Adipose tissue - AbsoluteAstronomy.com

Histology

Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...

, adipose tissue or body fat or fat depot or just fat is loose connective tissue

Connective tissue

"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

composed of adipocyte

Adipocyte

However, in some reports and textbooks, the number of fat cell increased in childhood and adolescence. The total number is constant in both obese and lean adult...

s. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblast

Lipoblast

A lipoblast is a precursor cell for a adipocyte.Alternate terms include adipoblast and preadipocyte.Early stages are almost indistinguishable from fibroblasts.-Liposarcoma:...

s. Its main role is to store energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

in the form of lipids, although it also cushions and insulates

Thermal insulation

Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...

the body. Far from hormonally inert, adipose tissue has in recent years been recognized as a major endocrine organ, as it produces hormone

Hormone

A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

s such as leptin

Leptin

Leptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...

, resistin

Resistin

Resistin also known as adipose tissue-specific secretory factor or C/EBP-epsilon-regulated myeloid-specific secreted cysteine-rich protein is a cysteine-rich protein that in humans is encoded by the RETN gene....

, and the cytokine

Cytokine

Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

TNFα. Moreover, adipose tissue can affect other organ systems of the body and may lead to disease. Obesity

Obesity

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...

or being overweight

Overweight

Overweight is generally defined as having more body fat than is optimally healthy. Being overweight is a common condition, especially where food supplies are plentiful and lifestyles are sedentary...

in humans and most animals does not depend on body weight but on the amount of body fat—to be specific, adipose tissue. Two types of adipose tissue exist: white adipose tissue

White adipose tissue

White adipose tissue or white fat is one of the two types of adipose tissue found in mammals. The other kind of adipose tissue is brown adipose tissue....

(WAT) and brown adipose tissue

Brown adipose tissue

Brown adipose tissue or brown fat is one of two types of fat or adipose tissue found in mammals....

(BAT). The formation of adipose tissue appears to be controlled in part by the adipose gene. Adipose tissue was first identified by the Swiss naturalist Conrad Gessner

Conrad Gessner

Conrad Gessner was a Swiss naturalist and bibliographer. His five-volume Historiae animalium is considered the beginning of modern zoology, and the flowering plant genus Gesneria is named after him...

in 1551.

Anatomical features

In humans, adipose tissue is located beneath the skin

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

(subcutaneous fat), around internal organ

Organ (anatomy)

In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

s (visceral fat), in bone marrow (yellow bone marrow) and in breast tissue. Adipose tissue is found in specific locations, which are referred to as 'adipose depots.' Adipose tissue contains several cell types, with the highest percentage of cells being adipocyte

Adipocyte

However, in some reports and textbooks, the number of fat cell increased in childhood and adolescence. The total number is constant in both obese and lean adult...

s, which contain fat droplets. Other cell types include fibroblasts, macrophages, and endothelial cells. Adipose tissue contains many small blood vessel

Blood vessel

The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s. In the integumentary system

Integumentary system

The integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages...

, which includes the skin, it accumulates in the deepest level, the subcutaneous layer, providing insulation from heat and cold. Around organs, it provides protective padding. However, its main function is to be a reserve of lipids, which can be burned to meet the energy needs of the body. Adipose depots in different parts of the body have different biochemical profiles. Under normal conditions, it provides feedback for hunger and diet to the brain.

Mice

In mice

Mouse

A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

, there are eight major adipose depots, four of which are within the abdominal cavity

Abdominal cavity

The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...

: The paired gonadal depots are attached to the uterus

Uterus

The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

and ovaries in females and the epididymis

Epididymis

The epididymis is part of the male reproductive system and is present in all male amniotes. It is a narrow, tightly-coiled tube connecting the efferent ducts from the rear of each testicle to its vas deferens. A similar, but probably non-homologous, structure is found in cartilaginous...

and testes in males; the paired retroperitoneal depots are found along the dorsal

Dorsum (biology)

In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

wall of the abdomen, surrounding the kidney, and, when massive, extend into the pelvis. The mesenteric depot forms a glue-like web that supports the intestine

Intestine

In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

s, and the omental depot, which originates near the stomach

Stomach

The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

and spleen

Spleen

The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...

, and, when massive, extends into the ventral abdomen. Both the mesenteric and omental depots incorporate much lymphoid tissue as lymph nodes and milky spots, respectively. The two superficial depots are the paired inguinal depots, which are found anterior to the upper segment of the hind limbs (underneath the skin) and the subscapular depots, paired medial mixtures of brown adipose tissue adjacent to regions of white adipose tissue, which are found under the skin

Skin

between the dorsal crests of the scapulae. The layer of brown adipose tissue in this depot is often covered by a “frosting” of white adipose tissue; sometimes these two types of fat (brown and white) are hard to distinguish. The inguinal depots enclose the inguinal group of lymph nodes. Minor depots include the pericardial, which surrounds the heart, and the paired popliteal depots, between the major muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s behind the knees, each containing one large lymph node

Lymph node

A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as...

. Of all the depots in the mouse, the gonadal depots are the largest and the most easily dissected, comprising about 30% of dissectible fat.

Obesity

In a severely obese person

Person

A person is a human being, or an entity that has certain capacities or attributes strongly associated with being human , for example in a particular moral or legal context...

, excess adipose tissue hanging downward from the abdomen is referred to as a panniculus

Panniculus

Panniculus is a medical term describing a dense layer of fatty tissue growth, consisting of subcutaneous fat in the lower abdominal area. It can be a result of obesity and can be mistaken for a tumor or hernia. Abdominal panniculus can be removed during abdominal panniculectomy, a type of...

(or pannus

Pannus

Pannus is a medical term for an abnormal layer of fibrovascular tissue or granulation tissue. Common sites for pannus formation include over the cornea, over a joint surface , or on a prosthetic heart valve...

). A panniculus complicates surgery of the morbidly obese. The panniculus may remain as a literal "apron of skin" if a severely obese person quickly loses large amounts of fat (a common result of gastric bypass surgery

Gastric bypass surgery

Gastric bypass procedures are any of a group of similar operations that first divides the stomach into a small upper pouch and a much larger lower "remnant" pouch and then re-arranges the small intestine to allow both pouches to stay connected to it. Surgeons have developed several different ways...

). This condition cannot be effectively corrected through diet and exercise alone, as the panniculus consists of adipocytes and other supporting cell types shrunken to their minimum volume and diameter. Reconstructive surgery is one method of treatment.

Abdominal fat

Visceral fat or abdominal fat also known as organ fat or intra-abdominal fat, is located inside the abdominal cavity

Abdominal cavity

, packed in between organs (stomach, liver, intestines, kidneys, etc.). Visceral fat is different than subcutaneous fat underneath the skin

Skin

, and intramuscular fat

Intramuscular fat

Intramuscular fat or Intramuscular triglycerides is located throughout skeletal muscle and is responsible for the marbling seen in certain cuts of beef. In humans, excess accumulation of intramuscular fat is associated with insulin resistance and type 2 diabetes...

interspersed in skeletal muscle

Skeletal muscle

Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

s. Fat in the lower body, as in thighs and buttocks, is subcutaneous, whereas fat in the abdomen

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

is mostly visceral. Visceral fat is composed of several adipose depots including mesenteric, epididymal white adipose tissue

White adipose tissue

White adipose tissue or white fat is one of the two types of adipose tissue found in mammals. The other kind of adipose tissue is brown adipose tissue....

(EWAT), and perirenal depots.

An excess of visceral fat is known as central obesity

Central obesity

Abdominal obesity, colloquially known as belly fat or clinically as central obesity, is the accumulation of abdominal fat resulting in an increase in waist size...

, or "belly fat", in which the abdomen protrudes excessively. There is a strong correlation between central obesity

Obesity

and cardiovascular disease

Cardiovascular disease

Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...

.
Excess visceral fat is also linked to type 2 diabetes

Diabetes mellitus type 2

Diabetes mellitus type 2formerly non-insulin-dependent diabetes mellitus or adult-onset diabetesis a metabolic disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. Diabetes is often initially managed by increasing exercise and...

, insulin resistance

Insulin resistance

Insulin resistance is a physiological condition where the natural hormone insulin becomes less effective at lowering blood sugars. The resulting increase in blood glucose may raise levels outside the normal range and cause adverse health effects, depending on dietary conditions. Certain cell types...

inflammatory diseases, and other obesity-related diseases.

Female sex hormone

Estrogen

Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

causes fat to be stored in the buttocks

Buttocks

The buttocks are two rounded portions of the anatomy, located on the posterior of the pelvic region of apes and humans, and many other bipeds or quadrupeds, and comprise a layer of fat superimposed on the gluteus maximus and gluteus medius muscles. Physiologically, the buttocks enable weight to...

, thighs, and hips

Hip (anatomy)

In vertebrate anatomy, hip refer to either an anatomical region or a joint.The hip region is located lateral to the gluteal region , inferior to the iliac crest, and overlying the greater trochanter of the femur, or "thigh bone"...

in women. Men are more likely to have fat stored in the belly due to sex hormone differences

Sex differences in humans

A sex difference is a distinction of biological and/or physiological characteristics associated with either males or females of a species. These can be of several types, including direct and indirect. Direct being the direct result of differences prescribed by the Y-chromosome, and indirect being...

. When women reach menopause

Menopause

Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...

and the estrogen produced by ovaries declines, fat migrates from their buttocks, hips and thighs to their waists; later fat is stored in the belly

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

.

High-intensity exercise is one way to effectively reduce total abdominal fat. One study suggests at least 10 MET

Metabolic equivalent

The metabolic equivalent of task , or simply metabolic equivalent, is a physiological concept expressing the energy cost of physical activities as multiples of resting metabolic rate and is defined as the ratio of metabolic rate during a specific physical activity to a reference rate of...

-hours per week of aerobic exercise

Aerobic exercise

Aerobic exercise is physical exercise of relatively low intensity that depends primarily on the aerobic energy-generating process. Aerobic literally means "living in air", and refers to the use of oxygen to adequately meet energy demands during exercise via aerobic metabolism...

is required for visceral fat reduction.

Epicardial fat

Epicardial

Epicardial is a term used by some cardiac surgeons meaning "on the outside of the cardiac muscle". Epicardial fat or Epicardial adipose tissue is one of the most important parts of pericardium.See also epicardium....

adipose tissue (EAT) is a particular form of visceral fat deposited around the heart and found to be a metabolically active organ that generates various bioactive molecules, which might significantly affect cardiac function. A marked component composition differences has been observed in comparing EAT with subcutaneous fat, suggesting a depote specific impact of stored fatty acids on adipocyte function and metabolism.

Subcutaneous fat

Most of the remaining non-visceral fat is found just below the skin in a region called the hypodermis. This subcutaneous fat is not related to many of the classic obesity-related pathologies, such as heart disease, cancer, and stroke, and there is even some evidence to suggest that it might be protective. The typically female (or gynecoid) pattern of body fat distribution around the hips, thighs, and buttocks, is subcutaneous fat, and therefore poses less of a health risk compared to visceral fat.

Like all other fat organs, subcutaneous fat is an active part of the endocrine system, secreting the hormones leptin and resistin.

Physiology

Free fatty acid is liberated from lipoprotein

Lipoprotein

A lipoprotein is a biochemical assembly that contains both proteins and lipids water-bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are lipoproteins...

s by Lipoprotein lipase

Lipoprotein lipase

Lipoprotein lipase is a member of the lipase gene family, which includes pancreatic lipase, hepatic lipase, and endothelial lipase. It is a water soluble enzyme that hydrolyzes triglycerides in lipoproteins, such as those found in chylomicrons and very low-density lipoproteins , into two free...

(LPL) and enters the adipocyte, where it is reassembled into triglyceride

Triglyceride

A triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....

s by ester

Ester

Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

ifying it onto glycerol

Glycerol

Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

. Human fat tissue contains about 87% lipids.

In humans, lipolysis is controlled through the balanced control of lipolytic B-adrenergic receptors and a2A-adrenergic receptor-mediated antilipolysis.

Fat is not laid down when there is surplus calories available and stored passively until it is needed; rather, it is constantly being stored in and released from the adipose tissue.

Storage in the adipose tissue is catalysed by insulin, the activity of which is stimulated by high blood sugar.

Fat cells have an important physiological

Physiology

Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

role in maintaining triglyceride and free fatty acid levels, as well as determining insulin resistance

Insulin resistance

. Abdominal

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

fat has a different metabolic

Metabolism

Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

profile—being more prone to induce insulin resistance. This explains to a large degree why central obesity

Central obesity

Abdominal obesity, colloquially known as belly fat or clinically as central obesity, is the accumulation of abdominal fat resulting in an increase in waist size...

is a marker of impaired glucose tolerance and is an independent risk factor for cardiovascular disease

Cardiovascular disease

(even in the absence of diabetes mellitus

Diabetes mellitus

Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...

and hypertension

Hypertension

Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...

). Studies of female monkeys at Wake Forest University

Wake Forest University

Wake Forest University is a private, coeducational university in the U.S. state of North Carolina, founded in 1834. The university received its name from its original location in Wake Forest, north of Raleigh, North Carolina, the state capital. The Reynolda Campus, the university's main campus, is...

(2009) discovered that individuals suffering from higher stress have higher levels of visceral fat in their bodies. This suggests a possible cause-and-effect link between the two, wherein stress promotes the accumulation of visceral fat, which in turn causes hormonal and metabolic changes that contribute to heart disease and other health problems.

Recent advances in biotechnology have allowed for the harvesting of adult stem cell

Adult stem cell

Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues...

s from adipose tissue, allowing stimulation of tissue regrowth using a patient's own cells. In addition, it was reported that adipose-derived stem cells from both human and animals can be efficiently reprogrammed into induced pluripotent stem cells without the need for feeder cells. The use of a patient's own cells reduces the chance of tissue rejection and avoids the ethical issues associated with the use of human embryonic stem cell

Embryonic stem cell

Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of the blastocyst, an early-stage embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist of 50–150 cells...

s.

Adipose tissue is the greatest peripheral

Peripheral

A peripheral is a device attached to a host computer, but not part of it, and is more or less dependent on the host. It expands the host's capabilities, but does not form part of the core computer architecture....

source of aromatase

Aromatase

Aromatase is an enzyme responsible for a key step in the biosynthesis of estrogens. It is a member of the cytochrome P450 superfamily , which are monooxygenases that catalyze many reactions involved in steroidogenesis. In particular, aromatase is responsible for the aromatization of androgens into...

in both males and females, contributing to the production of estradiol

Estradiol

Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

.

Adipose derived hormones

Adipose derived hormones

Adipose tissue is an important endocrine organ that secretes numerous protein hormones, including leptin, adiponectin, and resistin. These hormones generally influence energy metabolism, which is of great interest to the understanding and treatment of type 2 diabetes and obesity.Their relative...

include:

Adiponectin
Adiponectin
Adiponectin is a protein which in humans is encoded by the ADIPOQ gene.- Structure :...
Resistin
Resistin
Resistin also known as adipose tissue-specific secretory factor or C/EBP-epsilon-regulated myeloid-specific secreted cysteine-rich protein is a cysteine-rich protein that in humans is encoded by the RETN gene....
Plasminogen activator inhibitor-1
Plasminogen activator inhibitor-1
Plasminogen activator inhibitor-1 also known as endothelial plasminogen activator inhibitor or serpin E1 is a protein that in humans is encoded by the SERPINE1 gene....

(PAI-1)
TNFα
IL-6
Interleukin 6
Interleukin-6 is a protein that in humans is encoded by the IL6 gene.IL-6 is an interleukin that acts as both a pro-inflammatory and anti-inflammatory cytokine. It is secreted by T cells and macrophages to stimulate immune response, e.g. during infection and after trauma, especially burns or other...
Leptin
Leptin
Leptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
Estradiol
Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

(E2)

Adipose tissues also secrete a type of cytokine

Cytokine

s (cell-to-cell signalling proteins) called adipokine

Adipokine

The adipokines or adipocytokines are cytokines secreted by adipose tissue.Members include:* chemerin* interleukin-6...

s (adipocytokines), which play a role in obesity-associated complications.

Brown fat

A specialised form of adipose tissue in humans, most rodents and small mammals, and some hibernating animals, is brown fat or brown adipose tissue

Brown adipose tissue

Brown adipose tissue or brown fat is one of two types of fat or adipose tissue found in mammals....

. It is located mainly around the neck and large blood vessels of the thorax

Thorax

The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

. This specialised tissue can generate heat by "uncoupling" the respiratory chain of oxidative phosphorylation

Oxidative phosphorylation

Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...

within mitochondria. The process of uncoupling means that, when protons transit down the electrochemical gradient across the inner mitochondrial membrane, the energy from this process is released as heat rather than being used to generate ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

. This thermogenic process may be vital in neonates exposed to the cold, which then require this thermogenesis to keep warm, as they are unable to shiver

Shivering

Shivering is a bodily function in response to early hypothermia in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Muscle groups around the vital organs begin to shake in small movements in an attempt to create warmth by...

, or take other actions to keep themselves warm.

Attempts to simulate this process pharmacologically

Pharmacology

Pharmacology is the branch of medicine and biology concerned with the study of drug action. More specifically, it is the study of the interactions that occur between a living organism and chemicals that affect normal or abnormal biochemical function...

have so far been unsuccessful (even lethal). Techniques to manipulate the differentiation of "brown fat" could become a mechanism for weight loss

Weight loss

Weight loss, in the context of medicine, health or physical fitness, is a reduction of the total body mass, due to a mean loss of fluid, body fat or adipose tissue and/or lean mass, namely bone mineral deposits, muscle, tendon and other connective tissue...

therapy in the future, encouraging the growth of tissue with this specialized metabolism without inducing it in other organs.

Until recently, it was thought that brown adipose tissue was primarily limited to infant

Infant

A newborn or baby is the very young offspring of a human or other mammal. A newborn is an infant who is within hours, days, or up to a few weeks from birth. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth...

s in humans, but new evidence has now overturned that belief. Metabolically active tissue with temperature responses similar to brown adipose was first reported in the neck and trunk of some human adults in 2007, and the presence of brown adipose in human adults was later verified histologically in the same anatomical regions.

Genetics

The thrifty gene hypothesis

Thrifty gene hypothesis

The thrifty gene hypothesis was proposed by geneticist James V. Neel in 1962 to resolve a fundamental problem. Diabetes is clearly a very harmful medical condition. Yet it is quite common, and it was already evident to Neel that it likely had a strong genetic basis...

(also called the Famine Hypothesis) states that in some populations the body would be more efficient at retaining fat in times of plenty, thereby endowing greater resistance to starvation in times of food scarcity. This hypothesis has been discredited by physical anthropologists, physiologists, and the original proponent of the idea

James V. Neel

James Van Gundia Neel was an American geneticist who played a key role in the development of human genetics as a field of research in the United States. He made important contributions to the emergence of genetic epidemiology and pursued an understanding of the influence of environment on genes...

himself.

In 1995 Jeffrey Friedman

Jeffrey Friedman

Jeffrey Friedman is a political scientist and the editor of Critical Review: An Interdisciplinary Journal of Politics and Society.Friedman graduated from Brown University in 1983 with a double major in History and Philosophy, and received an MA in History at the University of California, Berkeley,...

, in his residency at Rockefeller University

Rockefeller University

The Rockefeller University is a private university offering postgraduate and postdoctoral education. It has a strong concentration in the biological sciences. It is also known for producing numerous Nobel laureates...

, discovered the protein leptin

Leptin

that the genetically obese mouse lacked. Leptin is produced in the white adipose tissue and signals to the hypothalamus

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

. When leptin levels drop, the body interprets this as loss of energy, and hunger increases. Mice lacking this protein eat until they are four times their normal size.

Leptin, however, plays a different role in diet-induced obesity in rodents and humans. Because adipocytes produce leptin, leptin levels are elevated in the obese. However, hunger remains, and, when leptin levels drop due to weight loss, hunger increases. The drop of leptin is better viewed as a starvation signal than the rise of leptin as a satiety signal. However, elevated leptin in obesity is known as leptin resistance. The changes that occur in the hypothalamus

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

to result in leptin resistance in obesity are currently the focus of obesity research.

Gene defects in the leptin gene (ob) are rare in human obesity. As of July, 2010, only fourteen individuals from five families have been identified worldwide that carry a mutated ob gene (one of which was the first ever identified cause of genetic obesity in humans) - two families of Pakistani origin living in the UK, one family living in Turkey, one in Egypt, and one in Austria - and two other families have been found that carry a mutated ob receptor. Others have been identified as genetically partially deficient in leptin, and, in these individuals, leptin levels on the low end of the normal range can predict obesity.

Several mutations of genes involving the melanocortin

Melanocortin

The melanocortins are a group of peptide hormones which include adrenocorticotropic hormone and the different forms of melanocyte-stimulating hormone . They can be synthetic . In humans they can be endogenously produced from proopiomelanocortin in the pituitary gland...

s (used in brain signaling associated with appetite) and their receptors have also been identified as causing obesity in a larger portion of the population than leptin mutations.

In 2007, researchers isolated the adipose gene, which those researchers hypothesize serves to keep animals lean during times of plenty. In that study, increased adipose gene activity was associated with slimmer animals. Although its discoverers dubbed this gene the adipose gene, it is not a gene responsible for creating adipose tissue.

Physical properties

Adipose tissue has a density of ~0.9 g/ml [0.9 kg/l]. Thus, a person with much adipose tissue will float more easily than a person with a lot of muscular tissue, since muscular tissue has a density of 1.06 g/ml [1.06 kg/l].

Cultural and social role

Excess adipose tissue on a human can lead to medical problems. For a discussion of the aesthetic and medical significance of body shape, see dieting

Dieting

Dieting is the practice of eating food in a regulated fashion to achieve or maintain a controlled weight. In most cases dieting is used in combination with physical exercise to lose weight in those who are overweight or obese. Some athletes, however, follow a diet to gain weight...

and obesity

Obesity

Body fat meter

A body fat meter is a widely available tool used to measure the percentage of fat in the human body. Different meters use various methods to determine the body fat to weight ratio

Body fat percentage

A person's body mass percentage is the total weight of the person's fat divided by the person's weight and consists of essential body fat and storage body fat. Essential body fat is necessary to maintain life and reproductive functions. The percentage of essential body fat for women is greater than...

. They tend to under-read body fat percentage.

In contrast with clinical tools, one relatively inexpensive type of body fat meter uses the principle of bioelectrical impedance analysis

Bioelectrical impedance analysis

Bioelectrical impedance analysis is a commonly used method for estimating body composition. Since the advent of the first commercially available devices in the mid-1980s the method has become popular owing to its ease of use, portability of the equipment and its relatively low cost compared to...

(BIA) to determine an individual's body fat percentage. To achieve this, the meter passes a small, harmless, electric current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

through the body and measures the resistance

Ohm

The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

, then uses information on the person's weight, height, age, and sex, to calculate an approximate value for the person's body fat percentage. The calculation measures the total volume of water in the body (lean tissue and muscle contain a higher percentage of water than fat), and estimates the percentage of fat based on this information. The result can fluctuate several percent depending on what one has eaten and how much water one has consumed prior to the analysis.