Smoking has long been considered the only modifiable risk factor that increases a person’s chances of developing rheumatoid arthritis (RA), an autoimmune disease causing pain, swelling and potential deformity in affected joints. Now doctors from the Mayo Clinic in Rochester, MN., say obesity also increases the odds of developing RA – at least for women, who are two to three times more likely than men to have the disease.

“This is really new. Smoking, and now it looks like obesity, appear to be factors for risk,” says Eric Matteson, MD, co-author of the study published online in Arthritis Care & Research that examined the link between obesity and RA. Dr. Matteson is chair of the Mayo Clinic’s division of rheumatology.

While no one knows for sure what causes this disease, a genetic predisposition is likely – especially in combination with environmental factors, such as previous infection, occupational exposure and smoking. As for obesity as a risk factor, the evidence has been contradictory. (However, weight gain can occur after diagnosis due to certain medications, a decrease in physical activity and/or a change in body composition due to RA.)

For this study, researchers compared medical records from 813 RA patients in the Rochester Epidemiology Project (which consists of data from RA patients in Olmsted County, Minn., gathered between 1980 and 2007) with the medical records of 813 participants (matched for age, sex and other factors) without the disease, who served as a control group. Nearly 70 percent of study participants were women, and about 30 percent of each group was classified as obese (with a body mass index of 30 or greater) when they entered the study.

The researchers write that after four decades of decline, the number of RA cases in Olmsted County started to rise again in 1995. While they note that the cause of the rise is unknown, they surmise it must be due to changes in environmental risk factors, as genetic factors do not change so quickly. While the “list of potential environmental influences on development of RA is extensive,” they chose to focus on obesity “due to its recent, dramatic increase in prevalence.”

To determine if there was a relationship, they calculated the proportion of RA cases that could likely have been prevented by eliminating obesity. They found that obesity explained more than half (52 percent) of the increase in RA among women between 1995 and 2007. Dr. Matteson says he can’t exclude the possibility that obesity could be a risk factor for RA in men, but his team didn’t have enough data from male patients to say that for sure.

The study places obesity as a more modest risk factor for RA than smoking, but a risk factor nonetheless. “Smoking would approximately more than double your risk of developing arthritis. Being overweight would add about 20 percent to that risk,” Dr. Matteson says. 

He notes the increased risk for developing RA existed for people who had ever been obese – not just those who were obese when they got RA.