Obesity as a risk factor for psoriasis

Evidence strongly suggests that through pro-inflammatory pathways, obesity predisposes individuals to the development of psoriasis and that obesity aggravates existing psoriasis. Although the exact mechanisms underlying this association are presently unclear, researchers have hypothesized that adipocyte secretion of pro-inflammatory mediators may worsen psoriasis. The first large study of 159 200 individuals demonstrating an association between psoriasis and obesity was published in 1986. These findings have since been supported by several studies and a large meta-analysis from 2012. Recently, the association was shown in a Mendelian randomization study using genetic variants as instrumental variables for body mass index (BMI). Furthermore, epidemiological data strongly suggest that obesity is an independent risk factor for psoriasis. This hypothesis is supported by two large prospective cohort studies. The first study followed 78 626 female nurses for 14 years, with identification of 892 new cases of psoriasis, and found a positive association between self-reported BMI measured at multiple time points and the risk of incident psoriasis. In the HUNT study, 33 734 Norwegians were followed for 10 years and it was shown that obesity, high abdominal fat and weight increased the risk of psoriasis. In this issue of the Journal of the European Academy of Dermatology and Venereology, Kim et al. published a population study where the authors used data from the Korean National Health Insurance Service. They included 8 million Koreans without a diagnosis of psoriasis and who had attended at least three health examinations between January 2005 and the end of 2010. The population was followed from 2009 to the time of a diagnosis of psoriasis or end of 2017. A surprisingly high number of new cases of psoriasis (187 128) (2.3%) were identified during a median follow-up time of 3.4 years. The authors calculated the visit-to-visit variability in bodyweight during the study period. Unfortunately, whether there was an increase or decrease in weight over time, or if the weight was unchanged, was unclear, which is a limitation of the study. The patients were classified into four groups according to the variability in weight, with Q4 having the highest variability and Q1 having the most stable weight. The primary finding was an association between variability in body weight and the risk of psoriasis, for example, with higher variability in weight, a higher risk of incident psoriasis was found. The increased risk was also present after adjusting for age, sex, alcohol intake, smoking, income level, physical activity, hypertension, diabetes and dyslipidaemia. This study supports the finding that high BMI is a risk factor for incident psoriasis. A strength of the study is that weight was measured and the diagnosis of psoriasis was verified, compared to other larger population studies, such as the Nurses’ Health Study II, where weight was self-reported, and the HUNT study from Norway, where the limitation was that the diagnosis of psoriasis was questionnaire based. A limitation of the study by Kim et al. is that it is not clear whether the variation leads to an increase in weight over time or an unchanged or even reduced weight over time. Furthermore, it is interesting that the group with the greatest variation in weight also smokes more, had higher alcohol intake, and had the highest proportion of diabetes. One could speculate that these persons also have other unknown confounders. In the HUNT study, a change in weight over time increased the risk of developing psoriasis, even if the subjects were classified as having normal weight. Persons with a weight gain of 5.0–9.9 and 10.0 kg or greater had a RR of 2.19 and 2.83, respectively, for developing psoriasis. In contrast, a weight reduction of 2.0 kg or more was associated with an RR of 0.52. This finding is in line with our weight reduction study, where substantial weight loss led to reduced severity of psoriasis. In this study, psoriasis patients had substantial weight variation as they lost approximately 15% of their body weight over 16 weeks. Overall, the study by Kim et al. supports the close association between psoriasis and obesity and that obesity is as risk factor for psoriasis. The study emphasizes the importance of lifestyle interventions aimed at controlling weight for the prevention and treatment of psoriasis.