All fats are made up of building blocks called fatty acids. These differ in their chemical composition. They are referred to as saturated, monounsaturated, or polyunsaturated fatty acids - saturates, monounsaturates, or polyunsaturates for short.

Fatty acids are made up of long chains of carbon atoms joined together by chemical bonds. Each carbon atom has four bonds and when they are all linked to other atoms by single bonds the fatty acid is said to be saturated. When the fatty acid contains one or more double bonds, it is said to be unsaturated.

A fatty acid with one double bond is referred to as monounsaturated and when there is more than one double bond the fatty acid is polyunsaturated.

Fatty foods contain different mixtures of these fatty acids, but often one type predominates. Foods high in saturates include cream, butter, full fat cheeses, fatty meats, pastry, cakes and biscuits.

Polyunsaturates also divide into two families - the n-6 (omega 6) family, found in vegetable oils such as corn and sunflower oils, and the n-3 (omega 3) family, found in some nuts and seeds and their oils, such as soya and walnuts, and in oily fish such as salmon, sardines, trout and mackerel. Some polyunsaturates are essential in the diet, as the body cannot synthesise them – they are known as Essential Fatty Acids.

How important are polyunsaturates?

Whereas a high saturate intake is associated with elevation of blood LDL cholesterol levels, the n-6 polyunsaturates lower blood cholesterol (both LDL and HDL). The n-3 polyunsaturates seem not to affect cholesterol levels either way. The monounsaturates have a small cholesterol-lowering effect but importantly they only lower LDL cholesterol and do not affect the level of the ‘protective’ HDL cholesterol.

The polyunsaturates in oily fish have also been shown to reduce heart disease risk, as they influence the clotting process and appear to protect against thrombosis. It is therefore recommended that these types of fish are eaten once or twice a week.*

*Nutritional Aspects of Cardiovascular Disease. Report of the Cardiovascular Review Group Committee on Medical Aspects of Food Policy. Report on Health and Social Subjects No 46. DH: HMSO 1994.