isotopes become circulated uniformly throughout the body and subsequently are secreted gradually in the urine over the next one to three weeks. The elimination rates of each isotope are proportional to metabolic carbon dioxide production, which is influenced by the activity level of an individual.
There are many different noncalorimetric techniques used to quantify energy expenditure. Noncalorimetric techniques can be classified as objective and subjective methods. These methods rely on the ability to estimate energy expenditure based on a significant relationship existing between a given variable and exercise energy expenditure. A noncalorimetric technique must be validated against a standardized method.
Objective methods use an instrument such as a pedometer to measure energy expenditure. Other common objective instruments include accelerometers and heart rate monitors. Subjective methods use questionnaires and diaries to estimate an individual's energy expenditure. The energy expenditure is based on the individual's interpretation and perception of the physical activities.
Scientific research has established a general doseresponse relationship between physical activity and multiple health outcomes, including cardiorespiratory fitness, obesity, dyslipidemia, type 2 diabetes, colon cancer, relative risk of developing cardiovascular disease, and relative risk of mortality from all causes. Based on these dose-response relationships, it has been noted that the health benefits of an exercise program are associated with the total weekly energy expenditure. Based on these findings, a number of organizations have recommended an initial target energy expenditure of 1,000 kilocalories per week (kcal/wk) for previously sedentary individuals. Exercise professionals, or others who design exercise programs, are encouraged to gradually progress individuals toward a goal energy expenditure of 3,000 kcal/wk. This upper-target energy-expenditure goal is based on evidence showing a graded inverse doseresponse relationship between relative risk of all-cause mortality and levels of weekly physical activity. Newer studies have shown that exercise programs with energy expenditure recommendations based on individual differences in body mass lead to significant improvements in cardiorespiratory fitness and other important risk factors for cardiovascular disease.
The magnitude of energy expenditure resulting from regular physical activity has important implications for public health. There has been considerable interest in the role of low energy expenditure and sitting in the risk for various chronic diseases. Physical inactivity has been identified as a major modifiable risk factor for numerous chronic diseases and premature mortality.
The research strategy most often used to study this area has been to compare the relative risk of disease or mortality in a group of individuals performing zero weekly physical activity to groups completing various levels of weekly physical activity. This method provides valuable, yet limited, insight into the effects of structured physical activity or exercise on relative risk. It does not differentiate between the potentially unique effects of sitting too much and how the consequences of this lifestyle may contribute independently to the relative risk of various chronic diseases and/or mortality.
As of 2016, research groups were beginning to concentrate more on the physiological, clinical, and public health implications of sitting too much. Observational studies strongly suggest daily sitting time, or low NEAT levels, may have a statistically significant direct relationship with obesity, metabolic syndrome, type 2 diabetes, and cardiovascular disease. Inactivity physiology is an emerging area of inquiry with limited, yet intriguing, findings. More research was required before public health statements could be formulated regarding recommended limits on sitting time to reduce disease and mortality risk.
The standard error of the estimate (SEE) is a measure of accuracy of the prediction equation. Smaller SEE values can be interpreted to mean that the predicted energy expenditure value is more likely to reflect the actual energy expenditure value for the type of exercise. Metabolic prediction equations, although not exactly precise, have numerous advantages. These include cost, simplicity, and the fact that these equations are relatively accurate in their estimations (e.g., decent SEE statistics). The estimated metabolic cost of an exercise can be mathematically converted into either metabolic equivalents (METs) or a caloric equivalent. Summarized data from diaries and questionnaires are analyzed further by linking a MET value to each physical activity performed over the collection period.
The Compendium of Physical Activities is a resource frequently used by exercise physiologists and clinical exercise physiologists to complete this task. Energy expenditure can then be predicted mathematically with the assumption that enough detail has been provided in the diary or questionnaire. The metabolic prediction equations are quite practical for use in a variety of fitness and clinical environments; and the equations can and should be implemented to permit more confidence in accurate prescription and estimation of expenditure.
See also Metabolic equivalent of tasks .
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American College of Sports Medicine, 401 W. Michigan St., Indianapolis, IN, 46202-3233, (317) 634-9200, Fax: (317) 634-7817, http://www.acsm.org .
Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA, 30329-4027, (800) 232-4636, http://www.cdc.gov .
World Health Organization, Avenue Appia 20, Geneva, Switzerland, 1211 27, 41 22 791 21 11, Fax: 41 22 791 31 11, http://www.who.int .
Lance C. Dalleck, BA, MS, PhD
Revised by William A. Atkins, BB, BS, MBA