Energy balance (biology)

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 + storage. It is also an aspect of bioenergetics, concerning energy flow through living systems.

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

Energy expenditure
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).

Gaining imbalance
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:
 * Overeating, resulting in increased energy intake
 * Sedentary lifestyle, resulting in decreased energy expenditure through external work

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
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
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 it is still not certain if calorie restriction has a positive effect on longevity for primates and humans. 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.