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Chapter: Introduction to Human Nutrition: Energy Metabolism

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Components of energy balance - Human Nutrition: Energy Metabolism

The energy that is consumed in the form of food is required by the body for metabolic, cellular, and mechanical work such as breathing, heart beat, and muscular work, all of which require energy and result in heat production.

Components of energy balance


Energy intake

 

Energy intake is defined as the caloric or energy content of food as provided by the major sources of dietary energy: carbohydrate (16.8 kJ/g), protein (16.8 kJ/g), fat (37.8 kJ/g), and alcohol (29.4 kJ/g).


Energy storage

 

The energy that is consumed in the form of food or drinks can either be stored in the body in the form of fat (the major energy store), glycogen (short-term energy/carbohydrate reserves), or protein (rarely used by the body for energy except in severe cases of starvation and other wasting conditions, or be used by the body to fuel energy-requiring events.


Energy expenditure

 

The energy that is consumed in the form of food is required by the body for metabolic, cellular, and mechanical work such as breathing, heart beat, and muscular work, all of which require energy and result in heat production. The body requires energy for a variety of functions. The largest use of energy is needed to fuel the basal metabolic rate (BMR), which is the energy expended by the body to maintain basic physiological functions (e.g., heart beat, muscle con-traction and function, respiration). BMR is the minimum level of energy expended by the body to sustain life in the awake state. It can be measured after a 12 hour fast while the subject is resting physically and mentally, and maintained in a thermoneutral, quiet environment. The BMR is slightly elevated above the metabolic rate during sleep, because energy expenditure increases above basal levels owing to the energy cost of arousal. Because of the difficulty in achieving BMR under most measurement situations, resting metabolic rate (RMR) is frequently measured using the same measurement conditions stated for BMR. Thus, the major difference between BMR and RMR is the slightly higher energy expended during RMR (~ 3%) owing to less subject arousal and non-fasting conditions. Because of this small difference, the terms basal and resting metabolic rate are often used interchangeably. RMR occurs in a continual process throughout the 24 hours of a day and remains relatively constant within individuals over time. In the average adult human, RMR is approximately 4.2 kJ/min. Thus, basal or resting metabolic rate is the largest component of energy expenditure and makes up about two-thirds of total energy expenditure.

 

In addition to RMR, there is an increase in energy expenditure in response to food intake. This increase in metabolic rate after food consumption is often referred to as the thermic effect of a meal (or meal-induced thermogenesis) and is mainly the energy that is expended to digest, metabolize, convert, and store ingested macronutrients, named obligatory thermo-genesis. The measured thermic effect of a meal is usually higher than the theoretical cost owing to a facultative component caused by an activation of the sympathoadrenal system, which increases energy expenditure through peripheral β-adrenoceptors. The energy cost associated with meal ingestion is pri-marily influenced by the composition of the food that is consumed, and also is relatively stable within indi-viduals over time. The thermic effect of a meal usually constitutes approximately 10% of the caloric content of the meal that is consumed. The third source of energy expenditure in the body is the increase in metabolic rate that occurs during physical activity, which includes exercise as well as all forms of physical activity. Thus, physical activity energy expenditure (or the thermic effect of exercise) is the term fre-quently used to describe the increase in metabolic rate that is caused by use of skeletal muscles for any type of physical movement. Physical activity energy expen-diture is the most variable component of daily energy expenditure and can vary greatly within and between individuals owing to the volitional and variable nature of physical activity patterns.

 

In addition to the three major components of energy expenditure, there may be a requirement for energy for three other minor needs.

The energy cost of growth occurs in growing indi-viduals, but is negligible except within the first few months of life.

 

Adaptive thermogenesis is heat production during exposure to reduced temperatures, and occurs in humans, e.g., during the initial months of life and during fever and other pathological conditions, but also as a contributor to daily energy expenditure.

 

Thermogenesis is increased by a number of agents in the environment, including in foods and bever-ages. Nicotine in tobacco is the most important one, and heavy smokers may have a 10% higher energy expenditure than nonsmokers of similar body size and composition and physical activity. Caffeine and derivatives in coffee, tea, and choco-late, capsaicin in hot chilies, and other substances in foods and drinks may possess minor thermo-genic effects that affect energy expenditure.

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