In biology, adipose tissue or body fat or just fat is loose connective tissue composed
mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular
fraction of cells including preadipocytes, fibroblasts, vascular endothelial cells and
a variety of immune cells). Adipose tissue is derived from preadipocytes. Its main role
is to store energy in the form of lipids, although it also cushions and insulates the
body. Far from hormonally inert, adipose tissue has, in recent years, been recognized as a
major endocrine organ, as it produces hormones such as leptin, estrogen, resistin, and the
cytokine TNFα. Moreover, adipose tissue can affect other organ systems of the body and
may lead to disease. Obesity or being overweight in humans and most animals does not depend
on body weight, but on the amount of body fat—to be specific, adipose tissue. The
two types of adipose tissue are white adipose tissue and brown adipose tissue. The formation
of adipose tissue appears to be controlled in part by the adipose gene. Adipose tissue –
more specifically brown adipose tissue – was first identified by the Swiss naturalist
Conrad Gessner in 1551.
Anatomical features In humans, adipose tissue is located beneath
the skin, around internal organs, in bone marrow and in the breast tissue. Adipose tissue
is found in specific locations, which are referred to as adipose depots. Apart from
adipocytes, which comprise the highest percentage of cells within adipose tissue, other cell
types are present, collectively termed stromal vascular fraction of cells. SVF includes preadipocytes,
fibroblasts, adipose tissue macrophages, and endothelial cells. Adipose tissue contains
many small blood vessels. In the integumentary system, 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 and to protect it from excess
glucose by storing triglycerides produced by the liver from sugars, although some evidence
suggests that most lipid synthesis from carbohydrates occurs in the adipose tissue itself. 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
Mice have eight major adipose depots, four of which are within the abdominal cavity.
The paired gonadal depots are attached to the uterus and ovaries in females and the
epididymis and testes in males; the paired retroperitoneal depots are found along the
dorsal 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 intestines and the omental
depot 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 and the subscapular depots, paired medial mixtures
of brown adipose tissue adjacent to regions of white adipose tissue, which are found under
the 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 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 muscles behind the knees, each containing one large
lymph node. 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 an obese person, excess adipose tissue
hanging downward from the abdomen is referred to as a panniculus. A panniculus complicates
surgery of the morbidly obese individual. It may remain as a literal "apron of skin"
if a severely obese person quickly loses large amounts of fat. 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 is located inside the abdominal cavity, packed between the organs.
Visceral fat is different from subcutaneous fat underneath the skin, and intramuscular
fat interspersed in skeletal muscles. Fat in the lower body, as in thighs and buttocks,
is subcutaneous and is not consistently spaced tissue, whereas fat in the abdomen is mostly
visceral and semi-fluid. Visceral fat is composed of several adipose depots, including mesenteric,
epididymal white adipose tissue, and perirenal depots. Visceral fat is considered adipose
tissue whereas subcutaneous fat is not considered as such.
An excess of visceral fat is known as central obesity, or "belly fat", in which the abdomen
protrudes excessively. Excess visceral fat is also linked to type 2 diabetes, insulin
resistance, inflammatory diseases, and other obesity-related diseases.
Men are more likely to have fat stored in the belly due to sex hormone differences.
Female sex hormone causes fat to be stored in the buttocks, thighs, and hips in women.
When women reach menopause and the estrogen produced by the ovaries declines, fat migrates
from the buttocks, hips and thighs to the waist; later fat is stored in the abdomen.
High-intensity exercise is one way to effectively reduce total abdominal fat. One study suggests
at least 10 MET-hours per week of aerobic exercise is required for visceral fat reduction.
Epicardial fat Epicardial adipose tissue 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.
Marked component differences have been observed in comparing EAT with subcutaneous fat, suggesting
a depot specific impact of stored fatty acids on adipocyte function and metabolism.
Subcutaneous fat
Most of the remaining nonvisceral 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 some evidence even suggests it might be
protective. The typically female 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.
The relationship between the subcutaneous adipose layer and total body fat in a person
is often modelled by using regression equations. The most popular of these equations was formed
by Durnin and Wormersley, who rigorously tested many types of skinfold, and, as a result,
created two formulae to calculate the body density of both men and women. These equations
present an inverse correlation between skinfolds and body density—as the sum of skinfolds
increases, the body density decreases. Factors such as sex, age, population size
or other variables may make the equations invalid and unusable, and, as of 2012, Durnin
and Wormersley's equations remain only estimates of a person's true level of fatness. New formulae
are still being created.
Ectopic fat Ectopic fat is the storage of triglycerides
in tissues other than adipose tissue, that are supposed to contain only small amounts
of fat, such as the liver, skeletal muscle, heart, and pancreas. This can interfere with
cellular functions and hence organ function and is associated with insulin resistance
in type-2 diabetes. It is stored in relatively high amounts around the organs of the abdominal
cavity, but is not to be confused as visceral fat.
The specific cause for the accumulation of ectopic fat is unknown. The cause is likely
a combination of genetic, environmental, and behavioral factors that are involved in excess
energy intake and decreased physical activity. Substantial weight loss can reduce ectopic
fat stores in all organs and this is associated with an improvement of the function of that
organ. Physiology
Free fatty acids are liberated from lipoproteins by lipoprotein lipase and enter the adipocyte,
where they are reassembled into triglycerides by esterifying it onto glycerol. Human fat
tissue contains about 87% lipids. There is a constant flux of FFA entering and
leaving adipose tissue. The net direction of this flux is controlled by insulin and
leptin—if insulin is elevated there is a net inward flux of FFA, and only when insulin
is low can FFA leave adipose tissue. Insulin secretion is stimulated by high blood sugar,
which results from consuming carbohydrates. In humans, lipolysis is controlled through
the balanced control of lipolytic B-adrenergic receptors and a2A-adrenergic receptor-mediated
antilipolysis. Fat cells have an important physiological
role in maintaining triglyceride and free fatty acid levels, as well as determining
insulin resistance. Abdominal fat has a different metabolic profile—being more prone to induce
insulin resistance. This explains to a large degree why central obesity is a marker of
impaired glucose tolerance and is an independent risk factor for cardiovascular disease. Studies
of female monkeys at Wake Forest University 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 cells from adipose tissue, allowing stimulation of tissue
regrowth using a patient's own cells. In addition, adipose-derived stem cells from both human
and animals reportedly 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 ethical issues associated with the use of human embryonic
stem cells. Adipose tissue is the greatest peripheral
source of aromatase in both males and females, contributing to the production of estradiol.
Adipose derived hormones include: Adiponectin
Resistin Plasminogen activator inhibitor-1
TNFα IL-6
Leptin Estradiol
Adipose tissues also secrete a type of cytokines called adipokines, which play a role in obesity-associated
complications. Perivascular adipose tissue releases adipokines such as adiponectin that
affect the contractile function of the vessels that they surround.
Brown fat
A specialized form of adipose tissue in humans, most rodents and small mammals, and some hibernating
animals, is brown fat or brown adipose tissue. It is located mainly around the neck and large
blood vessels of the thorax. This specialized tissue can generate heat by "uncoupling" the
respiratory chain of oxidative phosphorylation 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.
This thermogenic process may be vital in neonates exposed to cold, which then require this thermogenesis
to keep warm, as they are unable to shiver, or take other actions to keep themselves warm.
Attempts to simulate this process pharmacologically have so far been unsuccessful. Techniques
to manipulate the differentiation of "brown fat" could become a mechanism for weight loss
therapy in the future, encouraging the growth of tissue with this specialized metabolism
without inducing it in other organs. Until recently, brown adipose tissue was thought
to be primarily limited to infants 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 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,
originally advanced in the context of glucose metabolism and insulin resistance, has been
discredited by physical anthropologists, physiologists, and the original proponent of the idea himself
with respect to that context, although according to its developer it remains "as viable as
when [it was] first advanced" in other contexts. In 1995, Jeffrey Friedman, in his residency
at the Rockefeller University, together with Rudolph Leibel, Douglas Coleman et al. discovered
the protein leptin that the genetically obese mouse lacked. Leptin is produced in the white
adipose tissue and signals to the hypothalamus. When leptin levels drop, the body interprets
this as a 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 to result in
leptin resistance in obesity are currently the focus of obesity research.
Gene defects in the leptin gene are rare in human obesity. As of July, 2010, only 14 individuals
from five families have been identified worldwide who carry a mutated ob gene—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 melanocortins 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.
Thus, a person with more adipose tissue will float more easily than a person of the same
weight with more muscular tissue, since muscular tissue has a density of 1.06 g/ml.
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.
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
in order to determine an individual's body fat percentage. To achieve this, the meter
passes a small, harmless, electric current through the body and measures the resistance,
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, and estimates the percentage of fat based on this information. The result
can fluctuate several percentage points depending on what has been eaten and how much water
has been drunk before the analysis. Additional images
See also Adipose differentiation-related protein
Adiposopathy Apelin
Bioelectrical impedance analysis – a method to measure body fat percentage.
Blubber – an extra thick form of adipose tissue found in some marine mammals.
Body fat percentage Cellulite
Human fat used as pharmaceutical in traditional medicine
Obesity Starvation
Steatosis Stem cells
Subcutaneous fat Bariatrics
Classification of obesity Classification of childhood obesity
EPODE International Network The Worlds largest Obesity Prevention Network
World Fit A program of the United States Olympic Committee, and the United States Olympians
and Paralympians Association Obesity and walking
Social stigma of obesity References
Further reading MeSH A10.165.114
Stock, M. J.; Cinti, S.. "ADIPOSE TISSUE | Structure and Function of Brown Adipose Tissue". Encyclopedia
of Food Sciences and Nutrition. p. 29. doi:10.101600008-0. ISBN 9780122270550.
Vernon, R. G.; Flint, D. J.. "ADIPOSE TISSUE | Structure and Function of White Adipose
Tissue". Encyclopedia of Food Sciences and Nutrition. p. 23. doi:10.101600007-9. ISBN 9780122270550.