Eating protein, red meat and some fish is associated with a higher risk of rheumatoid arthritis because of these foods' salt content, the researchers suggest.
Swedish researchers from the Department of Public Health and Clinical Medicine, Rheumatology at Umeå University conducted the nested case-control study to test the results of a recent animal study that suggested a link between high sodium intake and rheumatoid arthritis.
In 2013, separate research teams at Yale University in New Haven, CT, Harvard Medical School in Boston, MA, the Broad Institute of Harvard and the Massachusetts Institute of Technology pooled their data on what factors are capable of inducing activity in an autoimmune cell called Th17.
Working on the knowledge that a gene known as SGK1 plays a role in the Th17 cells' development - as well as playing a role in absorbing salt into the gut and kidneys - the teams investigated how salt might affect the cells' development.
When they added salt to the diet of mice, it stimulated production of Th17 cells. Also, mice genetically engineered to have a form of multiple sclerosis had more severe disease if they were fed the high-salt diet. The next step was to confirm the results in humans.
Results became significant when narrowed to current smokers
The new study examined 386 people whose dietary habits had been recorded in the Västerbotten Intervention Programme (VIP), an average of 7.7 years before the onset of rheumatoid arthritis symptoms. From the same database, the researchers identified 1,886 control subjects for comparison.
In the VIP, participants' dietary habits, levels of physical exercise, and information on whether or not they smoked, as well as other information, was gathered along with blood samples.
When all the individuals were included in the analysis, the study did not find any significant association between sodium intake and development of rheumatoid arthritis, and so they were unable to confirm the findings of the animal study.
However, when the researchers narrowed their results to participants who were current smokers, the risk for rheumatoid arthritis among people with a high sodium intake more than doubled.
Author Björn Sundström explains:
"Additive interaction analyses suggested that approximately half (54%) of the increased risk from smoking in the development of rheumatoid arthritis is due to interaction with sodium intake. A large influence of sodium intake on smoking as a risk factor for rheumatoid arthritis 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.
These findings will provide new insights into the etiopathogenic process leading to the development of rheumatoid arthritis among smokers. The finding of sodium being a risk factor for the development of rheumatoid arthritis among smokers is intriguing, as it may explain discrepancies in previous studies of diet as a risk factor for rheumatoid arthritis."
Sundström adds that the reason why eating fruit and vegetables is associated with a lower risk of developing rheumatoid arthritis - while eating protein, red meat and some fish is associated with a higher risk - could be that fruit and vegetables are lower in sodium.
"These results could have implications for analyses of diet in other conditions in which inflammation is of importance," he says.