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 psoriasis5,14−22. To our knowledge, there have been few reports concerning the risk of psoriasis in patients with MetS8,9,23. One such report described a large-population based cross-sectional study involving 34,996 individuals, which was published by a Norwegian research team in 20189. 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 findings.
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 significantly 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 influence 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 other9. 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 inflammatory markers in MetS lead to the increased psoriasis risk. MetS and psoriasis share inflammatory pathways, such as the T helper-17‐mediated or T helper-1-mediated inflammation pathways3. Many similar cytokines contribute to the pathogeneses of psoriasis and impaired lipoprotein regulation24,25. Moreover, patients with central obesity have excessive adipose tissue, which secretes adipokines, drivers of various chronic inflammation processes such as psoriasis26. Another hypothesis states that the two diseases share genetic loci. One study demonstrated shared genes between psoriasis and dyslipidemia, hypertension, and coronary artery disease27, but other studies have found no genetic associations between psoriasis and MetS or coronary artery disease27–30. Our findings that psoriasis risk was decreased in the pre-MetS group supported the hypothesis that persistently elevated inflammatory markers in MetS lead to an increased risk of psoriasis. The inflammation in MetS differs from the traditional concept of tumor, redness, pain, and heat. It is helpful to characterize MetS inflammation 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 inflammasomes such as NLRP3 activation31) that are also increased in psoriasis32,33. Therefore, we infer that the continually increased inflammation in MetS plays a role in the development of psoriasis.
Additionally, we analyzed the associations of MetS components with psoriasis risk. For the waist circumference component, the HR in the continuous-MetS group was 1.146 (95% CI, 1.097–1.197), the strongest association compared with those of other MetS components. This was followed by the associations of HDL (HR, 1.106; 95% CI, 1.068–1.145) and TG (HR, 1.013; 95% CI, 0.982–1.046). The fasting glucose component did not show a significant association with psoriasis risk, and the blood pressure component was associated with lowered risk in both the pre-MetS and continuous-MetS groups, compared with the control group. These results differ from those of other studies, which showed positive associations between psoriasis risk and all MetS components8,21,34,35. Other recent, large, well-designed studies showed that dyslipidemia and abdominal obesity were the main MetS components positively associated with psoriasis risk; however, they found no significant associations of the MetS components of blood pressure and fasting glucose level with psoriasis risk9,14, 18–20,22,36, consistent with our results.
The strength of our study lies in its large cohort size, which was representative of the entire Korean population; therefore, the findings 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 misclassification 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 records10.