The study, published in the Rheumatology journal, found no link between sodium intake alone and development of the condition. Previous studies in animal models and human cells suggested that sodium could play a role in the development of autoimmune diseases, such as arthritis.
However, when taking into account a person's smoking habit – a well-established risk factor for rheumatoid arthritis – sodium was found to more than double the risk of developing the disease. It suggests sodium interacts with smoking to raise this risk.
The study analysed health and dietary data from 386 individuals in Sweden, taken from a national health screening database.
May explain discrepancies
The results suggest that just over half (54%) of the increased risk of developing rheumatoid arthritis from smoking could be attributed to an interaction with sodium.
Researchers said this interaction ‘may explain discrepancies in previous studies of diet as a risk factor for rheumatoid arthritis’.
Lead author Dr Björn Sundström, from Umeå University in Sweden, said the results would help ‘provide new insights’ into the development of the disease in smokers.
He said: ‘These results could have implications for analyses of diet in other conditions in which inflammation is of importance.’
The effect of sodium on arthritis could help explain why salty foods such as red meat and fish are associated with a higher risk of developing rheumatoid arthritis than foods containing less sodium, such as fruit and vegetables.