From Academic Kids
In biochemistry, fat is a generic term for a class of lipids. Fats are produced by organic processes in animals and plants. All fats are insoluble in water and have a density significantly below that of water (i.e. they float on water.) Fats that are liquid at room temperature are often referred to as oil.
Fats in nutrition
Fat is one of the three main classes of food and, at approximately 38 kJ (9 kilocalories) per gram, as compared to sugar with 17 kJ (4 kcal) per gram or ethanol with 29 kJ (7 kcal) per gram, the most concentrated form of metabolic energy available to humans. (Note that 1 kcal = 1 "Calorie", capitalised in nutrition-related contexts.) Vitamins A, D, E, and K are fat-soluble meaning they can only be digested, absorbed, and transported in conjunction with fats. Fats are sources of essential fatty acids, an important dietary requirement.
They also serve as energy stores for the body. In food, there are two types of fats: saturated and unsaturated. Fats are broken down in the body to release glycerol and free fatty acids. The glycerol can be converted to glucose by the liver and thus used as a source of energy. The fatty acids are a good source of energy for many tissues, especially heart and skeletal muscle.
The biological imperative
All varieties of fat have an extraordinary energy content. In animals, fat acts as an energy reserve, and is stored in fatty tissue, normally located subcutaneously or surrounding organs. Fatty tissue consist of fat cells, designed to store energy in the form of fat.
Energy is stored as fatty tissue when the nutrition/energy content of the blood remains higher than is consumed by muscular and other activity. When the energy content in the blood lessens, the fatty tissue reacts by releasing a corresponding amount of energy from the fat cells. This activity is controlled by insulin and other hormones in the body.
Adipose, or fatty, tissue is the human body's means of storing metabolic energy over extended periods of time. The location of the tissue determines its metabolic profile: "visceral fat" (around the abdomen) is prone to lead to insulin resistance, while "peripheral fat" (around the limbs) is much more harmless.
See the main article on fatty acid metabolism
The metabolism of lipids is a closely regulated system in virtually all lifeforms. It is effected by a variety of enzymes and, in higher organisms, regulated by hormones. Research is ongoing on the relative influence of various hormonal regulators on the anabolism (production) and catabolism (breakdown, also termed lipolysis) of fatty molecules.
- Chemical Structure of Fats and Fatty acids (http://www.scientificpsychic.com/fitness/fattyacids.html)ca:Greix