From osteoarthritis to heart disease to diabetes, obesity is implicated in a host of diseases. A study now adds one more condition to the list: psoriatic arthritis, or PsA.

Psoriatic arthritis, an autoimmune condition, is a form of inflammatory arthritis that affects an estimated 6 to 10 percent of people with of the skin condition psoriasis and up to 40 percent of those with extensive psoriasis. It can also affect people who do not have the skin disease.

It’s been known that being overweight or obese increases a person’s chances of developing psoriasis. But in the new study, published recently in the Annals of the Rheumatic Diseases, researchers report a link between body mass index, or BMI, and psoriatic arthritis, too.

“We’re now able to say psoriasis patients have an increased risk of psoriatic arthritis if they are heavy,” says study author Thorvardur Jon Love, MD, an assistant professor of rheumatology at Landspitali University Hospital in Reykjavik, Iceland. “The way we split them up by weight groups gives us a way to say there is a graded effect – the heavier people are, the greater the risk.”

Researchers gathered data from more than 75,000 psoriasis patients listed in a United Kingdom database between 1995 and 2010. They analyzed the BMI of patients when they were first diagnosed with psoriasis, and found that the higher the BMI, the greater the likelihood of subsequently developing PsA. Having a BMI between 25 and 29.9 increased the risk of developing PsA by nine percent, compared with someone of normal weight, with a BMI lower than 25. Obesity (BMI between 30 and 34.9) raised the risk by 22 percent, and morbid obesity (BMI of 35 or greater) by 48 percent.

“It’s a concern that the percentages are as large as they are and the risk increases as much as it does, especially when you get in the higher weight groups,” Dr. Love says.

The study also found that extra pounds increased the risk of developing psoriatic arthritis even among those who did not have psoriasis.

Researchers say the increased risk could be because fat tissue – also called adipose tissue – overproduces inflammatory cytokines (or proteins) in the body.