Smokers with high sodium intake face more than two-fold risk of developing from rheumatoid arthritis (RA), according to a new study by the Umeå University, Sweden.

A recent study on animals found an association between high sodium intake and RA. The Swedish researchers conducted this study to determine whether animal and human cell studies will trigger similar results.

For the current study on humans, the researchers analysed data from the Västerbotten Intervention Programme (VIP) - comprising of 386 individuals. All the participants reported their dietary habits an average of 7.7 years before the onset of RA symptoms. The results were compared to that of a group of 1886 matched controls from the same database.

As part of the VIP Programme, health-screening data (dietary habits, physical exercise, smoking, and other social factors) and blood samples were collected.

The researchers found that the risk for RA doubled in smokers with high sodium intake.

"Additive interaction analyses suggested that approximately half (54%) of the increased risk from smoking in the development of RA is due to interaction with sodium intake. A large influence of sodium intake on smoking as a risk factor for RA is also supported by the fact that we could not identify any significant proportion of risk from smoking in individuals with a low sodium intake," said Researcher Björn Sundström from the Department of Public Health and Clinical Medicine, Rheumatology, in a press release.

"These findings will provide new insights into the aetiopathogenic process leading to the development of RA among smokers. The finding of sodium being a risk factor for the development of RA among smokers is intriguing, as it may explain discrepancies in previous studies of diet as a risk factor for RA. These results could have implications for analyses of diet in other conditions in which inflammation is of importance."

The researchers said that fruit and vegetables are associated with a lower risk of developing RA, while protein, red meat, and fish is related to a higher risk. This could partially the link between dietary products and higher intake of sodium.             

The finding is published in the journal Rheumatology.