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Body Fat

What it is, part 2

Physiology

Free fatty acid is "liberated" from lipoproteins by lipoprotein lipase (LPL) and enters the adipocyte, where it is reassembled into triglycerides by esterifying it onto glycerol. Human fat tissue contains about 87% lipids.

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

Fat is not laid down when there is a surplus available and stored passively until it is needed; rather it is constantly being stored in and released from each cell.

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 profileóbeing 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 (even in the absence of diabetes mellitus and hypertension).

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. 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 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
  • Angiotensin
  • Plasminogen activator inhibitor-1 (PAI-1)
  • TNFα
  • IL-6
  • Leptin
  • Estradiol (E2) 

Adipose tissues also secrete a type of cytokines (cell-to-cell signaling proteins) called adipokines (adipocytokines) which play a role in obesity-associated complications.

Brown fat

A specialized form of adipose tissue in human infants, and some 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, leading to the breakdown of fatty acids. This thermogenic process may be vital in neonates exposed to the cold, who then require this thermogenesis to keep warm as they are unable to shiver, or take other actions to keep themselves warm.

Attempts to stimulate this process pharmacologically have so far been unsuccessful, but might in the future be a target of weight loss therapy.

Genetics

In 2007, researchers isolated the adipose gene, which ap≠par≠ently serves to keep animals lean dur≠ing times of plen≠ty. Increased adipose gene activity was associated with slimmer individuals.

Physical properties

Adipose tissue has a density of ~0.9g/ml. Thus, a person with much adipose tissue will float easier than a person with lot of muscular tissue, since muscular tissue has a density of 1.06 g/ml.

Cultural and social role

Excess adipose tissue on a human can lead to medical problems; however, a round or large figure does not of itself imply a medical problem, and is sometimes not primarily caused by adipose tissue. For a discussion of the aesthetic and medical significance of body shape, see dieting and obesity.

The term "adipose" was also used as the name of a monster in a 2008 episode of the British science fiction series Doctor Who , Partners in Crime. Aliens called "the Adipose" are part of a plan involving diet pills, hence the link with fat tissue.