Scientists have discovered why a diet high in oily fish like salmon and mackerel may help improve inflammatory conditions such as arthritis.

They have found a key anti-inflammatory fat in humans is derived from a fatty acid found in fish oil.

The researchers, from Harvard Medical School and Brigham and Women's Hospital, found the diet worked best when combined with low aspirin doses.

Details are published in the Journal of Experimental Medicine.

The inflammatory response protects the body against infection and injury, but when it goes wrong it can lead to conditions such as arthritis, and cardiovascular disease.

The Harvard team identified a new class of fats in the human body, called resolvins, which they showed can control inflammation.

They do this both by stopping the migration of inflammatory cells to sites of inflammation, and the turning on of other inflammatory cells.

Resolvins are made from the omega-3 fatty acids, found in high concentration in oily fish.

Their production also appears to be stimulated by taking aspirin.

One form of resolvin - E1 - is thought to play a particularly significant role in controlling inflammation.

The researchers identified this specific fat in blood plasma samples taken from volunteers given omega-3 fatty acids and aspirin.

Lead researcher Dr Charles Serhan told the BBC News: "Since we obtained these results I started to encourage my own children to eat foods rich in omega-3 fatty acids."

Drug concern

The study also reveals a potential pitfall of COX-2 inhibitors, drugs designed to block inflammation, which have been shown to have negative cardiovascular side effects.

The enzyme COX-2 is involved in making resolvin E1, and the researchers suspect that taking the drugs may disrupt the body's ability to synthesise the fat.

Thus, while the drugs are designed to reduce inflammation, it is possible that they actually undermine one of the body's most important methods for achieving the same effect.

However, the researchers stress the experiment to prove this idea has yet to be done with humans.

A spokesperson for the Arthritis Research Campaign said: "A number of studies have suggested that oily fish can help people with inflammatory arthritis, and this new study appears to confirm this.

"Our charity has also funded a small preliminary study which shows that oily fish or fish oil capsules can also play a part in reducing the pain and inflammation in osteoarthritis - a degenerative type of arthritis.

"We always suggest that people with arthritis eat two or three portions of oily fish a week, or take 1,500mg fish oil capsules a day."