Polyunsaturated fats are also categorised according to where the double bonds occur along the carbon chain. If the first double bond occurs on the third carbon from the methyl end of the carbon chain, the fatty acid is called an Omega-3 fatty acid (also written as n-3). These polyunsaturated fatty acids occur in high concentrations in seafoods—especially oily fish, some seeds and breast milk. These have become popular lately as healthy oils which help prevent heart disease. There is also now some evidence that they may not be converted to body fat as readily as other forms of dietary fat, although the research in this area is still in its early days. If the first double bond occurs on the sixth carbon from the methyl end of the chain, it is called an Omega-6 fatty acid (also written as n-6). The polyunsaturated fatty acids in the vegetable oils mentioned earlier, and also in margarines made from these oils, are the Omega-6 type.

There are specific rules for describing fatty acids, according to their structure and the position of their double bonds. In the last example above, the double bonds occur on the sixth and ninth carbon atoms from the methyl end of the carbon chain. It would be written as C18: 2n-6 because it has 18 carbons (C18), two double bonds (:2) and the first double bond occurs on the sixth carbon from the end (n-6).

Qmega-3s and Omega-6s are often in the news because nutritionists now believe they need to be present in the diet in the right balance. (Some of the products which come from the Omega-3 and Omega-6 fatty acids oppose each other in such complex processes as blood clotting and inflammation. This opposition keeps the balance between, for example, the blood clotting and blood thinning mechanisms.)

In many modem diets they are not well balanced, with many more Omega-6s than Omega-3s being eaten. The two major competitors are an Omega-6 fatty acid called linoleic acid (C18:2n-6) and an Omega-3 called Underlie acid (C18:3n-3): both are essential for humans. linoleic add definitely has advantages over the saturated fats for blood cholesterol, but too much of it will not only add to total fat intake, but may upset the balance between Omega-6 and Omega-3- Rich sources of linoleic acid are vegetable oils and margarines. Rich sources of linolenic acid are fish and shellfish. Oleic acid (C18:n-9), the monounsaturated fatty acid found in olive and canola oils, has its only double bond on the 9th carbon from the methyl end of the chain. It does not compete with the other two essential fatty acids and is generally regarded as an all-round good guy in its effect on cholesterol and other metabolic processes, but it does add to total fat intake.

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