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.
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.
The metabolism of lipids is a closely regulated system in virtually all lifeforms. It is affected 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.
A subject of particularly close study is cholesterol, levels and types of which are influenced by the fatty acid metabolism and is known for its role in development of atherosclerosis.