Measurement of Energy

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.

Further reading

Baldwin RL and Sainz RD ( 1995 ) Energy partitioning and modelling in animal nutrition . Annual Review Nutrition 15, 191-211

Blaxter K ( 1987) Energy metabolism in animals and man. Cambridge University Press.
Brockway JM (1987) Derivation of formulae used to calculate energy expenditure in man Human Nutrition: Clinical Nutrition 41C, 463-471
Brunengraber H, Kelleher JK, Rosiers CD, ( 1997), Application of mass isotopomer analysis to nutritional research. Annual Review Nutrition 17, 559-96
Heusner, A.A. (1985) Body size and energy metabolism. Annual Review of Nutrition, 5, 267–93.
Leibel, R.L., Edens, N. K. and Fried, S.K. (1989) Physiologic basis for the control of body fat distribution in humans. Annual Review of Nutrition, 9, 17–43.
Livingstone, M.B.E. et al. (1992) Daily energy expenditure in free-living children: comparison of heart rate monitoring with the doubly labelled water method. American Journal of Clinical Nutrition, 46, 343–52.
Livingstone MBE ( 1997) Heart – rate monitoring : the answer for assessing energy expenditure and physical activity in population studies. British Journal of Nutrition 78, 869-871.
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Schulman RG and Rothman DL ( 2001) Measurements of intermediary metabolism., 13 C NMR intermediary metabolism : implications for sytemic physiology. Annual Review Physiology 63 15-48
Stubbs RJ, Johnstone AM, O’Reilly LM, Popper SD ( 1998) Methodological issues relating to the measurement of food , energy and nutrient intake in human laboratory – based studies. Proceedings of the Nutrition Society 57, 357-72
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Symposium ( 1997) Mechanisms of energy compensation. Proceedings of the Nutrition Society 56, 1-50
Westerterp KR ( 1998 ) Energy requirements assessed using the doubly- labelled water method. British Journal Nutrition , 80, 217-8
Wagenmakers AJM ( 1999 ) Metabolic aspects of human nutrition at rest and during physical stress; recent methodological and technical developments. Proceedings of the Nutrition Society 58, 839-1033.
Weibel ER ( 2002) The pitfalls of power laws. Nature 417, 131-2.
Woo, R., Daniels-Kush, R. and Horton, E.S. (1985) Regulation of energy balance. Annual Review of Nutrition, 5, 411–33.
World Health Organisation (1985) Energy and Protein requirements , report of the joint FAO/WHO/UNO Expert consultation . Technical report series 724 WHO Geneva.
Zemel BS, Riley EM and Stallings VA, ( 1997 ) Evaluation of methodology nutritional assessment in childreen : anthropometry , body composition and expenditure. Annual Review Nutrition 17, 211-35


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