In biology, energy homeostasis, or the homeostatic control of energy balance, is a
biological process that involves the coordinated
homeostatic
In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
regulation of food intake (energy inflow) and energy expenditure (energy outflow).
The human brain, particularly the
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
, 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 Human nutrition, nutritional needs for a sustaine ...
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 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, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular e ...
and
glycogen 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
body fat
Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular e ...
,
triglyceride
A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''wikt:tri-#Prefix, tri-'' and ''glyceride'').
Triglycerides are the main constituents of body fat in humans and other ...
s, or
glycogen, transferred to cells and converted to chemical energy in the form of
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms o ...
(ATP – a
coenzyme
A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that ass ...
) or related compounds, or dissipated as
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
.
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
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
,
and choice, which is determined by the sets of brain structures that are responsible for
stimulus control (i.e.,
operant conditioning and
classical conditioning
Classical conditioning (also known as Pavlovian or respondent conditioning) is a behavioral procedure in which a biologically potent stimulus (e.g. food) is paired with a previously neutral stimulus (e.g. a triangle). It also refers to the learni ...
) and
cognitive control
In cognitive science and neuropsychology, executive functions (collectively referred to as executive function and cognitive control) are a set of cognitive processes that are necessary for the cognitive control of behavior: selecting and suc ...
of eating behavior.
Hunger is regulated in part by the action of certain
peptide hormone
Peptide hormones or protein hormones are hormones whose molecules are peptide, or proteins, respectively. The latter have longer amino acid chain lengths than the former. These hormones have an effect on the endocrine system of animals, including 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,
leptin,
ghrelin
Ghrelin (; or lenomorelin, INN) is a hormone 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 ghrelin are hi ...
, 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. NPY has been identified as the most abundant peptide present in the ma ...
, among others) in the
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
.
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. It is reported in energy units per unit time ranging from watt (joule/second) to ml O2/min or joule per hour per kg body mass J/(h·kg). Pro ...
(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 produc ...
. 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 a person's total energy expenditure. In combination with the basal metabolic rate, it can be used to compute the amount o ...
(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, 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 co ...
and weight gain such as
obesity
Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's ...
. 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 in relation to the energy that is expended via physical activity or expelled via excretion, leading to weight gain and often obesity. Overeating is the defining characteristic of bing ...
, 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, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
and/or
muscle, 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 w ...
. In time,
overweight and
obesity
Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's ...
may develop, with resultant complications.
Negative balance
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.
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 in non-scientific publications is often used interchangeably with anorexia nervosa, many possible causes exist for a loss of appetite, some of which may be harmless, while others ...
,
anorexia nervosa
Anorexia nervosa, often referred to simply as anorexia, is an eating disorder characterized by low weight, food restriction, body image disturbance, fear of gaining weight, and an overpowering desire to be thin. ''Anorexia'' is a term of Gr ...
,
digestive disease
Gastrointestinal diseases (abbrev. GI diseases or GI illnesses) refer to diseases involving the gastrointestinal tract, namely the oesophagus, 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 the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidis ...
can also be a cause.
Requirement
Normal energy requirement, and therefore normal energy intake, depends mainly on age, sex and
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 a person's total energy expenditure. In combination with the basal metabolic rate, it can be used to compute the amount o ...
(PAL). The
Food and Agriculture Organization
The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
(FAO) of the
United Nations
The United Nations (UN) is an intergovernmental organization whose stated purposes are to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and be a centre for harmoniz ...
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 (caloric restriction or energy restriction) is a dietary regimen that reduces intake of energy from caloric foods & beverages without incurring malnutrition. "Reduce" can be defined relative to the subject's previous intake b ...
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.
See also
*
Dynamic energy budget
The dynamic energy budget (DEB) theory is a formal metabolic theory which provides a single quantitative framework to dynamically describe the aspects of metabolism (energy and mass budgets) of all living organisms at the individual level, based o ...
*
Earth's energy balance
Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but ma ...
References
External links
*Diagram of regulation of fat stores and hunge
Daily energy requirement calculator
{{Authority control
Nutrition
Metabolism
Biochemistry