The risk of getting diabetes is about 50 percent higher in patients with autoimmune forms of arthritis, such as rheumatoid arthritis, or RA, and psoriatic arthritis, according to a study in the Annals of the Rheumatic Diseases.

In the study, researchers mined a database in British Columbia, Canada, for medical records collected between 1996 and 2006. They then compared the data on 48,718 RA patients, 40,346 patients with psoriatic arthritis or psoriasis and 442,033 patients who had no known rheumatic diseases, to see how their risks of developing diabetes differed. Patients were classified as diabetic if they’d been prescribed at least one diabetic medication or received a diagnosis of diabetes.

After analyzing the data, researchers found that the relative risk for diabetes was increased by approximately 50 percent in patients with RA or psoriatic arthritis compared to people who don’t have these diseases.

“With psoriatic arthritis, it’s been known there’s an increased risk of diabetes, so in some respects, our psoriatic cohort was our positive control group. Finding the increased risk in psoriatics confirmed that our method was sound,” explains study author Daniel Solomon MD,  from the Division of Rheumatology at Brigham and Women’s Hospital in Boston. “With RA we didn’t really know what to expect because there hasn’t been a rigorous study focusing on this issue.”

Dr. Solomon says the risk of getting diabetes increases with age and body mass index (BMI), and it’s likely another condition, along with heart disease, lung disease, osteoporosis and some kinds of cancer, that people with RA are at increased risk for. 

But he says this study isn’t saying RA patients should worry that they will definitely get it.

“Patients with RA shouldn’t say, 'Oh my goodness. Now I’m at risk.' Most of us are at risk of diabetes,” Dr. Solomon says. “But they are at an increased risk. It’s another reason for staying slim and active.”

Dr. Solomon says he did find one surprise: The increased diabetes risk in these patients doesn't a result from steroid use. Corticosteroids are known to cause diabetes in people who use them long-term.

“Most people say of course, people with RA use steroids and therefore that’s why they have this increased risk,” Dr. Solomon says. “But we stratified on steroid use and we found similar risk in people with RA who are users and non users. That doesn’t seem to be a reason for the elevated risk."

Other experts say they are surprised to see that steroids didn't play a role in the elevated risk.

“That is amazing," says Meenakshi Jolly, MD, a rheumatologist at Rush University Medical Center in Chicago. "I would have thought people using more steroids are usually the people who have more active disease. That means they had inflammation going on a long time and I would have guessed they would have more diabetes, and they didn’t find it, which is interesting.” 

Dr. Solomon says the exact cause of the increased diabetes risk is not known. His study didn’t attempt to solve that mystery, but he speculates it’s connected to inflammation and possibly inactivity.

“One hypothesis is that the inflammation of RA is associated with insulin resistance – the body doesn’t process insulin correctly,” Dr. Solomon says.

The link is not entirely understood, but when patients have rheumatoid arthritis, the immune system gets confused and attacks the joints and other organs, instead of defending them. If the organ that’s attacked is the pancreas, where insulin is made, it can lead to scarring and decreased production of insulin, which may lead to diabetes. 

Even though the link is not fully understood, Dr. Solomon says that there may be a message here for doctors who care for RA patients: "It’s worthwhile for doctors to ask the question if patients with RA may have diabetes and to screen for it.”