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The study also found that the rate of psoriasis went up with increasing weight. For example, obese people with a body mass index (BMI) over 35 had almost double the odds of psoriasis than normal weight people did. BMI is a body fat measurement based on height and weight. A BMI of 30 or over is considered obese.
"Psoriasis is a complex disorder," said lead researcher Dr. Ann Sophie Lonnberg, of the University of Copenhagen. "The genetic background for the disease and its many comorbidities [co-existing conditions] have not yet been fully uncovered," she said.
This study can't prove that psoriasis causes type 2 diabetes or obesity or vice versa, Lonnberg added. However, the study suggests the association between psoriasis and obesity could partly be tied to a common genetic cause, she explained.
"The reason psoriasis and obesity are associated is not only due to a common lifestyle, but they are also associated due to common genes," Lonnberg said. "It is important to treat psoriasis and obesity and diabetes, since they are risk factors for heart disease and could have serious effects on overall health."
For the study, Lonnberg and her colleagues collected data on nearly 34,000 twins, aged 20 to 71. Just over 4 percent had psoriasis, slightly more than 1 percent had type 2 diabetes and over 6 percent were obese, the findings showed.
People with psoriasis tended to weigh more than those without the skin condition, the researchers said. The risk for obesity was also greater among those with psoriasis -- 11 percent of people with psoriasis were obese, but only 8 percent of non-obese study participants had psoriasis, the findings showed.
The researchers also looked at 720 twin pairs in which one twin had psoriasis and the other didn't. The twins with psoriasis weighed more than the twins without psoriasis, and were also more likely to be obese, the study found. The prevalence of type 2 diabetes, however, was the same in twins with and without psoriasis, according to the report.
The study was published in the April 27 online edition of the journal JAMA Dermatology.
"Psoriasis is not just a disease of the skin -- patients and health care professionals need to be aware of systemic health issues associated with psoriasis," said Dr. Joel Gelfand. He's an associate professor of dermatology at the University of Pennsylvania Perelman School of Medicine in Philadelphia, and author of an accompanying journal editorial.
Other studies have suggested that people with psoriasis are more likely to develop type 2 diabetes even if they don't have major risk factors for the blood sugar disease, and that this risk increases with the severity of the psoriasis, Gelfand said.
"Some of this risk may be due to shared genetics between psoriasis and diabetes. It is also thought that chronic inflammation in psoriasis may predispose patients to diabetes," Gelfand explained.
He suggested that people with psoriasis -- particularly those aged 40 to 70 with more extensive skin disease -- should receive medical screenings for diabetes.
"Patients with psoriasis who are overweight or obese may lower their risk of diabetes while making the skin disease less active if they are able to achieve and maintain a more healthy body weight," Gelfand said.
Another doctor thinks genetics may help explain what she has seen in her own practice.
"I have seen that psoriasis is linked with diabetes, which suggests that a genetic link may help explain why it's a lot harder to control diabetes in patients with psoriasis," said Dr. Doris Day. She is a dermatologist at Lenox Hill Hospital in New York City.
"We are understanding more about psoriasis and coming up with better treatments for it," she said. "If you have psoriasis, you need to see a dermatologist, a cardiologist and an endocrinologist to make sure you have other conditions under control," Day advised.
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