Energy balance (biology)

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In biology, energy balance is the biological homeostasis of energy in living systems. It is measured with the following equation:
Energy intake = internal heat produced + external work + energy stored.[1] It is also an aspect of bioenergetics, concerning energy flow through living systems.

In the US, it is generally measured using the energy unit Calorie (i.e. kilogram calorie), which equals the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ.

Energy intake[edit]

Energy intake is part of the diet, which is mainly regulated by hunger and food energy of what is consumed.

Energy expenditure[edit]

Energy expenditure is mainly a sum of internal heat produced and external work.

The internal heat produced is, in turn, mainly a sum of basal metabolic rate (BMR) and the thermic effect of food.

External work may be estimated by measuring physical activity level (PAL).

Imbalance between intake and expenditure[edit]

Gaining imbalance[edit]

A gaining energy imbalance is a result of energy intake being higher than what is consumed in external work and other bodily means of energy expenditure.

The main preventable causes are:

A gaining imbalance results in energy being stored, primarily as fat, causing weight gain. In time, overweight and obesity may develop, with resultant complications.

Losing imbalance[edit]

A losing energy imbalance is a result of energy intake being less than what is consumed in external work and other bodily means of energy expenditure.

The main cause is undereating due to a medical condition such as decreased appetite, anorexia nervosa, digestive disease, or due to some circumstance such as fasting, famine, or overpopulation.

Energy requirement[edit]

Normal energy requirement, and therefore normal energy intake, depends mainly on age, sex and physical activity level (PAL).

The Food and Agriculture Organization (FAO) of the United Nations has compiled a detailed report on human energy requirements: Human energy requirements (Rome, 17–24 October 2001) An older but commonly used and fairly accurate method is the Harris-Benedict equation.

Yet, there are currently ongoing studies to show if calorie restriction to below normal values have beneficial effects, and even though they are showing positive indications in primates[2][3] it is still not certain if calorie restriction has a positive effect on longevity for primates and humans.[2][3] Calorie restriction may be viewed as attaining energy balance at a lower intake and expenditure, and is, in this sense, not generally an energy imbalance, except for an initial imbalance where decreased expenditure hasn't yet matched the decreased intake.

See also[edit]


  1. ^ G. P. Talwar, L. M. Srivastavaby, ed. (2003). Textbook Of Biochemistry and Human Biology (3 ed.). p. 472. ISBN 81-203-1965-6. 
  2. ^ a b Anderson RM, Shanmuganayagam D, Weindruch R (2009). "Caloric restriction and aging: studies in mice and monkeys". Toxicol Pathol 37 (1): 47–51. doi:10.1177/0192623308329476. PMID 19075044. 
  3. ^ a b Rezzi S, Martin FP, Shanmuganayagam D, Colman RJ, Nicholson JK, Weindruch R (May 2009). "Metabolic shifts due to long-term caloric restriction revealed in nonhuman primates". Exp. Gerontol. 44 (5): 356–62. doi:10.1016/j.exger.2009.02.008. PMC 2822382. PMID 19264119. 

External links[edit]