Fats or lipids are the most concentrated form of all energy giving foods. They constitute an important part of our daily diet and supply about 10-30% of our energy needs.
Like carbohydrates, fats are composed of organic compounds of carbon, hydrogen and oxygen. Fats have a much smaller proportion of oxygen than do carbohydrates and differ mostly in their structure and properties. Some fats also contain phosphorus and nitrogen compounds. If the substance is liquid at 20°C, it is called oil, if it is solid at that temperature, it is known as fat. Pure fat is composed of a molecule of glycerol and three molecules of fatty acids. The hydrogen atoms of the acid are replaced and the new substance is called ester. As three fatty acid molecules take part in the reactions, such ester is also known as triglycerides.
A glycerol or glycerin is a thick syrupy, sweetish liquid soluble in water which can react with a fatty acid.
A fatty acid is an organic acid containing one replaceable hydrogen atom and having a general formula RCOOH which R represents hydrogen or a group of carbon and hydrogen atoms.
The nature of the fat or oil depends on the kind of fatty acids linked to the glycerol. The hardness, melting point and flavor of fat is related to the length of carbon chain and the level of saturation of the fatty acids.
Fats or lipids may be classified as follows:
1. Simple lipids:
Include fatty acid, fats which are esters of fatty acids with glycerol (triglycerides) and waxes which are esters of fatty acids with long chain monohydroxy alcohols.
2. Compound lipids:
These are esters of glycerol and fatty acids, with substitution of other components such as carbohydrate, phosphate and / or nitrogenous groupings phospholipids such as lecithin and cephalin which are generally found in tissues of the blood. The tissues of the nervous system and the liver are exceptionally rich in phospholipids. They help in emulsifying fats which makes their digestion much easier. Glycolipids such as the cerebrosides contain a molecule of glucose or galactose. Lipoprotein includes a variety of lipid molecules bound to protein molecules in order to facilitate transport in the aqueous medium of the blood.
3. Derived lipids:
This group includes fatty acids, alcohols, carotenoids and the fat-soluble vitamins A, D, E, and K. Fatty acids may be saturated or unsaturated in nature. If the combining power of the carbon atom (Valency=4) in an acid is satisfied, it is a saturated acid for example stearic acid.
The presence of one or more than double or triple bonds indicates that the valencies of the carbon atom are not satisfied. Such an acid is known as an unsaturated acid, for example Oleic acid.
Like amino acids, there are certain fatty acids which are essential for our body, but cannot be synthesized by them. So, our diet should provide these essential fatty acids to the body. Examples are Linoleic and Linolenic acids.
Cholesterol, an alcohol lipid or sterol is synthesized by the body in the liver. It is necessary for the formation of many essential substances such as steroid hormones and bile salts. It is also an integral part of all membrances of the body including myelin, which forms a protective sheath around nerve fibers.