In biology, energy homeostasis, or the homeostatic control of energy balance, is a

biological process Biological processes are those processes that are necessary for an organism to live and that shape its capacities for interacting with its environment. Biological processes are made of many chemical reactions or other events that are involved in ...

that involves the coordinated homeostatic regulation of food intake (energy inflow) and energy expenditure (energy outflow). The human brain, particularly the

hypothalamus The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...

, plays a central role in regulating energy homeostasis and generating the sense of

hunger In politics, humanitarian aid, and the social sciences, hunger is defined as a condition in which a person does not have the physical or financial capability to eat sufficient food to meet basic nutritional needs for a sustained period. In t ...

by integrating a number of biochemical signals that transmit information about energy balance. Fifty percent of the energy from glucose metabolism is immediately converted to heat. Energy homeostasis is an important aspect of bioenergetics.

Definition

In the US, biological energy is expressed using the energy unit

Calorie The calorie is a unit of energy that originated from the caloric theory of heat. The large calorie, food calorie, dietary calorie, kilocalorie, or kilogram calorie is defined as the amount of heat needed to raise the temperature of one liter o ...

with a capital C (i.e. a kilocalorie), which equals the energy needed to increase the temperature of 1 kilogram of water by 1 °C (about 4.18 k J). Energy balance, through biosynthetic reactions, can be measured with the following equation: :''Energy intake (from food and fluids) = Energy expended (through work and heat generated) + Change in stored energy (

body fat Adipose tissue (also known as body fat or simply fat) is a loose connective tissue composed mostly of adipocytes. It also contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, Blood vessel, vascular endothel ...

and

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. It is the main storage form of glucose in the human body. Glycogen functions as one of three regularly used forms ...

storage)'' 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 energy is consumed, one of three particular effects occur within the body: a portion of that calorie may be stored as

triglyceride A triglyceride (from '' tri-'' and '' glyceride''; also TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans and other vertebrates ...

s, or

, transferred to cells and converted to chemical energy in the form of

adenosine triphosphate Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...

(ATP – a

coenzyme A cofactor is a non-protein chemical compound or Metal ions in aqueous solution, metallic ion that is required for an enzyme's role as a catalysis, catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can ...

) or related compounds, or dissipated as

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

Energy

Intake

Energy intake is measured by the amount of calories consumed from food and fluids. Energy intake is modulated by hunger, which is primarily regulated by the

, and choice, which is determined by the sets of brain structures that are responsible for

stimulus control In behavioral psychology, stimulus control is a phenomenon in operant conditioning that occurs when an organism behaves in one way in the presence of a given Stimulus (psychology), stimulus and another way in its absence. A stimulus that modifies b ...

(i.e.,

operant conditioning Operant conditioning, also called instrumental conditioning, is a learning process in which voluntary behaviors are modified by association with the addition (or removal) of reward or aversive stimuli. The frequency or duration of the behavior ma ...

and

classical conditioning Classical conditioning (also respondent conditioning and Pavlovian conditioning) is a behavioral procedure in which a biologically potent Stimulus (physiology), stimulus (e.g. food, a puff of air on the eye, a potential rival) is paired with a n ...

) and cognitive control of eating behavior. Hunger is regulated in part by the action of certain

peptide hormone Peptide hormones are hormones composed of peptide molecules. These hormones influence the endocrine system of animals, including humans. Most hormones are classified as either amino-acid-based hormones (amines, peptides, or proteins) or steroid h ...

s and

neuropeptide Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like the ...

s (e.g.,

insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the insulin (''INS)'' gene. It is the main Anabolism, anabolic hormone of the body. It regulates the metabol ...

leptin Leptin (from Ancient Greek, Greek λεπτός ''leptos'', "thin" or "light" or "small"), also known as obese protein, is a protein hormone predominantly made by adipocytes (cells of adipose tissue). Its primary role is likely to regulate long ...

ghrelin Ghrelin (; or lenomorelin, INN) is a hormone primarily produced by enteroendocrine cells of the gastrointestinal tract, especially the stomach, and is often called a "hunger hormone" because it increases the drive to eat. Blood levels of ghrel ...

, and

neuropeptide Y Neuropeptide Y (NPY) is a 36 amino-acid neuropeptide that is involved in various physiological and homeostatic processes in both the central and peripheral nervous systems. It is secreted alongside other neurotransmitters such as GABA and glu ...

, among others) in the

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 Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest.. In other words it is the energy required by body organs to perform normal It is reported in energy units per unit time ranging from watt ( ...

(BMR) and the

thermic effect of food Specific dynamic action (SDA), also known as thermic effect of food (TEF) or dietary induced thermogenesis (DIT), is the amount of energy expenditure above the basal metabolic rate due to the cost of processing food for use and storage. Heat produ ...

. External work may be estimated by measuring the

physical activity level The physical activity level (PAL) is a way to express a person's daily physical activity as a number and is used to estimate their total energy expenditure. In combination with the basal metabolic rate, it can be used to compute the amount of fo ...

(PAL).

Imbalance

The Set-Point Theory, first introduced in 1953, postulated that each body has a preprogrammed fixed weight, with regulatory mechanisms to compensate. This theory was quickly adopted and used to explain failures in developing effective and sustained weight loss procedures. A 2019 systematic review of multiple weight change interventions on humans, including

dieting Dieting is the practice of eating food in a regulated way to decrease, maintain, or increase body weight, or to prevent and treat diseases such as diabetes and obesity. As weight loss depends on calorie intake, List of diets, different kinds of ...

, exercise and overeating, found systematic "energetic errors", the non-compensated loss or gain of calories, for all these procedures. This shows that the body cannot precisely compensate for errors in energy/calorie intake, contrary to what the Set-Point Theory hypothesizes, and potentially explaining both

weight loss Weight loss, in the context of medicine, health, or physical fitness, refers to a reduction of the total body mass, by a mean loss of fluid, body fat (adipose tissue), or lean mass (namely bone mineral deposits, muscle, tendon, and other conn ...

and weight gain such as

obesity Obesity is a medical condition, considered by multiple organizations to be a disease, in which excess Adipose tissue, body fat has accumulated to such an extent that it can potentially have negative effects on health. People are classifi ...

. This review was conducted on short-term studies, therefore such a mechanism cannot be excluded in the long term, as evidence is currently lacking on this timeframe.

Positive balance

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 Overeating occurs when an individual consumes more calories than the energy that is expended via physical activity or expelled via excretion, or when they consume food past the point of satiation, often leading to weight gain and often obesity. O ...

, resulting in increased energy intake *

Sedentary lifestyle Sedentary lifestyle is a Lifestyle (social sciences), lifestyle type, in which one is physically inactive and does little or no physical movement and/or exercise. A person living a sedentary lifestyle is often sitting or lying down while enga ...

, resulting in decreased energy expenditure through external work A positive balance results in energy being stored as

fat In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers specif ...

and/or

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

, causing

weight gain Weight gain is an increase in body weight. This can involve an increase in muscle mass, fat deposits, excess fluids such as water or other factors. Weight gain can be a symptom of a serious medical condition. Description Weight gain occurs ...

. In time,

overweight Being overweight is having more body fat than is optimally healthy. Being overweight is especially common where food supplies are plentiful and lifestyles are sedentary. , excess weight reached epidemic proportions globally, with more than ...

and

may develop, with resultant complications.

Negative balance

A negative balance or caloric deficit 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 is a medical term for a loss of appetite. While the term outside of the scientific literature is often used interchangeably with anorexia nervosa, many possible causes exist for a loss of appetite, some of which may be harmless, while o ...

anorexia nervosa Anorexia nervosa (AN), often referred to simply as anorexia, is an eating disorder characterized by Calorie restriction, food restriction, body image disturbance, fear of gaining weight, and an overpowering desire to be thin. Individuals wit ...

digestive disease Gastrointestinal diseases (abbrev. GI diseases or GI illnesses) refer to diseases involving the gastrointestinal tract, namely the esophagus, stomach, small intestine, large intestine and rectum; and the accessory organs of digestion, the liver, ...

, or due to some circumstance such as fasting or lack of access to food.

Hyperthyroidism Hyperthyroidism is a endocrine disease in which the thyroid gland produces excessive amounts of thyroid hormones. Thyrotoxicosis is a condition that occurs due to elevated levels of thyroid hormones of any cause and therefore includes hyperth ...

can also be a cause.

Requirement

Normal energy requirement, and therefore normal energy intake, depends mainly on age, sex and

(PAL). The

Food and Agriculture Organization The Food and Agriculture Organization of the United Nations; . (FAO) is a specialized agency of the United Nations that leads international efforts to defeat hunger and improve nutrition and food security. Its Latin motto, , translates ...

(FAO) of the

United Nations The United Nations (UN) is the Earth, global intergovernmental organization established by the signing of the Charter of the United Nations, UN Charter on 26 June 1945 with the stated purpose of maintaining international peace and internationa ...

has compiled a detailed report on human energy requirements. 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 Calorie restriction (CR, also known as caloric restriction or energy restriction) is a dietary regimen that reduces the energy intake from foods and beverages without incurring malnutrition. The possible effect of calorie restriction on body w ...

to below normal values have beneficial effects, and even though they are showing positive indications in nonhuman primates it is still not certain if calorie restriction has a positive effect on longevity for humans and other primates. 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.

Society and culture

There has been controversy over energy-balance messages that downplay energy intake being promoted by food industry groups.

References

External links

*Diagram of regulation of fat stores and hunge

Daily energy requirement calculator
{{Authority control Nutrition Metabolism Biochemistry