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Psoriasis, Type 2 Diabetes Mellitus, and Obesity: Weighing the Evidence.

Psoriasis is a common, chronic inflammatory disease that is associated with an increased risk of cardiovascular, metabolic, and renal disease in a manner that varies with the severity of psoriasis and is often independent of traditional risk factors for these other diseases.1,2 The clinical significance of these associations is emphasized by premature death, particularly in patients with more severe psoriasis, in whom excess mortality is comparable to that seen in rheumatoid arthritis treated with disease-modifying medications.3 The association of psoriasis with type 2 diabetes mellitus and obesity has been extensively studied and has been the subject of numerous metaanalyses that clearly establish an association of psoriasis with both obesity and diabetes. Lønnberg and colleagues4 have provided new insights into these associations by studying monozygotic and dizygotic twins with and without psoriasis. Their main findings were that psoriasis is associated with diabetes independent of sex, age, smoking, and body mass index (BMI) (calculated as weight in kilograms divided by height in meters squared) (odds ratio, 1.53; 95% CI, 1.03 to 2.27) and that increases in BMI are associated with increasing odds of twins reporting a diagnosis of psoriasis. Similar findings were reported for hospitalized patients in whom psoriasis was diagnosed. The unique twin design of the study by Lønnberg and colleagues, in which an increasing BMI was associated with a diagnosis of psoriasis, allowed the investigators to identify a genetic correlation between psoriasis and BMI of 0.12 (95% CI, 0.08 to 0.19). A genetic correlation of similar magnitude of psoriasis and type 2 diabetes of 0.13 (95% CI, −0.06 to 0.31) was also observed, but this finding was not statistically significant (the prevalence of diabetes in this sample was quite low, yielding only 6 patients with psoriasis who also had diabetes). The magnitude of the genetic correlation is modest and similar to what has been reported for age at menarche and type 2 diabetes (−0.13).5 Nevertheless, these findings are consistent with emerging genetic evidence linking psoriasis to diabetes. For example, genetic variation in IL12B, IL23R, and IL23A has an influence not only on the risk for psoriasis but also on its severity and type 2 diabetes.6 Other researchers have suggested a role for CDKAL1 in conferring susceptibility to both psoriasis and diabetes.7 Only a few studies have examined the incidence (risk) of diabetes in patients with psoriasis while adjusting for major confounding variables (eg, BMI and hypertension), and only a subset of these were able to evaluate effects of the severity of psoriasis on the association of diabetes with psoriasis (using treatment patterns).8 The results of these studies suggest that psoriasis is associated with an increased risk of diabetes independent of major risk factors in a manner that correlates with the severity of psoriasis. Although the design used by Lønnberg et al4 does not address diabetes risk (ie, incidence) or directly evaluate a dose-response association of the severity of psoriasis to diabetes, its careful control for BMI, which most large population-based studies are unable to capture, adds to the weight of evidence suggesting psoriasis as a risk factor for diabetes. Additional population-based studies have similarly noted an increased prevalence of insulin resistance in patients with psoriasis in a manner that varies positively with increasing body surface area affected by psoriasis and independent of major risk factors such as BMI, hypertension, and dyslipidemia.9 Although inflammation is often invoked as a mechanistic link to insulin resistance, a simultaneous analysis of patients with psoriasis and rheumatoid arthritis with adjustment for major risk factors demonstrated an increased risk for diabetes only in patients with psoriatic disease.10 The risk of diabetic complications in patients with psoriasis (compared with diabetic patients without psoriasis) is rather unexplored, but emerging studies suggest that microvascular and macrovascular complications are more common in patients who have psoriasis and diabetes, that psoriasis is associated with worse hemoglobin A1c, and that increasing body surface area affected by psoriasis is associated with an increased risk for diabetic complications.11-13 These epidemiologic findings have clear implications for clinical practice. The dermatologist is on the front line in educating patients with psoriasis about their disease, treatment options, and risks of comorbidities. The US Preventive Services Task Force (updated 2015) recommends screening for an abnormal blood glucose concentration as part of the assessment for cardiovascular risk in adults aged 40 to 70 years who are overweight or obese. Moreover, the American Diabetes Association’s 2016 Standards of Medical Care recommend testing (with a fasting plasma glucose or oral glucose tolerance test or an assay for hemoglobin A1c) for diabetes in asymptomatic adults of any age who are overweight or obese and who have 1 or more additional risk factors for diabetes. Given the association of psoriasis—particularly more severe disease—and increases in BMI, most of our patients would be recommended for diabetes screening based on standard recommendations. More important, level A evidence (ie, derived from randomized clinical trials) indicates that lifestyle changes and Related article Opinion