Eating one portion of fatty fish every week, or four of lean fish, could halve the risk of developing rheumatoid arthritis, according to a study published in the journal Annals of the Rheumatic Diseases.

Researchers from the Karolinska Institute in Sweden sent a questionnaire to all women between 1987 and 1990 who were a part of The Swedish Mammography Cohort Study and born between 1914 and 1948.

The questionnaires requested information on their diet, height, weight, parity and educational level.

In 1997, a follow-up questionnaire was sent to 56,030 women still alive requesting the same information, but with additional information on smoking history, physical activity, and use of dietary supplements and aspirin.

As part of the dietary information request, the women were asked to complete food frequency questionnaires about how often they ate a choice of 67 foods in 1987, and 96 foods in 1997. These choices included a variety of lean and fatty fish.