Cod liver oil is, as the name suggests, extracted from the livers of cod. Its claim to health-giving properties lies in the fact that it's rich in an essential fatty acid called omega-3. In fact, it contains two types of omega-3 - eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). It's also a rich source of vitamins A and D.

Essential fatty acids

The body needs fatty acids for many normal activities, including skin repair, nerve and immune system function. They also help to carry fat around the body and in the manufacture of prostaglandins.

Essential fatty acids (EFAs) are those we must get from our diet because they're not made by the body. They're vital in the formation of cell walls, and are needed whenever the body is building or restoring tissues.

They also play a part in allowing nutrients and other chemicals to pass in and out of cells.

There are two types of EFA: omega-3 and omega-6. We need both, but what seems to matter most is getting the right balance.

Humans evolved on a diet containing roughly equal amounts of omega-3 and omega-6. Now, however, we consume 20 or 30 times more omega-6, partly because of the increased amount of certain vegetable oils in our diet.

An excess of omega-6 alters the body's physiology to a state where the blood is thicker, the blood vessels more likely to go into spasm and clots more likely to form. This increases the risk of heart attack and other forms of cardiovascular disease. Omega-3 can counteract this, but you need to get enough.

Sources in the diet

Omega-3 is derived from linolenic acid. Good sources include soya beans and rapeseed oil. Oily fish, such as sardines, herring, mackerel, trout and salmon, are particularly rich in omega-3.

We need about 1g to 2g of omega-3 a day, which can be provided by 100g of herring or a handful of walnuts. The Department of Health recommends eating two portions of oily fish a week.

Omega-6 is derived from linoleic acid, which is found in many vegetable oils, especially sunflower, olive and corn oils. An adult needs about 4g a day, the equivalent of two teaspoons of sunflower oil or a handful of almonds or walnuts.

It's possible to get enough EFAs through the diet. Of course, that's not easy if children are choosy about what they eat. If you want to be absolutely certain your son is getting enough, you may want to consider giving him supplements.

Extra health benefits

Recently, EFAs have been promoted for the treatment of a wide range of conditions.

Omega-3 may help to treat:

• Heart disease
• Arthritis
• Psoriasis

Extra omega-6 EFAs may help to protect against:

• Inflammatory conditions
• Breast pain
• Menopausal symptoms
• Premenstrual syndrome

Omega-3 and the brain

There's a long-held belief that fish oil is good for growing brains and it seems there may be some truth in that. Certainly, omega-3 is important for nerve function.

More specific research has shown that children low in omega-3 are more likely to be hyperactive, have learning disorders and display behavioural problems. Omega-3 deficiency has also been linked to dyslexia, violence, depression and memory problems.

Some people have found omega-3 supplements help children with dyslexia, attention deficit disorder and similar conditions.

This article was last medically reviewed by Dr Trisha Macnair in December 2007