In biology, energy homeostasis, or energy balance, is an aspect of bioenergetics concerning the energy flow through living systems, by metabolism. Energy homeostasis involves the human body using chemical and neural signals to adjust the amount of energy flows; and to regulate caloric intake by signaling the brain to regulate the sensation of hunger. Fifty percent of the energy from glucose metabolism is immediately converted to heat.
In the US, biological energy is expressed using the energy unit Calorie with a capital C (i.e. kilocalorie), which equals the energy needed to increase the temperature of 1 kilogram of water by 1 °C (about 4.18 kJ).
Energy balance, through biosynthetic reactions, can be measured with the following equation:
Energy intake (food) = Energy expended (heat + work) + Energy stored.
The first law of thermodynamics states that energy can be neither created nor destroyed. But energy can be converted from one form of energy to another. So when a calorie of food eaten enters a body, ultimately 100% of that calorie will be converted to heat, resulting in three particular short-term effects: a portion of that calorie is either stored as fat, transferred to the body's cells as chemical energy, by Adenosine triphosphate (ATP), a coenzyme and used for mechanical work or chemical synthesis, or immediately dissipated through heat.
Energy intake is calories consumed as food, which is determined by hunger (hypothalamus) and choice (cerebral cortex). Hunger is determined by the impact of the hormones leptin and ghrelin on the hypothalamus.
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 the physical activity level (PAL).
A positive balance 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 negative balance is a result of energy intake being less than what is consumed in external work and other bodily means of energy expenditure.
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.
- Keith N. Frayn (2013). Metabolic Regulation: A Human Perspective. John Wiley & Sons. ISBN 1118685334.
- Kevin G. Murphy & Stephen R. Bloom (December 14, 2006). "Gut hormones and the regulation of energy homeostasis" 444 (7121). Nature. pp. 854–859. doi:10.1038/nature05484. PMID 17167473.
- 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.
- 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.
- Diagram of regulation of fat stores and hunger 
- Daily energy requirement calculator
- BodyEngine - Visual fitness tool which calculates RMR (resting metabolic rate ), EER and BMI