Metabolic Syndrome Diagnosis Changes and Psoriasis Risk: A Nationwide Population-Based Study


 Background

Metabolic syndrome (MetS) is associated with psoriasis, but it remains unclear whether the risk of psoriasis remains in patients whose MetS diagnosis changes.
Objectives

To assess the associations between psoriasis risk and changes in MetS components.
Methods

We obtained data from the National Health Insurance Service of Korea and divided the participants into four groups: individuals without MetS (control); individuals with MetS in 2009 but without it in 2012 (pre-MetS); individuals without MetS in 2009, but with newly diagnosed MetS in 2012 (post-MetS); and individuals with MetS during the 2009–2012 period (continuous-MetS). We calculated the risk of psoriasis for each group.
Results

Psoriasis risks were similar in the control and pre-MetS groups, but were significantly higher in the post-MetS group (hazard ratio [HR], 1.08495%; 95% confidence interval [CI], 1.045–1.124) and in the continuous-MetS group (HR, 1.106; 95% CI, 1.068–1.145) than in the control group. Among MetS components, waist circumference showed the strongest association with psoriasis, followed by high-density lipoprotein and triglyceride levels.
Conclusion

Psoriasis risk was higher in patients with sustained or newly developed MetS than in those who did not have MetS (regardless of prior MetS status).


Introduction
Psoriasis is a chronic, immune-mediated skin disease that affects an estimated 125 million people worldwide 1 . The pathophysiology of psoriasis involves a variety of in ammatory cells that secrete cytokines, thereby stimulating the activity of myeloid dendritic cells. The predominant feature involves IL-23-mediated activation of the Th17 pathway 2 . Genetic factors play an important role in the development of psoriasis, and environmental factors can exacerbate the disease. Psoriasis is a disease of the skin and joints, as well as a systemic in ammatory process associated with a range of comorbidities 3 . Metabolic syndrome (MetS), which includes hyperglycemia, atherogenic dyslipidemia, elevated blood pressure, and abdominal obesity 4,5 , increases the risks of cardiovascular disease and all-cause mortality 6 .
During the past two decades, studies concerning the association between MetS and psoriasis have been actively conducted 7 . In one nationwide population-based study, MetS was positively associated with an increased rate of psoriasis, and this trend became more pronounced in individuals with more MetS components 8 . Furthermore, a prospective study showed that MetS (especially its obesity component) was associated with an increased risk of incident psoriasis 9 . Many studies concerning the association between the two diseases exist, but none have investigated whether the risk of psoriasis is affected by changes in the MetS diagnosis.
Here, we assessed the associations between psoriasis risk and changes in MetS components using data from the Korean National Health Insurance Service (KNHIS) database.

Data Source
We extracted data from the National Health Claims database of the KNHIS 10 . This database represents the entire Korean population, and maintains data comprising inpatient and outpatient medical service records, diagnostic and procedural codes, prescribed medication claims, and patient demographics. All Korean nationals receive a unique identi cation number at birth; accordingly, their health care records cannot be duplicated or omitted. The KNHIS uses the standard codes of the International Statistical Classi cation of Diseases and Related Health Conditions, 10th revision (ICD-10). The KNHIS also provides general health check-ups and a cancer-screening program. Therefore, all insured Koreans and their dependents enjoy free age-relevant health checkups. We ensured that the data extracted from the KNHIS were complete and anonymized, and the Institutional Review Board of the KNHIS approved our request for these data (NHIS-2019-1-075). In addition, the Ethics Committee of Seoul St. Mary's Hospital, at the Catholic University of Korea, approved our study design and waived the requirement for informed consent from the participants (KC18ZESI0639).

Study Design and Population
We included data from 5,644,324 adults who underwent health examinations including measurement of MetS components in 2009 and in 2012. We used the following de nition of MetS in this study (in accordance with the revised criteria of the National Cholesterol Education Program Adult Treatment Panel III) 11 : MetS is present in any individual with three or more of the ve MetS components (waist circumference ≥ 90 cm for men and ≥ 85 cm for women, in accordance with the Korean Society for the Study of Obesity cut-off point for abdominal obesity 12 ; triglyceride [TG] levels ≥ 150 mg/dL or the use of medication for elevated TG; high-density lipoprotein [HDL] cholesterol levels < 40 mg/dL for men and < 50 mg/dL for women or the use of medication to reduce LDL-cholesterol; systolic blood pressure ≥ 130 mmHg or diastolic blood pressure ≥ 85 mmHg or the use of antihypertensive medication; fasting glucose levels ≥ 100 mg/dL or the use of medication for elevated glucose levels). We divided the participants into We assessed the associations between MetS components (waist circumference, blood pressure, fasting glucose, and TG and HDL levels) and psoriasis for each group. We tracked the data for all study participants (their health care records) during the 6-year period from 2012 to 2017 to identify patients who developed psoriasis (ICD-10: L40).

Clinical, Laboratory, and Anthropometric Measurements
The KNHIS regular medical heath examination program includes information concerning variables that may alter the general health status. The following data were obtained in this study: sex, age, smoking and alcohol consumption statuses, and physical activity level. Detailed smoking status, alcohol consumption, and physical activity histories (including the amounts and frequencies) were obtained from checkup program questionnaires. We classi ed individuals as nonsmokers, ex-smokers, and current smokers based on their smoking status. We de ned alcohol consumption habits into three categories: abstinence (no alcoholic drinks consumed within the past year), moderate drinking (< 30 g pure alcohol per day), and heavy drinking (≥ 30 g pure alcohol per day). We classi ed individuals' physical activity habits as vigorous-moderate or absent. Vigorous-moderate physical activity was de ned as ≥ 1 day per week of moderate or vigorous intensity exercise. Venous blood samples for the measurement of fasting glucose, lipid pro les, and liver enzyme levels were obtained at each examination site after an overnight fast. Body mass indexes (BMIs) were calculated based on each participant's weight (in kilograms) divided by the square of their height (in meters); these measurements were taken during the checkup programs. The checkup programs also included measurements of systolic and diastolic blood pressures, and of waist circumference 13 .

Statistical Analysis
For the study population characteristics in the four groups, we analyzed continuous variables using the independent t-test and dichotomous variables using the χ 2 test. We analyzed variables with skewed distributions after logarithmic transformation. The data are shown as means ± standard deviations (continuous variables), numbers and percentages (dichotomous variables), or geometric means and 95% con dence intervals (CIs) (continuous variables with skewed distributions). We performed univariate and multivariate Cox proportional hazard regression analyses to evaluate the associations of MetS component changes with psoriasis risk. We also performed a multivariate Cox proportional hazard regression analysis adjusting for age, sex, smoking status, alcohol consumption, physical activity, and BMI. We used log-rank tests to analyze the hazard ratios (HRs) for psoriasis for each group. All statistical analyses were performed using SAS software (version 9.4, SAS Institute, Cary, NC, USA). P values < 0.05 were considered statistically signi cant.

Characteristics of the study population
We compared the results of health examinations in 2009 and 2012 among the four groups (control group, n = 3,439,976; pre-MetS group, n = 430,044; post-MetS group, n = 752,360, and continuous-MetS group, n = 1021944). Table 1 shows total patient demographics by group.

Discussion
In this large-scale nationwide population-based study, we calculated the psoriasis risks based on MetS diagnosis changes over a period of 4 years. Our results showed that the psoriasis risk in the pre-MetS group was similar to that in the control group; however, the risks were greater in the post-MetS and continuous-MetS groups than in the control group. This tendency was slightly different for each component of the MetS. First, for the waist circumference criterion, the psoriasis risk was higher in all three groups (pre-MetS, post-MetS, and continuous-MetS) than in the control group. For TG and HDL cholesterol criteria, the psoriasis risk was increased only in the post-MetS and continuous-MetS groups. For the fasting blood glucose criterion, the psoriasis risks were similar in all groups. Finally, for the blood pressure criterion, the risk of psoriasis decreased in the pre-MetS and continuous-MetS groups.
Many reports have studied the association between psoriasis and MetS, but most of those studies were designed to evaluate the risk of MetS in patients with psoriasis 5,14−22 . To our knowledge, there have been few reports concerning the risk of psoriasis in patients with MetS 8,9,23 . One such report described a largepopulation based cross-sectional study involving 34,996 individuals, which was published by a Norwegian research team in 2018 9 . The relative risk of psoriasis in the individuals with MetS was 1.66, compared with the control group. They also examined the associations of each MetS criterion with the development of psoriasis and found increased psoriasis risks in patients with waist circumference, TG, and HDL components, but not in those with high blood pressure or blood glucose levels. Those results are consistent with our ndings.
Our study differs from others in that the patients with MetS were followed for a period of 4 years, and were divided into four groups for systematic analysis of the associations between psoriasis and MetS components. In the pre-MetS group, we found similar psoriasis risks in the control group, but the post-MetS and continuous-MetS groups had signi cantly higher psoriasis risks than the control group. This trend was consistent for the waist circumference and TG MetS components. For the HDL criterion, the HR was higher in the post-MetS and continuous-MetS group than in the pre-MetS group. This implies that components of MetS, particularly obesity and dyslipidemia, may in uence the development of psoriasis.
The association between psoriasis and MetS has been reported, but it is unclear whether one of the two conditions precedes or induces the other 9 . Central obesity and dyslipidemia contribute to an increased psoriasis risk. Several hypotheses have been suggested to explain the increased psoriasis risk in individuals with MetS. A convincing hypothesis is that the increasing levels of in ammatory markers in MetS lead to the increased psoriasis risk. MetS and psoriasis share in ammatory pathways, such as the T helper-17-mediated or T helper-1-mediated in ammation pathways 3 . Many similar cytokines contribute to the pathogeneses of psoriasis and impaired lipoprotein regulation 24,25 . Moreover, patients with central obesity have excessive adipose tissue, which secretes adipokines, drivers of various chronic in ammation processes such as psoriasis 26 . Another hypothesis states that the two diseases share genetic loci. One study demonstrated shared genes between psoriasis and dyslipidemia, hypertension, and coronary artery disease 27 , but other studies have found no genetic associations between psoriasis and MetS or coronary artery disease [27][28][29][30] . Our ndings that psoriasis risk was decreased in the pre-MetS group supported the hypothesis that persistently elevated in ammatory markers in MetS lead to an increased risk of psoriasis. The in ammation in MetS differs from the traditional concept of tumor, redness, pain, and heat. It is helpful to characterize MetS in ammation as "low-grade" or chronic. The condition is primarily caused by nutrient and metabolic excesses, which trigger various pathologic mechanisms (e.g., Toll-like receptor pathways or in ammasomes such as NLRP3 activation 31 ) that are also increased in psoriasis 32,33 . Therefore, we infer that the continually increased in ammation in MetS plays a role in the development of psoriasis. The strength of our study lies in its large cohort size, which was representative of the entire Korean population; therefore, the ndings are likely to be generalizable to the general public in Korea. In addition, by dividing the MetS patients into four groups based on MetS improvements or aggravations, we were able to characterize the causal relationship between MetS and psoriasis. In addition, we were able to adjust our regressions for confounding factors, such as smoking, drinking, physical activity and BMI, which were included in the source database. However, there was a notable limitation in this study.
Because the diagnosis of MetS and psoriasis was dependent on ICD-codes, coding or misclassi cation errors may have been included. However, a study verifying the frequency of such errors found that approximately 70% of the diagnostic codes were consistent with those in the medical records 10 .

Conclusions
Our ndings from this population-based, cross-sectional study in Korea demonstrated the effects of changes in MetS diagnosis on psoriasis risk. Our study differs from prior investigations because we focused on MetS diagnosis changes, and our results can help to better elucidate the association between psoriasis and MetS. Remarkably, the risk of psoriasis was higher in patients with sustained or newly induced MetS than in those without MetS (regardless of prior MetS status). Therefore, we emphasize the need to personally explain these ndings to patients. The data used in this study are owned by the Korean National Health Insurance Service, and the public sharing of these data is restricted. The authors accessed these data after requesting them from the National Health Insurance Data Sharing Service (NHISS). The authors of this study enjoyed no special access privileges before requesting the data, and they con rm that other quali ed researchers should also be able to request access to these data from the NHISS. For more information about this process, please see https://nhiss.nhis.or.kr/bd/ab/bdaba032eng.do.

Declarations
The English in this document has been checked by at least two professional editors, both native speakers  Figure 1 Relationship between changes in the number of MetS components from rst to second visits and the risk of psoriasis