Researchers set out to see if a salty diet might be linked to the onset of rheumatoid arthritis, but found a connection only among smokers - who were more than twice as likely as anyone with a low-salt diet to develop the condition.
"Although we could not confirm our original hypothesis, we were surprised by the large influence of sodium intake on smoking as a risk factor for rheumatoid arthritis," Björn Sundström told Reuters Health in an email.
"Smoking is a strong risk factor for developing rheumatoid arthritis, and this risk is further amplified by less than ideal food habits with high sodium intake," said Sundström, a researcher in the departments of public health and clinical medicine at Umea University who led the new study.
Rheumatoid arthritis is a chronic disease that causes pain and swelling in the joints. About 1.5 million Americans, three-quarters of them women, have been diagnosed with the condition, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases.
Read: Smoking a strong risk factor for rheumatoid arthritis
RA cause is unknown
The cause of rheumatoid arthritis isn't known, but it results from a person's own immune system attacking joint tissues. Genetics and lifestyle factors, such as smoking, hormone and cholesterol levels and obesity, have all been identified as risk factors.
Previous laboratory research on animals and human cells also suggests sodium may provoke inflammatory molecules that are often elevated in people several years before RA appears, the authors write in the journal Rheumatology.
To see if there is a link between sodium in the diet and risk for developing RA, Sundström and colleagues looked at health records and lifestyle information for almost 100,000 participants in a screening and intervention program that began in 1991 in Vasterbotton County in northern Sweden.
The study team identified 386 cases of people who developed rheumatoid arthritis by 2011. Then for each case, they identified five participants who were similar in age and sex, but did not develop disease, for comparison.
The researchers accounted for dietary habits, education, cholesterol and triglyceride levels, weight and other factors for all the participants and found no significant associations between sodium in the diet and who went on to develop RA.
Smoking and high sodium diet double risk
But when they looked just at smokers, they found those who consumed the most sodium were 2.26 times as likely as smokers who consumed the least sodium to develop RA.
Smokers who consumed the least sodium had similar risk to non-smokers in the study, leading the researchers to conclude that smoking and heavy sodium intake interact somehow to promote RA.
"Ideally, these results needs to be repeated in an independent population," Sundström, said.
More research is also needed to identify the biological pathways through which sodium intake can affect smoking as a risk factor, he said.
Read: Lifestyle factors linked to RA
"The study provides the first evidence in rheumatoid arthritis that sodium intake may influence risk for onset of the disease," Dr. Lars Klareskog told Reuters Health in an email.
Klareskog, a rheumatologist and researcher at the Karolinska Institute and Karolinska University Hospital in Stockholm, was not involved in the study.
"In addition, the study demonstrates that the impact of high salt diet is restricted to individuals who smoke," he said.
Klareskog noted that smoking is an important risk factor for rheumatoid arthritis.
"The contribution of smoking to risk for rheumatoid arthritis overall is such that 25 percent of all rheumatoid arthritis in our country, Sweden, would not have happened without smoking," he said. One third of all cases with the more severe form of rheumatoid arthritis would not have happened without smoking, he added.
Reducing salt intake
The present study suggests that reducing salt intake may be added to this list of lifestyle advice for avoiding rheumatoid arthritis, Klareskog said.
"Confirmatory studies are, however, needed before recommendations on salt intake can be made to the public as ways to protect against getting rheumatoid arthritis," he said.
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