Energy Expenditure

Energy expenditure refers to the amount of energy that an organism uses in carrying out all its functions and activities . These functions can be as varied as maintenance of body temperature, conduction of nerve impulses, repair and formation of tissues, mechanical work of muscles to move, digest food, or breathe.

In the field of human nutrition, it is measured in calories and specifically refers to the way in which the body uses the energy stored in the chemical bonds of different nutrients . Thus, there is also talk of energy requirement as a measure of the daily amount of energy that food must supply to meet the body’s energy needs. The energy balance refers to the balance between the energy requirement and the energy intake of the food consumed.

Components of energy expenditure

Total energy expenditure is usually divided into three major 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 cause total energy expenditure to be very different from one individual to another, and even very different for the same person from one day to the next.

Energy expenditure at rest

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

Resting energy expenditure is usually slightly higher than basal metabolic rate alone. The basal metabolic rate is measured in the morning upon waking up 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, for its part, 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 is the most energy consuming) and the work of involuntary muscles (10%. Diaphragm, heart, movements intestinal peristaltics, etc.).

The main factors influencing basal metabolism are:

• Age and stage of growth : the basal metabolism is higher at an early age, mainly due to the greater energy expenditure in the growth of organs and tissues. Basal energy expenditure usually increases from birth to puberty, also influenced by increased body size and growth rate, and decreases from adulthood to old age. During pregnancy the basal metabolism increases considerably, also during lactation.
• Body Weight and Build – Higher body weight and size is often accompanied by a higher basal metabolic rate. This is because more heat is lost as there is a greater body surface area. The metabolic rate is also higher in strong builds due to the increased energy requirement to maintain resting lean muscle mass.
• State of health : 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 each degree of body temperature above 37ºC.
• Hormonal factors : The levels of certain hormones can affect the basal metabolic rate, especially thyroxine (produced in the thyroid) and norepinephrine (a hormone and neurotransmitter). The higher the levels of thyroxine and norepinephrine, the higher the basal metabolic rate.
• Environmental factors : One of the environmental factors that most affects the 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 thermic effect of food, accounts for 5-10% of total energy expenditure. It is the energy expenditure for the digestion, absorption, distribution and storage of ingested food . One of the most widely used measurement methods is the difference in resting energy expenditure before and after a meal.

The foods that generate the most thermic effect are foods rich in protein, 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 up to twofold, a phenomenon known as adaptive thermogenesis .

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

Expenditure for voluntary physical activity

Energy expenditure due to voluntary physical activity represents it produced by any movement of the body, such as getting out of bed, going up the stairs or playing a 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 of Energy expenditure

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

The main methods of measuring energy expenditure are:

1. Calorimetry – Calorimetry measures the heat given off in a chemical reaction. Since energy is neither created nor destroyed, but rather transformed, measuring the heat released by the body can estimate the energy expenditure made. This would be direct calorimetry . Indirect calorimetry does not measure the heat given off by the body directly but through the consumption of oxygen and/or the production of carbon dioxide.
2. 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.
3. Doubly labeled water : This technique uses non-radioactive isotopes of hydrogen and oxygen introduced into the body via water molecules. The rate of elimination of these isotopes can be measured through urine and blood samples and gives a fairly accurate approximation of the volume of oxygen consumed and carbon dioxide produced. It is an acceptable precision technique used even in newborns.
4. Accelerometry : there are various 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.
5. Questionnaires to estimate the level of physical activity : the questionnaires can be used to make estimates on the level of physical activity and also of daily energy consumption in questionnaires on nutritional habits. Its main utility is in large-scale epidemiological studies.

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. Could be:

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

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