Fats Explained

Explanation of terms

Fat is a substance which contains one or more fatty acids and is the principal form in which the body stores energy. It is also used as an insulating material both just beneath the skin and around some of the internal organs. Fat is essential in the diet to supply an adequate amount of essential fatty acids and for the absorption of the fat-soluble vitamins: A, D, E and K.

The chemical and physical properties of a fat are determined by the relative amounts of the various fatty acids of which it is composed. Generally, the more saturated the fatty acid content, the harder the fat will be at room temperature; the more unsaturated its content, the runnier it will be. All fats are a mixture of different fatty acids.

Fatty Acids

All fats are composed of fatty acids, of which there are dozens in nature. A fatty acid is an organic acid having a chain of carbon atoms coupled to hydrogen atoms at the side and a carboxyl radicle at one end. The carbon atoms are joined together with mainly a single bond plus a number of 'double bonds'. It is the difference between these that differentiates the various fatty acids.

Saturated fatty acids have no double bonds and this makes them stable.

  H H H H H H H H H H H H H H H H H
  | | | | | | | | | | | | | | | | |
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-COOH
  | | | | | | | | | | | | | | | | |
  H H H H H H H H H H H H H H H H H

The diagram above is of Stearic acid . It has 18 carbon atoms and no double bonds. It is designated 18:0 . Because all the carbon atoms are surrounded by hydrogen atoms, it is called saturated . As the hydrogen atoms are all close together, it is difficult to bend. It is this resistance to bending that makes most saturated fats solid at room temperature.

Unsaturated fatty acids are those where two or more of their carbon atoms are connected with 'double bonds'.

Mono- unsaturated fatty acids are unsaturated fatty acids with one double bond.

  H H H H H H H H H H H H H H H H H
  | | | | | | | | | | | | | | | | |
H-C-C-C-C-C-C-C-C-C=C-C-C-C-C-C-C-C-COOH
  | | | | | | | |     | | | | | | |
  H H H H H H H H     H H H H H H H

Here we have another 18 carbon fatty acid: Oleic acid , the major fatty acid in olive oil. This has one ' double bond ' in the middle. Fatty acids with one double bond are called mono -unsaturated. It is designated by the term 18:1 . Because of the double bond this fatty acid can bend and is liquid at room temperature. Oleic acid is also the most abundant fatty acid in animal fats and in human fat.

Poly- unsaturated fatty acids are those with two or more double bonds.

  H H H H H H H H H H H H H H H H H
  | | | | | | | | | | | | | | | | |
H-C-C-C-C-C-C=C-C-C=C-C-C-C-C-C-C-C-COOH
  | | | | |     |     | | | | | | |
  H H H H H     H     H H H H H H H

The one above is Linoleic acid , the most abundant fatty acid in vegetable seed oils such as sunflower, safflower, soya and corn oils. This has two double bonds, the first after the sixth carbon atom. This makes it an Omega-6 fatty acid. If the first double bond follows the third carbon atom, the fatty acid is an Omega-3 fatty acid. The shorthand for linoleic acid is 18:2.

Some fatty acids come in two configurations: Omega-3 and Omega-6. Linolenic acid , an 18-carbon molecule with three double bonds, is an example. Alpha-linolenic acid is Omega-3 , while Gamma-linolenic acid is Omega-6 .

All polyunsaturated fatty acids are liquid at room temperature.

The unsaturated fatty acids are not stable; they react gradually with oxygen in the air and become rancid. This oxygenation process produces molecules known as free radicals which are known to have undesirable effects on the body. The more unsaturated a fatty acid is, the more liable it is to oxidation. This is why polyunsaturated margarines must be kept refrigerated.

Cis and trans bonds

Generally, in Nature, fatty acids' double bonds are as shown above with the single hydrogen atoms at a double bond on the same side of the molecule. This is called a cis configuration. However, in the process of margarine manufacture, liquid oils are not much good. They have to be made more solid. The process which does this is called hydrogenation . During hydrogenation, some of the double bonds are twisted so that the hydrogen atoms lie on opposite sides, as in the diagram below. This configuration is called trans.

  H H H H H   H H   H H H H H H H H
  | | | | |   | |   | | | | | | | |
H-C-C-C-C-C-C=C-C-C=C-C-C-C-C-C-C-C-COOH
  | | | | | |   | |   | | | | | | |
  H H H H H H   H H   H H H H H H H

(In the diagram of linoleic acid above, both double bonds are shown as trans bonds.)

Fats are attacked by oxygen. Just like iron left out in the rain, fats oxydise (rust). But oxygen can only attack fats where there are double bonds. This is why saturated fats don't spoil but polyunsaturated margarines must be kept in the fridge. And the more double bonds a fatty acid has, the more it oxydises and the more ' free radicals ' it throws off.

Fats and fatty acids

All fats contain a mixture of different fatty acids, both saturated and unsaturated. The fatty acid content of some typical foods are tabled below. The total percentages are less than 100% because of the glycerol and other compounds that are present. Note that, although animal fats are generally thought of as being saturated fats, most are less than fifty percent saturated.

Last updated 24 March 2001