Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Nutrition, Digestion, and Absorption
Energy Expenditure

Under conditions of energy equilibrium (caloric balance), energy intake must equal energy expenditure. Energy expenditure varies widely under different conditions and can be measured by placing an animal in an insulated chamber and determining heat loss alongside excretory products. It is usually more convenient to estimate oxygen consumption, since in most cases 1 L of consumed O₂ corresponds to approximately 4.83 kcal (20 kJ) of expended energy.

Individual energy expenditure depends on several factors, including:

Class="center">Table 53.3. Recommended energy intake for men and women. (Data from Recommended Dietary Allowances, 9th ed. Food and Nutrion Board, National Research Council — National Academy of Sciences, 1980.)

1) basal metabolic rate, corresponding to the energy expended to maintain basic physiological Functions under standard conditions (the subject must be at rest, awake, in a thermoneutral environment, and measurements should be taken at least 12 h after the last meal). Basal metabolic rate is proportional to fat-free body mass and body surface area. It is higher in men than in women, elevated in young children as well as in individuals with fever and hyperthyroidism, and reduced in hypothyroidism and starvation;

2) the thermic effect (specific dynamic action) of food. It accounts for approximately 5—10% of total energy expenditure and is associated with the additional energy cost of Digestion and The stimulation of METABOLISM driven by the influx of new substrates;

3) physical activity, which is the factor responsible for the largest and most highly variable component of energy expenditure; the range of energy expenditure between resting state and peak physical activity in athletes can reach a 10-fold increase;

4) ambient Temperature. In animals possessing brown adipose tissue, increased energy expenditure at low temperatures is driven by shivering thermogenesis and other mechanisms. When environmental temperature exceeds body temperature, excess energy is expended on cooling the body.