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Numerous studies show that smoking increases the risk of rheumatoid arthritis (RA) but, Rheumatology reports, it is now thought that an interaction between smoking and sodium chloride accounts for about half this risk.

Researchers from Sweden compared 386 people (who stated their dietary habits a median of 7.7 years before the onset of RA symptoms) with 1,886 matched controls.

Sodium intake did not seem to influence the risk of developing RA in the population as a whole or among non-smokers. However, smokers in the top tertile of sodium intake (median intake 2.15g daily equivalent to 5.46g sodium chloride) were more than twice as likely to develop RA as those in the lowest tertile (1.15g sodium and 3.84g salt a day).

In the highest tertile, the interaction between smoking and salt accounted for 54 per cent of the increased RA risk. The authors speculate that the results could have implications for other inflammatory diseases.