Women who are overweight or obese are at greater risk of developing rheumatoid arthritis (RA), according to a new study presented at the 2012 annual meeting of the American College of Rheumatology. This adds to existing evidence that maintaining a healthy weight is necessary to avoid potential health problems, including RA.

“Controlling your weight not only prevents diabetes and cardiovascular diseases, but it can also prevent RA,” says study researcher Bing Lu, MD, assistant professor of medicine at Brigham and Women's Hospital in Boston.

“Previous studies have had inconsistent results, but we were able to confirm that obesity increases RA risk based on two large studies, with a much better study design and a very large sample size,” says Dr. Lu.

For the analysis, Dr. Lu and his colleagues reviewed two large studies, the Nurses’ Health Study (NHS), which ran from 1976 to 2008, and the Nurses’ Health Study II (NHSII), which ran from 1989 to 2009.

The NHS group included 121,700 female registered nurses, who were between the ages of 30 and 55 years at the start of the study; the NHSII group included 116,608 nurses between the ages of 25 and 42 years at the start of the study.

Their body fat was calculated by using the Body Mass Index (BMI), a ratio of weight to height. Women were classified as being of normal weight if their BMI was 25 or less, overweight if their BMI was between 25.0 to 29.9 and obese if their BMI was 30 or greater.
 

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.”