Energy Metabolism

Introduction

Definition and conceptualization of energy balance

The average adult human consumes close to 1 000 000 calories (4000 MJ) per year. Despite this huge energy intake, most healthy individuals are able to strike a remarkable balance between how much energy is consumed and how much energy is expended, thus resulting in a state of energy balance in the body. This accurate balance between energy intake and energy expenditure is an example of homeostatic control and results in maintenance of body weight and body energy stores. This regulation of energy balance is achieved over the long term despite large fluctuations in both energy intake and energy expenditure within and between days. The accuracy and precision by which the body maintains energy balance is high-lighted by the fact that even a small error in the system can have detrimental consequences over time. If energy intake chronically exceeds energy expenditure by as little as 105 kJ/day, then, over time, a person will become substantially obese. The achievement of energy balance is driven by the first law of thermo- dynamics, which states that energy can be neither destroyed nor created. This principle necessitates that when energy intake equals energy expenditure, body energy stores must remain constant.

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.

Energy balance

Energy balance occurs when the energy content of food is matched by the total amount of energy that is expended by the body. When energy intake exceeds energy expenditure, a state of positive energy balance occurs. Thus, positive energy balance occurs when excessive overfeeding relative to energy needs occurs, and the body increases its overall energy stores. Examples of positive energy balance include periods around major festivals when overeat-ing and inactivity generally prevail, and during preg-nancy and lactation when the body purposefully increases its stores of energy. When energy intake is lower than energy expenditure, a state of negative energy balance occurs, for example during periods of starvation. In this regard, evidence suggests that, under conditions of substantial energy imbalance, be it positive or negative, energy expenditure may reach a level that is beyond what could be predicted by body weight changes. This so-called “adaptive thermogen-esis” might contribute to the occurrence of resistance to lose fat in the context of obesity treatment or the achievement of a new body weight plateau following overfeeding. It is important to note that energy balance can occur regardless of the levels of energy intake and expenditure; thus, energy balance can occur in very inactive individuals as well as in highly active individuals provided that adequate energy sources are available. It is also important to think of energy balance in terms of the major sources of energy, i.e., carbohydrate, protein, and fat. For example, carbohydrate balance occurs when the body balances the amount of carbohydrate ingested with that expended for energy.