Energy is continuously required for cell repair and growth and intermittently for work, though intake of food to provide this energy is intermittent. There is loss of nutrient energy when food is converted to mechanical energy; about 65% is dissipated as heat.
Twelve people sitting talking in a room produce heat at 60 kJ per minute, equivalent to a 1 kw electric fire.
Total energy expenditure ( TEE ) has three components
Basal metabolic rate (BMR) . At complete rest and without physical work (basal metabolism), energy is required for the activity of the internal organs and to maintain body temperature. This is the single largest contributor to total energy expenditure at 60-70% of TEE. During sleep the overall metabolic rate approximates to the BMR
Thermogenic component. The energy expended through the physiological response following the ingestion of food, exposure to cold or stimulants.
Physical activity. Only 25–35% of nutrient energy is used for mechanical work and less than 10% is for basic physiological activity, e.g. cardiac and respiratory contractions..
The energy requirements of an individual is the energy intake which will balance energy expenditure when the individual has a body size, composition and level of physical activity consistent with long term good health and will allow for the maintenance of economically necessary and socially desirable activity. In children, pregnant and lactating women , the energy level includes the energy need associated with the deposition of tissues and the secretion of milk at rates consistent with good health.
Regulation of energy balance
The regulation of body weight is dependent upon a balance between nutrient intake and utilisation, though there are other important factors.
1. Energy is continuously required for cell repair and growth, but only intermittently for work, though food intake to provide this energy is intermittent.
2. At complete rest and without physical work (basal metabolism), energy is still required for the activity of the internal organs and to maintain body temperature. This is called basal metabolic rate (BMR). Basal metabolic rate = weight/surface area in metres2. BMR in a 70-kg man is approximately 60–75% of the total daily expenditure, i.e. 1500 kcal/day. Determinants of metabolic rate which are invariable are: age, sex and genetic constitution. The variable elements are: the diet that antedated the test, body composition and weight, temperature, hormones, smoking, drugs and stress.
3. Heat loss is proportional to body surface area.
4. In the regulation of energy balance, nutrient intake and energy expenditure are related in the formula: E = Ein – Eout. Inappropriately high intakes or low expenditure produce energy excesses, increase fat storage and result in a gain in body weight. Ein is the energy available for metabolism of the foods and Eout is formed from two components: Eout = Eexer + Ether, where Eexer is the energy available for metabolism of the foods lost from the body in urine and stools, and Ether is heat production (thermogenesis).
5. The energy expenditure for light work is less than 170 W (2.5 kcal/min) (e.g. golf, assembly work, gymnastic exercises, brick laying, painting). That for moderate work is 350–500 W (5–7.4 kcal/ min) (e.g. general labouring with a pick and shovel, agricultural work, ballroom dancing and tennis). That for very hard work is 650–800 W (10–12.5 kcal/min) (e.g. lumber work, furnace stoking, cross-country running, hill climbing).
6. The energy requirements of children vary with age. Pregnant and lactating women have increased energy needs.
7. Measurements of energy expenditure include heart rate monitoring, direct and indirect calorimetry and the doubly labelled water method.
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