Researchers surveyed Swedish women about their diets and found over the course of more than seven years, long-term consumption of more than one serving of fatty fish each week was tied to a lower risk of developing the condition.
"This study is the first to attribute the protective effect of fish against rheumatoid arthritis to its content of omega-3 fatty acids," Daniela Di Giuseppe, a doctoral student at the Karolinska Institute in Stockholm and lead author of the study, told Reuters Health in an email.
Rheumatoid arthritis is an autoimmune disease that causes joint inflammation, deformities and disability. People with the condition also have a higher risk of heart disease, some infections, anxiety, and depression and blood cancers like leukaemia.
According to the American College of Rheumatology, between 0.5 and 1% of the US population have rheumatoid arthritis. Women are two to three times more likely than men to develop the disease, which most commonly starts affecting people in their 60s.
Di Giuseppe and her colleagues followed over 32 000 women born between 1914 and 1948 who were part of the Swedish Mammography Cohort. Information about fish consumption was gathered from diet questionnaires sent to women in 1987 and 1997.
National registries were used to identify new diagnoses of rheumatoid arthritis between 2003 and 2010.The researchers separated women into five groups based on the amount of fish-based omega-3 fatty acids in their diet, ranging from 0.21 grams or less per day to at least half a gram daily.
Eating 0.21 grams per day of omega-3 fatty acids equates to about one serving per week of salmon and other fatty fish, or four servings per week of lean fish such as cod. During follow-up, 205 women developed rheumatoid arthritis, the researchers reported in Annals of the Rheumatic Diseases.
Long-term consumption of any fish at least once per week, compared to less than one weekly serving, was tied to a 29% lower risk of rheumatoid arthritis. However, that finding could have been due to chance, the researchers found.
Women who reported getting more than 0.21 grams of omega-3 fatty acids derived from fish per day both in 1987 and 1997 had a 52% decreased risk of developing the disease, compared to those who ate the least.
The researchers found a threshold effect, suggesting more omega-3s may not always be better. Below 0.35 grams per day, the risk of rheumatoid arthritis increased, but above it, the benefits seemed to taper off.
The results are consistent with other studies that have found threshold effects, and with recommendations from the US Department of Agriculture and the Department of Health and Human Services, which advise eating at least two servings of fish per week.
The researchers concluded that "moderate consumption of fish is sufficient to reduce risk of diseases". Genes and lifestyle may both play a role in rheumatoid arthritis. Dr Simon Helfgott, a rheumatoid arthritis researcher at Harvard Medical School in Boston, said the top three things people can do to prevent the disease are not smoke, avoid gum disease by having good oral hygiene and improve their diet.
"When we say diet there's really only one influence that seems to affect rheumatoid arthritis and that's fish consumption," Helfgott, who was not involved in the new research, told Reuters Health. Omega-3 fatty acids are used by the body to make molecules that help regulate inflammation, known as eicosanoids.
The current thinking is that eicosanoids derived from essential fatty acids in meat promote more inflammation than those from omega-3 fatty acids in fish, researchers said."This study lends credence to a strongly considered hypothesis in rheumatology circles, which is that we might be able to intervene in preventing rheumatoid arthritis in some individuals," Helfgott said.