What is energy expenditure?

In biology, energy expenditure refers to the amount of energy an organism uses in carrying out all of its functions and activities. These functions can be as varied as maintaining body temperature, conducting nerve impulses, repairing and forming tissues, mechanically working muscles to move, digest food, or breathe.

In the field of human nutrition, energy expenditure is measured in calories and refers specifically to the way the body uses the energy stored in the chemical bonds of different nutrients. Thus, energy needs are also referred to as a measure of the daily amount of energy that food must provide to meet the energy needs of the body. The energy balance refers to the balance between the energy requirement and the energy contribution of the food consumed.

Components of energy expenditure

Total energy expenditure It is generally divided into three main components: resting energy expenditure, food-induced thermogenesis, and voluntary physical activity. Each of these components is affected by numerous personal and environmental variables that can make total energy expenditure very different from one individual to another, and even very different for the same person on a day-to-day basis.

resting energy expenditure

Resting energy expenditure includes the energy expenditure necessary to maintain the body’s vital functions and body temperature in a neutral environment and in a resting state. The main component is the basal metabolic rate, which is why it is also called basal energy expenditure, which can represent up to 75% of total energy expenditure.

Resting energy expenditure is usually slightly higher than the basal metabolic rate alone. Basal metabolic rate is measured in the morning upon waking, when at least 12 hours have passed since the last meal and the person is in a state of complete physical and mental rest. Resting energy expenditure, on the other hand, is measured at any time of day when at least 4 hours have passed since the last meal and physical activity.

Most of the basal energy is consumed in cellular metabolism (50%), molecular synthesis (40%, protein synthesis consumes more energy) and involuntary muscle work (10% Diaphragm, heart, peristaltic bowel movements, etc).

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The main factors that influence basal metabolism are:

Age and growth stage: basal metabolism is higher at an early age, mainly due to the higher energy expenditure in the growth of organs and tissues. Basal energy expenditure normally increases from birth to puberty, also influenced by increasing body size and growth rate, and decreases from adulthood to old age. During pregnancy, basal metabolism increases considerably, also during lactation. Body Weight and Constitution Increase in body weight and size is often accompanied by a higher basal metabolic rate. This is because more heat is lost by having a larger body surface. The metabolic rate is also higher in strong builds due to the increased energy requirement to maintain lean muscle mass at rest. Health status: In general, during an illness, the basal metabolic rate tends to increase. For example, the basal metabolic rate can increase by up to 7% for every degree of body temperature above 37°C. Hormonal factors: Levels of certain hormones can affect basal metabolic rate, especially thyroxine (produced in the thyroid) and norepinephrine (a hormone and neurotransmitter). At higher levels of thyroxine and norepinephrine, higher basal metabolic rate. Environmental Factors: One of the environmental factors that most affects basal metabolic rate is temperature. The higher the temperature, the higher the basal metabolic rate.

food induced thermogenesis

Food-induced thermogenesis, also known as diet-induced thermogenesis or the thermic effect of food, accounts for 5-10% of total energy expenditure. It is the expenditure of energy for the digestion, absorption, distribution and storage of ingested food. One of the most commonly used measurement methods is as the difference in resting energy expenditure before and after a meal.

The foods that generate the most thermic effect are foods rich in proteins, up to 20% higher than foods rich in carbohydrates and fats (25% increase in energy expenditure at rest compared to 5%). If food intake is followed by physical activity, food-induced thermogenesis can increase by up to two-fold, a phenomenon known as adaptive thermogenesis.

Other factors that include this component of energy expenditure are age, genetic predisposition, insulin sensitivity and digestion disorders.

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Spending on voluntary physical activity

The energy expenditure due to voluntary physical activity represents the energy consumption produced by any movement of the body, such as getting out of bed, climbing stairs or playing some sport. It is the most variable fraction of total energy expenditure and can range from 10% in a sedentary person to 50% in an athlete.

measurement methods

Energy expenditure can be measured by several methods, generally classified into direct measurements and indirect measurements. Direct methods are generally much more accurate, but are more expensive and difficult to perform in large populations due to the need for specialized equipment and facilities. Indirect methods are generally much cheaper and easier to perform; although they are much less accurate, they are useful in population studies.

The main methods for measuring energy expenditure are:

Calorimetry: Calorimetry measures the heat released in a chemical reaction. As energy is neither created nor destroyed, but transformed, measuring the heat released by the body, it is possible to estimate the energy expenditure performed. This would be direct calorimetry. o indirect calorimetry It does not measure the heat emitted by the body directly, but through the consumption of oxygen and/or the production of carbon dioxide. heart rate monitoring: This indirect method uses the relationship between the individual’s heart rate and oxygen consumption. You need individual calibration curves as this relationship is highly variable between different people. It is mainly used in children. doubly labeled water: this technique uses non-radioactive isotopes of hydrogen and oxygen introduced into the body through water molecules. The rate of elimination of these isotopes can be measured from urine and blood samples and provides a very accurate approximation of the volume of oxygen consumed and carbon dioxide produced. It is an acceptable precision technique used even in newborns. Accelerometry: There are several devices that measure the movement and acceleration of the body. With these data it is possible to estimate the level of physical activity and estimate energy expenditure. Questionnaires for estimating the level of physical activity: the questionnaires can be used to estimate the level of physical activity and also the daily energy consumption in questionnaires about nutritional habits. Its main utility is in large-scale epidemiological studies.
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energy balance

The difference between energy requirement, determined almost entirely by total energy expenditure, and energy intake through the diet is known as energy balance. It might be:

positive balance: more energy is ingested than expended. One of its main consequences is weight gain. More calories are ingested than burned, and the excess is stored in stores of glycogen and fat as a reserve. Negative balance: The negative balance, or caloric deficit, makes you lose weight, but it can become very dangerous in the extreme or if it is accompanied by a nutritional deficit. neutral balance: consumption and expenditure are practically the same.

The body has mechanisms for regulating energy flow, and therefore energy balance. Together, these mechanisms form the process known as energy homeostasis, which includes chemical and neuronal signals to regulate the different processes involved in energy expenditure and intake. For example, signals from the central nervous system to generate a feeling of fullness and stop eating.

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Byrd-Bredbenner, C., Beshgetoor, D., Moe, G., & Berning, J. (2010). Perspectives in Nutrition (8th Edition). Mexico: McGraw Hill. Levine J. Measuring energy expenditure. Nutrition in Public Health, 2005.

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