White adipose tissue

From Wikipedia, the free encyclopedia

White adipose tissue (WAT) or white fat is one of the two types of adipose tissue found in mammals (compare to brown adipose tissue). In humans, white adipose tissue composes as much as 20% of the body weight in men and 25% of the body weight in women.

Adipose tissue is specialized connective tissue that functions as the major storage site for fat in the form of triglycerides. Adipose tissue is found in mammals in two different forms: white adipose tissue and brown adipose tissue. The presence, amount, and distribution of each varies depending upon the species. Most adipose tissue is white, the focus of this review. White adipose tissue serves three functions: heat insulation, mechanical cushion, and most importantly, a source of energy. Subcutaneous adipose tissue, found directly below the skin, is an especially important heat insulator in the body, because it conducts heat only one third as readily as other tissues. The degree of insulation is dependent upon the thickness of this fat layer. For example, a person with a 2-mm layer of subcutaneous fat will feel as comfortable at 15°C as a person with a 1-mm layer at 16°C. Adipose tissue also surrounds internal organs and provides some protection for these organs from jarring. As the major form of energy storage, fat provides a buffer for energy imbalances when energy intake is not equal to energy output. It is an efficient way to store excess energy, because it is stored with very little water. Consequently, more energy can be derived per gram of fat (9 kcal.gm-1) than per gram of carbohydrate (4 kcal.gm-1) or protein (4 kcal.gm-1). In addition, if terrestrial animals stored their excess energy as carbohydrate, the increased mass would interfere with mobility. The average woman with 20% body fat has about one month of energy stored as fat.

There are some constraints on the use of fat as fuel. Most animals cannot convert lipid into carbohydrate. Tissues that function predominantly anaerobically (e.g.. erythrocytes) must rely on carbohydrate for energy and need to have an ample supply available. Additionally, under normal conditions the brain is dependent upon glucose for energy and does not use fatty acids. In unusual metabolic circumstances, the brain can use ketone bodies (a by-product of incomplete fat metabolism) when they are present in sufficiently high quantities. Finally, a typical diet contains a high proportion of carbohydrate, and the transport of insoluble lipids through blood requires a specific mechanism, so it may be "easier" metabolically for tissues to use glucose under typical dietary conditions.

Albright, A.L. and Stern, J.S. (1998). Adipose tissue. In: Encyclopedia of Sports Medicine and Science, T.D.Fahey (Editor). Internet Society for Sport Science: http://sportsci.org. 30 May 1998.