Did Nanny really know best when she thrust that disgusting dose of cod-liver oil down her reluctant charge's throat? And should modern-day nannies follow suit?

If they are after a quiet life, it could be worth trying. Cod-liver oil, and fish oil generally, contains a particular type of fat - docosahexaenoic acid, or DHA - which is otherwise found only in breast milk. Studies on hyperactive children have revealed that their levels of DHA are unusually low. They are also less likely than other children to have been breast- fed. Animal studies have shown that low DHA produces antisocial behaviour, and students have been shown to become less aggressive under stress when taking fish oil. Put these findings together, and force-feeding the stuff starts to seem quite reasonable, especially as it now comes in easy-to- swallow, fish-smell-free capsules.

In fact, fish oil is worth taking at any age because it confers an extraordinarily wide range of benefits. It protects against cardiovascular disease and helps to prevent auto-immune diseases such as diabetes, rheumatoid arthritis, psoriasis, inflammatory bowel disease, and allergies, including asthma. It helps to prevent thyroid disorder, period pains and depression, and there is even some evidence to suggest that it guards against certain cancers. The only bad effect discovered so far is that it slightly increases the risk of haemorrhage. All this medicinal benefit is thought to be conferred by DHA and one other fat, eicosapentanoic acid, or EPA. Both of them are omega-3 unsaturated fats (the name relates to the chemical structure of the molecule), and they make up about 25 per cent of the oil which can be squeezed from the flesh of cold-water fish. (Cod-liver oil contains proportionately less, but it has the bonus of vitamins A and D.) Omega-3 fats work in several ways. One effect is to damp down the inflammatory reaction of individual cells. This reduces pain and relieves symptoms of allergy and auto-immune disorders. They help prevent heart attacks by making blood more runny, lowering blood pressure and reducing the fatty sludge which gunges up arteries. They also help to stop a heart already damaged by an attack from going into a fatal spasm.

You don't need to take much omega-3 to benefit. Several large studies have shown that eating about two oily fish meals a week - or taking the equivalent in supplements - is enough to reduce the risk of a fatal heart attack by up to 50 per cent and to prevent death from all causes by 30 per cent. The best source of omega-3 oil is fresh fish. Mackerel, herrings, sardines, pilchards, salmon, anchovies and trout contain most; two small meals of any of these will set you up for the week. Tuna is good when it is fresh, dark and seared, but the pale, dry flesh you get in tins has lost most of its oil.

Other fish survive canning and freezing well, but boiling or over-cooking can take much of the goodness out.

If you can't stand fish, supplements will do, providing they are taken before the use-by date. Once the oil is removed from the fish, it goes off quite quickly.

The optimum dose of omega-3 oil is thought to be about 800mg a day. It is difficult to overdose on fish oil - some people take more than 20 times that amount without problems - but in theory it could create a risk of haemorrhage. For that reason it is best avoided if you are already taking aspirin or Warfarin for a heart condition.