Their technology could be adapted for use in livestock that produce meat, milk and eggs fortified with omega-3 fatty acids, they say. This could rival fish oil supplements.

Omega-3 fatty acids have been shown to significantly reduce the risk of heart disease and related cardiovascular events. They are also thought to play an important role in mental health, with studies suggesting that they could reduce risk of Alzheimer's disease. But Western diets tend to contain about ten times more omega-6 fatty acids than omega-3.

However producers of the most common source of omega-3 fatty acids, fish oils, face a technical challenge when adding the fishy ingredient to foods. There have also been recent scares over the safety of eating fish - in the US high consumption of certain fish can raise intake of mercury to toxic levels, say some scientists, while in Europe a study linked farmed salmon to increased cancer risk.

Researchers at Harvard Medical School describe how they transferred the fat-1 gene, which encodes an enzyme that converts omega-6 to omega-3 fatty acids, into mice. Mammals cannot usually change omega-6 fats into omega-3s.

The animals were raised on a diet rich in omega-6 and lacking omega-3 fatty acids. In a control group, tissue polyunsaturated fatty acids consist primarily of linolenic and arachidonic acids, both omega-6 fatty acids. But the engineered mice produced a balanced mix of omega-6 to omega-3 fatty acids in muscle, milk, and other tested organs, said the researchers.

In a report by Reuters Health, the team said it is now attempting to cross fat-1-producing animals with obese mice that develop diabetes to find out if this could change the disease process or development.

Several research groups have investigated changes to animal diets that could product higher quantities of beneficial oils in the resulting food products. But genetic engineering is unlikely to have a fast, easy route to approval in Europe.