Eating fish can help prevent asthma, say scientists after testing over 600 people working in a fish processing factory in a small village in South Africa.

The current theory is that the dramatic change in diet worldwide is behind the rise of the disease, researchers said.

"Around 334 million people worldwide have asthma, and about a quarter of a million people die from it every year," said Andreas Lopata, a scientist from James Cook University in Australia.

"There has been a global move from fresh fish to fast food," he said.

He said the fishing village was chosen for the testing because it had a population with high fish consumption and low socioeconomic status, so it would be likely that marine oils from fish and other seafood would be the main source of n-3, rather than supplements.

"We found that certain types of n-3 (from marine oils) were significantly associated with a decreased risk of having asthma or asthma-like symptoms by up to 62 per cent, while high n-6 consumption (from vegetable oils) was associated with an increased risk by up to 67 per cent," he said.

It was more evidence of the suspected inflammatory role of n-6 in the development of asthma, and more evidence that n-3 gave significant protection.

"Even if you factor in contaminants such as mercury found in some fish populations, the benefits of fish and seafood intake far more outweighs the potential risks," said Lopata.

Further work needed to be done on what effect specific types of n-3 have and how their beneficial role could be optimised, and on minimising the negative effects of n-6.