Oils and fats recognised as essential nutrients in both human and animal diets. They provide the most concentrated source of energy of any foodstuff, supply essential fatty acids (which are precursors for important hormones, the prostaglandins), contribute greatly to the feeling of satiety after eating, are carriers for fat soluble vitamins, and serve to make foods more palatable.
Fats and oils are present in varying amounts in many foods. The principal sources of fat in the diet are meats, dairy products, poultry, fish, nuts, and vegetable oils and fats.
The common chemical characteristic of such oils and fats is that they are composed by units commonly called "triglycerides" resulting from the combination of one unit of glycerol and three units of fatty acids.
The common physical properties of such oils and fats are that they float on water but are not soluble in it; they are greasy to the touch, and have lubricating properties; they are not readily volatile; and may be burned without leaving any residue, i.e., ash.
The distinction between a fat and an oil is purely an accidental one, depending upon the environment in which the substance happens to be placed. If the substance is solid at ordinary temperatures, it is termed a fat; if fluid, an oil. This is merely a distinction of convenience, since all oils are solidified at lower temperatures and all fats melted at higher temperatures. In each climate, however, the distinction is of importance in industrial and in culinary uses; it has also some importance in nutrition, since fats are somewhat less digestible than oils.