Eating oily fish may reduce risk of developing rheumatoid arthritis (RA), but psychosocial work stress and smoking may increase the risk, a new study has found.
The findings, all taken from a large population-based case-control study in Sweden called EIRA (Epidemiological Investigation of Rheumatoid Arthritis), shed light on the important role of environmental and social factors in the development of RA.
For the first time, the intake of oily fish has been demonstrated to have a protective effect against the development of RA, reducing an individual's risk by 20-30pc.
Studying 1,899 subjects with a confirmed diagnosis of RA (fulfilling ACR criteria) and 2,145 controls (randomly selected and matched for age, sex and residential area), investigators concluded that the odds ratio (OR) for developing RA was 0.8 (0.7-1.0) for those who consumed oily fish 1-7 times per week or 1-3 times per month, compared with those who never, or seldom consumed oily fish. Interestingly, no significant association with RA risk was observed for consumption of fish oil supplements.
Tobacco smoking is an established risk factor for RA, but the investigators found that there is a dose dependency for the level of smoking (i.e. the number of cigarettes smoked across a given period) on the odds ratio of developing anti-citrulline (anti-CCP) positive RA.
The highest odds ratios were seen in those carrying a risk variant of the susceptibility gene PTPN22. In the study, 1,240 cases and 798 controls were identified as smokers from a total group of 1,419 cases and 1,674 controls via an extensive questionnaire regarding lifestyle factors, including smoking habits.
These subjects were then classified into three different groups according to the number of pack years smoked - less then 10, 10-20 or more than 20 pack years (where one pack-year is equivalent to having smoked one pack per day for one year) and genotyped to determine the presence of the PTPN22 risk allele.
Psychosocial stress at work, defined as low decision latitude (or low level of control) was found to be associated with a higher risk for RA. Collected via a validated questionnaire, this was demonstrated in both self-reported data and JEM (job exposure matrix)-derived data.These results were only marginally changed when the investigators adjusted the odds ratios for social class and smoking for the 1,221 cases and 1,454 controls who participated in the study.
Mrs Annmarie Wesley of the Institute of Environmental Medicine, Stockholm, Sweden, EIRA investigator and lead author of the oily fish intake study, commented: "The findings from these studies add to an increasing body of evidence to support the assertion that lifestyle modifications can have a significant impact on an individual's risk for developing RA, one of the most common autoimmune diseases, affecting approximately 1pc of adults worldwide."
"We hope that the data will contribute to the growing understanding of the aetiology of RA and, ultimately, its treatment and prevention."
The study was presented at EULAR 2008, the Annual Congress of the European League Against Rheumatism in Paris, France.