Rates of rheumatoid arthritis have increased over the past few decades. According to a recent study, obesity may be to blame for about half of this increase.
Results from the study showed that people who were obese were more likely to develop rheumatoid arthritis than those who were not obese.
These findings suggest that unless the obesity epidemic is controlled, rates of rheumatoid arthritis will continue to rise, leading to increased demands for arthritis care, the authors said.
"Control your weight to reduce your risk of arthritis."
The study was conducted by Sherine E. Gabriel, MD, of Mayo Clinic, and colleagues.
"Obesity is an underrecognized risk factor for rheumatoid arthritis. In recent years, both the prevalence of obesity and the incidence of rheumatoid arthritis have been rising," the authors wrote in background information to the study.
In other words, more and more Americans are becoming obese. At the same time, more Americans are developing rheumatoid arthritis.
"Our objective was to determine whether the 'obesity epidemic' could explain the recent rise in the incidence of rheumatoid arthritis," they wrote.
The study included 813 patients with rheumatoid arthritis and 813 without arthritis. About 30 percent of each group was obese.
Results showed that people with a history of obesity had 1.24 times the odds of developing rheumatoid arthritis compared to those without a history of obesity.
Between 1985 and 2007, the rate of new cases of rheumatoid arthritis increased by 9.2 per 100,000 cases among women. Obesity accounted for about 52 percent of this increase.
According to the authors, obesity's role in rheumatoid arthritis has nearly been forgotten in recent years, as environmental and genetic factors have received more attention.
"The strongest environmental factors influencing the development of rheumatoid arthritis are arguably cigarette smoking and use of oral contraceptives. However, the prevalence of these factors has been stable in recent years, so they are unlikely to explain the recent rise in [new cases of rheumatoid arthritis]," the authors wrote.
While this study did not explain why obesity may lead to rheumatoid arthritis, the authors had some ideas. One reason may be the relationship between obesity and inflammation, one of the defining characteristics of rheumatoid arthritis.
Another reason is that obese patients often lack sufficient vitamin D. Some research has show than low levels of vitamin D may increase the risk of developing rheumatoid arthritis.
A third reason may be the relationship between obesity and sex hormones, which have also been shown to play a role in the development of rheumatoid arthritis.
Finally, past studies have found genetic links between obesity and autoimmune diseases like psoriatic arthritis. Like rheumatoid arthritis, psoriatic arthritis involves autoimmune processes (the immune system's attack on healthy tissues) and inflammation.
"In conclusion, obesity is associated with a modest risk for developing rheumatoid arthritis. Given the recent rapid increase in the prevalence of obesity, this risk factor appears to have a significant impact on the incidence of rheumatoid arthritis and may account for a large proportion of the recent increase in incidence of rheumatoid arthritis among women," the authors wrote.
The study was published December 27 in Arthritis Care & Research, a journal of the American College of Rheumatology. Research was funded in part by the National Institutes of Health and Rochester Epidemiology Unit. No conflicts of interest were reported.