Smokers with high sodium intake were more than twice as likely to develop rheumatoid arthritis as were those with low sodium intake in a nested case-control study.
The study, which lends support to the possibility that an interaction between smoking and a diet heavy in sodium can increase RA risk, involved records from 386 individuals who had provided information on their dietary habits for a long-term, population-based study a median 7.7 years before going on to develop RA. Researchers from Umeå (Sweden) University, led by Björn Sundström, Ph.D., compared these with records for 1,886 age- and sex-matched controls who did not have RA and had taken part in the same population-based study (Rheumatology 2014 Sept. 10 [doi:10.1093/rheumatology/keu330]).
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About half of the "risk from smoking in the development of RA is due to interaction with sodium intake," the investigators said.
Smokers in the highest tertile of sodium intake in the cohort (median intake was 2.15 g/day, equivalent to 5.46 g NaCl) had an odds ratio of 2.26 (95% confidence interval, 1.06-4.81; P = .036) for developing RA, compared with smokers in the lowest tertile, who had a median intake of 1.51 g/day (equivalent to 3.84 g NaCl). The significant association between high sodium intake and the development of RA was found only for smokers, not for the total study population, although smokers with low-sodium diets saw no added likelihood of developing RA, compared with nonsmokers.
Smoking is an established risk factor for RA, but in this study, “additive interaction analyses suggested that approximately half of the amount of risk from smoking in the development of RA is due to interaction with sodium intake,” Dr. Sundström and his associates wrote, with an attributable proportion of 0.54 (95% CI, 0.26-0.82). Although the researchers did not identify the possible pathways by which an interaction between smoking and high sodium intake might increase risk of RA, “these findings will provide new insights into the etiopathogenic process leading to the development of RA among smokers,” they wrote.
Their study was funded by the Swedish Research Council, the Swedish Rheumatism Association, the Swedish Controlling Chronic Inflammatory Diseases with Combined Efforts (COMBINE) program, and the Swedish royal family. None of the coauthors declared conflicts of interest.