Eating salmon once a week 'reduced risk of rheumatoid arthritis by half' Oily fish such as salmon or mackerel had most marked effect, study found Same benefit comes from four portions a week of lean fish - cod or plaice Eating salmon at least once a week could halve the risk of developing rheumatoid arthritis. Oily fish such as salmon or mackerel had the most marked effect, reducing the risk by 52 per cent, a study found. The same benefit comes from eating four portions a week of lean fish such as cod or plaice. Eating salmon at least once a week could halve the risk of developing rheumatoid arthritis The difference is in the consumption of omega-3 fatty acids, which are present in much higher level in oily fish. Even eating just one portion of any type of fish each week for 10 years leads to a 29 per cent cut in risk of arthritis, compared with those eating less fish. Middle-aged and older women are traditionally more at risk of rheumatoid arthritis. The Swedish study, published in the Annals of the Rheumatic Diseases, involved 32,000 women born between 1914 and 1948. They completed surveys on what they ate, in 1987 and again in 1997. During this period, 205 of the women developed rheumatoid arthritis. After adjusting for factors such as smoking habits, alcohol intake and age, researchers at the Institute of Environmental Medicine at the Karolinska Institute in Stockholm found women with consistently high daily intake in both 1987 and 1997 of omega-3 fish oils had a 52 per cent lower risk of developing the condition. Oily fish such as salmon or mackerel (pictured) had the most marked effect, reducing the risk by 52 per cent, a study found. The same benefit comes from eating four portions a week of lean fish such as cod or plaice The study attributed the benefit to the long-chain omega-3 polyunsaturated fatty acid (Pufa) content in fish. Rheumatoid arthritis, which affects around 580,000 men and women across England and Wales, is a chronic inflammatory disease that causes pain and swelling in the joints. It is thought that omega-3 fatty acids block the body's response to inflammation. Oily fish contain the highest levels of omega-3 fatty acids, which are considered essential because the body cannot make them from other sources and must obtain them through diet. The best source is fish such as mackerel, herring, salmon, sardines or trout.