Fundamentals of Human Nutrition/Energy expenditure

13.1 Energy Expenditure

Physical Activity The most variable component of energy expenditure is physical activity since it is dictated by intentional lifestyle choices involving skeletal muscle action and the body systems that support these actions. Ranging from 5-40% of daily energy expenditure, this percentage will remain in the lower end of the spectrum for those with sedentary jobs who incorporate little physical activity and at the higher end of the spectrum for those who have physically strenuous jobs and use their leisure time to exercise (Powers, Scott K.; Howley, Edward T.; 2015). Estimating the amount of energy expended during physical activity relies on two categories: body composition, specifically muscle mass and body weight, and the nature of the activity (Whitney, Ellie; Rolfes, Sharon Rady; 2016). Skeletal muscles must produce more force and the body must work harder, or produce more power (Watts per unit of time) and consequently expend more energy in order to move an individual with greater total body weight (Schoeller, Dale A., 2009). Furthermore, several epidemiological studies comparing body weight and physical activity have concluded that body fat is more equally distributed in those who are consistently active (Rising R et al., 1994). This indicates that exercise can engage the whole body to expend energy and more equally distribute macronutrients to produce energy for skeletal muscles and contributing body systems. The other body systems, including the nervous system, cardiovascular system, endocrine system, and pulmonary system, are all highly involved in regulating and supporting skeletal muscle activity. These systems need energy to transmit sensory information and motor responses via electrical impulses, inhalation of oxygen and exhalation of carbon dioxide, deliver oxygen and nutrients to skeletal muscle cells, and initiate fatigue in order to protect other body systems from damage or visceral failure (Schoeller, Dale A., 2009). Additionally, not only does energy expenditure depend on the duration, frequency, and intensity of the activity, but the type of macronutrients the body uses for energy also depend on these factors (Whitney, Ellie; Rolfes, Sharon Rady; 2016). When energy expenditure is expressed in term of kilocalories, body weight and the type of activity are used to calculate the number of kcalories expended per minute for a given bout of exercise. Thus, the longer, the more intense, and the more frequent the physical activity, the greater the energy expenditure.

Research Spotlight: Energy Expenditure via Exercise and Weight Loss

In terms of weight loss, a study by Ross et al. concluded that among all of the components of energy expenditure exercise made up only a modest fraction of the entire energy deficit, indicating that diet contributes more to weight loss than exercise (2003).

Physical Activity References:

Ross, R., Freeman, J.A., & Janssen I. (2000). Exercise alone is an effective strategy for reducing obesity and related comorbidities. Exercise Sport and Sciences Reviews. 28. 165-170

Rising, R., Harper, I.T., Fontvielle, A.M., Ferraro R.T., Spraul, M., & Rauvin, E. (1994) Determinants of total daily energy expenditure variability in physical activity. American Journal of Clinical Nutrition. 59. 800-804.

Shoeller, Dale A. (2009). Energy expenditure in the lean and obese [Video]. Retrieved from: London: Henry Stewart Talks

Whitney, Ellie, & Rolfes, Sharon Rady. (2016). Understanding Nutrition. “8.3 Energy Out: the kCalories the Body Expels”. Cengage Learning. 14e. 240-242.

Energy Expenditure Regarding Basal Metabolic Rate

People expend energy throughout the day while participating in physical activities as well as just resting. Energy is expended for one’s basal metabolism, ability to participate in physical activities, and even to consume food (Whitney & Rolfs, 2013). In this section we will discuss how energy expenditure is related to one’s basal metabolic rate (BMR). The basal metabolic rate is the rate at which the body expends energy for activities that support the body and are necessary for one’s survival (Whitney & Rolfs, 2013). One’s BMR represents the energy needed to carry out fundamental metabolic functions such as breathing, ion transport, maintaining the normal turnover of enzymes, etc. (Baggott, 2008). Approximately two-thirds of the energy one expends in a day goes towards one’s basal metabolism. One’s BMR also supports functions like the heart beating and the kidneys filtering wastes (Whitney & Rolfs, 2013). Basal metabolic rate can also be defined as the minimum rate of energy expended in an awake, relaxed person, lying on a bed in a thermoneutral environment after an overnight fast (Ravussin & Bogardus, 1989). Each person has a specific basal metabolic rate. BMR is typically measured with the subject in a fasted state sitting or lying quietly in a room with a comfortable temperature. It is often expressed in terms of surface area, kJ m-2 hr-1, which allows for a fair comparison among infants, adults, those who are lean, and those who are obese (Baggott, 2008). Basal metabolic rate is variable between the sexes and also varies with age. BMR is highest in people who are growing such as children and pregnant women (Whitney & Rolfs, 2013). One’s BMR peaks in the infancy stage and declines rapidly through one’s childhood and adolescent years. Lean body mass is a major determinant of one’s BMR. Women tend to have more body fat and less lean muscle mass compared to men; thus, women tend to have a lower BMR than those of comparable men. With increasing age, one’s BMR starts to fall slowly. This decline is largely due to the loss of muscle. The loss of muscle is not inevitable and is often caused by absolute or relative inactivity. Weight-bearing exercises can help prevent some muscle loss among the elderly (Baggott, 2008). Additionally, BMR is high in those who have a fever, are under stress, or have highly active thyroid glands. One-way to increase one’s BMR is to participate in endurance and strength-training activities regularly in order to maximize lean body mass (Whitney & Rolfs, 2013). It has also been found that one’s fat-free mass and fat mass are also significant contributors to one’s BMR (Johnstone, Murison, Duncan, Rance, & Speakman, 2005). Because BMR accounts for 60-80% of one’s daily energy expenditure small deficits or excess in BMR can add up to a large number of calories over time. Weight gain tends to occur with aging and the age-related decrease in basal metabolic rate has been accounted for by a change in one’s body composition (Ravussin & Bogardus, 1989). Along the same note, from a nutritional standpoint, decreased caloric requirements can lead to decreased food intake, and a lower calorie intake can lead to a deficiency of essential nutrients (Baggott, 2008). Overall it is important to note that one’s basal metabolic rate is very important in everyday life, and maintaining a healthy lifestyle is important to keeping one’s basal metabolic rate in an ideal range.

References Baggott, J. (2008). IV. Life Stages. Retrieved November 14, 2015, from Johnstone, A., Murison, S., Duncan, J., Rance, K., & Speakman, J. (2005). The American Journal of Clinical Nutrition. Retrieved December 1, 2015, from Ravussin, E., & Bogardus, C. (1989). Relationship of genetics, age, and physical fitness to daily energy expenditure and fuel utilization. Retrieved December 1, 2015, from Whitney, E., & Rolfes, S. (2013). Energy Balance and Body Composition. In Understanding Nutrition (Fourteenth ed., p. 240, 241). Stamford, Connecticut: Cengage Learning.

Adaptive Thermogenesis

Adaptive thermogenesis is the change in basal metabolic rate of the body to compensate for the adjustment of energy expenditure based on changes in the environment, such as diet, trauma, stress, and extreme weather. The human body shows a stronger opposition to weight loss than to weight gain (Hill, Peters, & Wyatt, 2012). This is why weight is very hard to keep off after it is lost. This theory is studied frequently when looking at the topic of obesity amongst individuals. When an obese individual restricts their kilocalorie intake significantly for an extended period of time, their body tends to adapt to the diet change and eventually the individual will come to find that it is much more difficult to keep losing weight. When food intake and body weight decrease, there is a decline in metabolic rate as well. After about three or four weeks the body will adapt to the new kilocalorie intake, and the individual will stop losing weight even though they are still on the lower kilocalorie diet (Muller & Bosy-Westphal, 2013). This theory is also commonly referred to as a “weight-loss plateau”. The human body uses extra energy to adapt to new outside conditions and this can greatly change the energy expended in an individual, but only in extreme cases. This is why the energy from adaptive thermogenesis is not included when estimating energy requirements for most healthy individuals.

Resting Metabolic Rate

Resting Metabolic Rate (RMR) is the measure of energy expended in a relaxed state, similar to Basal Metabolic Rate (BMR). However, RMR is measured with requirements that aren’t as strict as BMR, such as a lack of a specific requirement of recent food intake and physical activity of the day before. Because of this, RMR values are typically higher than BMR values for the same individual. The RMR depends on a wide-range of factors, such as Age, Height, Growth, Body Composition, Dietary intake, and other outside factors that change how much energy is expended in a day. While some studies have shown that RMR is significantly increased with exercise and strength training, other studies have not come to the same conclusion (Speakman & Selman, 2003). The varying outcome is potentially related to the time at which the RMR is measured and the less-strict RMR measurement requirements. There are two main ways to measure RMR or how many calories a person burns in a given day. One method is called indirect calorimetry. This uses respiratory gases to determine the volume of oxygen uptake and volume of carbon dioxide output to then be put into the Weir Equation. The other method, which tends to be less accurate, is to use an RMR formula that uses height, weight, and age to calculate an individual’s RMR, but some of these formulas have been proven to give values up to 1000 kilocalories off of the actual value (Kelly, 2012).


Speakman, J., & Selman, C. (2007). Physical activity and resting metabolic rate. Proceedings of the Nutrition Society, 62, 621-634. doi:10.1079/PNS2003282

Hill, J. O., Wyatt, H. R., & Peters, J. C. (2012). Energy Balance and Obesity. Circulation, 126(1), 126–132.

Müller, M.J. and Bosy-Westphal, A. (2013), Adaptive thermogenesis with weight loss in humans. Obesity, 21: 218–228. doi: 10.1002/oby.20027

Kelly, Mark (2012). Resting Metabolic Rate: Best ways to Measure It – And Raise It, Too. Retrieved from

Thermic Effect of Food

The thermic effect on food is the energy your body uses to digest, and process the macronutrients you eat through absorption, transportation, storage and your metabolism. This used to be known as the Specific Dynamic Action. Everything you eat will give you calories that are burned through digestion and increase your metabolism. This process happens about 2 hours after you eat a meal. This type of energy expenditure accounts for about 10% of the total calories you ingest (Whitney, E., & Rolfes, S., 2013). The amount varies, however, with the different types of macronutrients.

Carbohydrates have a thermic effect of food of 5-30%.

Proteins have a thermic effect of food of 30%.

Fats have a thermic effect of food of 0-5% (Whitney, E., & Rolfes, S., 2013).

This amount can also vary with how much food you eat at once and the types of food you eat. Eating large loads of food versus eating smaller, more frequent meals has an effect on thermic effect of food. Meals with more calories have a higher thermic effect of food than smaller meals (Castillo, P., Tai, M., & Pi-Sunyer, F., 1991). Some people may think they can lose weight by eating less but that is not the case. Not eating will, in turn, slow down your metabolism and keep you from your goal. Eating more will burn more fat, as long as you’re eating the right things. This means eating foods that have a greater thermic effect of food.

People of different body compositions also have different thermic effects of food. Swaminathan, R., King, R., Holmfield, J., Siwek, R., Baker, M., & Wales, J. (1985) found that obese people have a lower thermic effect of food for fats and for a meal in whole than lean people do. This means that lean people will burn off their calories faster than heavy set people. Another study done by Segal, K., Gutin, B., Nyman, A., & Pi-Sunyer, F. (1985) stated that the thermic effect on food was higher for the people who were lean versus the people who were obese. All the participants that were compared had the same body mass but differing body composition.

It has also been proven that exercise increases the effects of thermic effect on food after your workout (Segal et all., 1985). The food will be used as energy and to restore your muscles rather than stored as fat. This also agrees with what is stated above because it is most common for people who work out to be leaner than people who are sedentary.


Castillo, P., Tai, M., & Pi-Sunyer, F. (1991). The American Journal of Clinical Nutrition. Retrieved November 24, 2015, from

Segal, K., Gutin, B., Nyman, A., & Pi-Sunyer, F. (1985). Thermic effect of food at rest, during exercise, and after exercise in lean and obese men of similar body weight. Retrieved November 26, 2015, from

Swaminathan, R., King, R., Holmfield, J., Siwek, R., Baker, M., & Wales, J. (1985). Thermic effect of feeding carbohydrate, fat, protein and mixed meal in lean and obese subjects. Retrieved November 26, 2015, from

Whitney, E., & Rolfes, S. (2013). MindTap - Cengage Learning. Retrieved November 23, 2015, from!&parentId=58605136

Energy Expenditure: Basal Metabolic Rate

Basal metabolic rate (BMR) is the minimal rate of energy expenditure per unit time by endothermic animals at rest. (Pike, Brown, 1975). Metabolism is all of the internal and external processes that the body needs to do for proper function. Some of the processes that the body carries out constantly without rest are breathing, blood flow, body temperature control, and the binding of myosin and actin filaments for muscle contraction. When the body generates heats from metabolism, it is called thermo genesis. The amount of heat generated from the body can be measured to estimate energy expenditure.

There are a few trends that are associated with basal metabolism. First off, as you increase in age, your basal metabolic rate goes decreases. "This is associated with a decrease in muscle mass, which normally decreases metabolic rate as well."(Saltin, Gollnick, 1983). When comparing two different people of the same age, the person with more lean muscle mass will be have a higher basal metabolic rate. They have more muscles; so that puts a larger demand on the body to circulate blood faster, increase the rate of gas exchange for respiration, and there is more force exerted when muscles contract.

Of the body's total energy expenditure, 70% comes from basal metabolic processes from various organ systems in the body. 20% came from physical activity and the last 10% came from thermo genesis. Thermo genesis is defined as the digestion of food. Intake of oxygen and coenzymes are necessary to have all of the body processes work properly for survival. Carbon dioxide needs to be expelled through the Krebs cycle as well. In a general breakdown of the organs that account for energy expenditure, the liver accounts for 27%. This has the highest percentage of energy expenditure because the liver takes part in many different metabolic processes. Next the brain accounts for 19% because it is the control center of the body and is responsible for almost every action in the body. Skeletal muscles account for 18% of energy expenditure and is responsible for the contraction of muscles during movement. The kidneys account for 10% of energy expenditure and are responsible for the excretion of urine. The heart is responsible for 7% of the expenditure of energy because it has the task of pumping blood throughout the whole body. And other organs account for 19% collectively. (Thorstensson, 1976 ) The primary organ responsible for the regulation of metabolism is the hypothalamus. Its functions include control of the autonomic nervous system, production of certain hostile feelings, and regulating the body temperature and food intake as well.


Pike, Ruth L; Brown, Myrtle Laurestine (1975). Nutrition: An Integrated Approach (2nd ed.). New York: Wiley.

Saltin, Bengt; Gollnick, Philip D. (1983). "Skeletal muscle adaptability: Significance for metabolism and performance".Stephen R. Handbook of Physiology. Baltimore: Williams & Wilkins.

Thorstensson (1976). "Muscle strength, fibre types and enzyme activities in man". Acta physiologica Scandinavica.