Researchers found that women in the NHS group who were overweight or obese increased their risk of developing RA by 19 percent and 18 percent, respectively, compared with women who were of normal weight. In the NHSII group, overweight and obese women were also at significantly increased risk.

But based on the available data, not all experts are convinced that weight plays a causal role in developing this autoimmune disease that affects 1.5 million Americans. 

“The study only shows an association,” says Allen Anandarajah, MD, clinical director and associate professor of medicine in the division of allergy, immunology and rheumatology at the University of Rochester, in New York. “It could be argued that they gained weight due to decreased mobility or function as a result of RA, or even possibly secondary to some of the treatments for RA, like steroids.” 

Other experts, however, believe that excess weight is a risk factor for developing the disease, not just a common consequence of having it. In a similar study earlier this year, researchers including Eric Matteson, MD, the chair of rheumatology at the Mayo Clinic in Rochester, Minn., found that obesity in women was a factor that increased the risk of RA by about 20 percent. 

“This study adds to our information about the risks of obesity,” says Dr. Matteson. “Obesity is a risk factor for other diseases, but also autoimmune diseases.”

Although the link between obesity and autoimmune diseases remain unclear, Dr. Matteson believes fat cells may play a role in inflammation. “We know that fat cells produce inflammatory proteins,” he says. “So there’s probably a connection between having an excess amount of fat cells and turning on the autoimmune cells in the body.”