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According to a recently published report in the British Journal of Dermatology, no significant relationship was found between 25-hydroxyvitamin D and psoriasis, although an interaction analysis indicated that having both a high BMI and vitamin D deficiency significantly increased the risk of active psoriasis more than the sum of each of these 2 factors. 

Case-control studies in hospital settings have shown decreased levels of 25-hydroxyvitamin D to be associated with an increased risk of psoriasis; however, evidence from larger population-based studies that include mild cases of psoriasis have been limited. To narrow this research gap, investigators from Norway explored the potential association between 25-hydroxyvitamin D levels and psoriasis in the population-based Troms酶 Study and determined whether any relationship was modified by body mass index (BMI). 

Analyzing Troms酶 data

The investigators conducted a cross-sectional analysis using data from the Troms酶 Study 2015鈥�2016 among 19,520 individuals from the general population 40鈥�79 years of age. The Troms酶 Study is a single-center, population-based study consisting of health surveys of people living in Troms酶, Norway. The cohort included 10,203 women (52.3%); mean age was 56.3 years and mean 25-hydroxyvitamin D, 63.4 nmol/L. A level of 25-hydroxyvitamin D <25 nmol/L was considered vitamin D deficient; 25鈥�50 nmol/L, insufficient; and >50 nmol/L, sufficient.

25-hydroxyvitamin D levels were measured using a validated liquid chromatography鈥搈ass spectrometry method. 

The research team used fractional polynomials to analyze the relationship between 25-hydroxyvitamin D and psoriasis and logistic regression to estimate the odds ratio (OR) for lifetime and active psoriasis. Adjusted models considered age, sex, month of blood sampling (to account for seasonal variation in 25-hydroxyvitamin D levels), and BMI as potential confounding variables. 

Additive interaction was assessed between 25-hydroxyvitamin D and BMI using 4 groups according to dichotomized 25-hydroxyvitamin D (鈮� or <25 nmol/L) and BMI (> or 鈮�27.5 kg/m2). ORs were then calculated for psoriasis, with 25-hydroxyvitamin D 鈮�25 nmol/L plus BMI 鈮�27.5 kg/m2 defined as the reference group. 

Possible increased risk with high BMI plus vitamin D deficiency

In total, 2088 participants (10.7%) reported lifetime psoriasis and 1179 (6.0%) reported active psoriasis during the previous year.

No significant relationship was evident between measured 25-hydroxyvitamin D and lifetime psoriasis in an adjusted model (OR per 10 nmol/L increase in 25-hydroxyvitamin D 1.02, 95% CI 0.99鈥�1.04; P=.18). The authors pointed out that a nonlinear association between 25-hydroxyvitamin D levels and active psoriasis was suggested, but did not reach statistical significance (P=.098); different thresholds for 25-hydroxyvitamin D yielded similar results.

BMI >27.5 kg/m2 in combination with 25-hydroxyvitamin D <25 nmol/L had a superadditive effect (or an effect larger than the sum of each variable) on the risk of active psoriasis (OR 1.92, 95% CI 1.18鈥�3.12), although the authors remarked that this finding should be interpreted with caution due to a small number of cases in this subgroup. This superadditive effect was not observed for lifetime psoriasis (OR 1.41, 95% CI 0.93鈥�2.15).

鈥淥besity associated low-grade inflammation has been linked to psoriasis, and [vitamin D deficiency] may contribute to the proinflammatory state in obesity,鈥� the authors wrote. 鈥淭hus, we consider it plausible that the two factors may act synergistically in driving inflammation.鈥� 

Associations attenuated?

The authors noted several methodologic limitations that may have weakened the association between 25-hydroxyvitamin D levels and psoriasis in their analysis. For instance, their study may have been underpowered to identify a threshold effect in the lower levels of 25-hydroxyvitamin D. 

In addition, the higher prevalence of physical impairments due to comorbidities among individuals with psoriasis and their hesitation to reveal skin lesions could have hindered participation and led to a study population underrepresented by individuals with psoriasis. As a third example, the expected misclassification of active and lifetime psoriasis can be assumed not to be impacted by 25-hydroxyvitamin values and 鈥渕ay possibly bias the estimated ORs in our study towards the null.鈥� 

鈥淧roviding advice to prevent vitamin D deficiency,鈥� the research team concluded, 鈥渕ay be considered in the follow-up of overweight patients with psoriasis.鈥�¹

Published: June 20, 2024