Their findings are published online in the American College of Rheumatology journal Arthritis Care & Research.

In rheumatoid arthritis, the immune system attacks tissues, inflaming joints and sometimes also affecting other organs and causing fever and fatigue. Rheumatoid arthritis tends to initially impact the hands and feet and then spread to the knees, ankles, hips and shoulders. It is more common in women than in men. Complications can include heart problems, lung disease, osteoporosis and carpal tunnel syndrome.

To examine a potential link with obesity, researchers pulled medical records covering 1980-2007 from the Rochester Epidemiology Project and studied 813 adults with rheumatoid arthritis and 813 adults as the control group, matched by age, gender and calendar year. Height, weight and smoking status also were noted; roughly 30 percent of the patients in each group were obese and 68 percent were women.

Rheumatoid arthritis cases rose by 9.2 per 100,000 women from 1985-2007, the study found. Obesity accounted for 52 percent of the increase. Smoking also is a substantial risk factor for developing rheumatoid arthritis, but smoking's prevalence remained constant over the years studied, ruling it out as an explanation for the rise in rheumatoid arthritis, the study found.

More research is needed to determine how obesity may lead to rheumatoid arthritis. The exact nature of the link between obesity and autoimmune diseases such as rheumatoid arthritis is not clear, says co-author Eric Matteson, M.D., chair of the Division of Rheumatology at Mayo Clinic in Rochester.

"We know that fat tissues and cells produce substances that are active in inflammation and immunity. We know too that obesity is related to many other health problems such as heart disease and diabetes, and now perhaps to autoimmunity," Dr. Matteson says. "It adds another reason to reduce and prevent obesity in the general population."

The study was funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases. Co-authors included biostatistician Cynthia Crowson, rheumatologist and epidemiologist Sherine Gabriel, M.D., and rheumatologist John M. Davis III, M.D., all of Mayo Clinic in Rochester.