Fats play a significant role in our diet. They are highly concentrated sources of energy and carry vitamins A, D, E and K. They also perform a not unimportant function in making food more palatable. Ideally, fats should account for only about 30 per cent of the diet although they frequently from up to 45 per cent.

The chemical composition of fats and oils is identical. We tend to think of fats as being hard and of animal origin, and oils as being liquid and of vegetable origin. In general this is true, but there are exceptions like vegetable-based hard margarines.

Most of the fats in our diet are made up of triglycerides which are broken down by digestive enzymes in the intestine into glycerol and fatty acids. Many of them are essential in maintaining the health of certain body functions.

Fatty acids are made up of chains of carbon atoms, each of which has four bonds. Two of these bonds are used to attach to the carbon atoms on either side. The other two are usually attached to hydrogen atoms, but sometimes there is a free bond which then forms a double bond between two carbon atoms. If all the spare bonds are attached to hydrogen atoms, the fatty acid is described as 'saturated'; if there is one double food bond between carbon atoms on the chain, the fatty acids is 'monounsaturated'; if there is more than one double bond, the fatty acid is 'polyunsaturated'.

All fats and oils contain all three types of fatty acids. Animal fats tend to have a higher proportion of saturated fats, while vegetable fats contain a higher proportion of polyunsaturated fats. Saturated fats have been shown to raise blood cholesterol levels to a dangerous point. Polyunsaturated and in particular monounsaturated fats seem to have the reverse effect, and are even considered by some to undo the harm done by saturated fats.