The association between serum 25-hydroxyvitamin D levels and psoriasis in a large population-based cohort, a cross-sectional analysis of The Tromsø Study 2015-16.

BACKGROUND: Case-control studies indicate an association between lower serum 25-hydroxyvitamin D (25[OH]D) levels and psoriasis. Data from larger population-based cohorts including mild cases are sparse.

OBJECTIVES: To investigate the association between 25(OH)D and psoriasis in a large population-based cohort, and assess possible effect modification by overweight.

METHODS: Data from the Tromsø Study 2015-16 (Tromsø7), including 19520 subjects from the general population aged 40-79 years old, were analysed cross-sectionally. We assessed the shapes of the 25(OH)D/psoriasis-relationships using fractional polynomials. Odds ratios (ORs) for life-time and active psoriasis were estimated using logistic regression. Adjusted models included month of blood sampling, body mass index (BMI), age and sex. Two-way and additive interaction between BMI and 25(OH)D were explored.

RESULTS: Among a total of 19520 subjects (10203 women [52.3%]; mean [SD] age, 56.3 [10.4] years; mean [SD] 25[OH]D, 63.4 [21.9] nmol/L), 2088 (10.7%) reported life-time psoriasis and 1179 (6.0%) reported active psoriasis the past 12 months.There was no association between 25(OH)D and life-time psoriasis (OR [95% confidence interval] per 10 nmol/L increase in 25[OH]D: 1.02 [0.99, 1.04]). The relationship between 25(OH)D and active psoriasis was suggested to be non-linear, but the model was not significant (p = 0.098).There was evidence for a super-additive effect (i.e. larger than the sum of the factors) of BMI >27.5 kg/m2 and 25(OH)D <25 nmol/L on the odds for active psoriasis (OR 1.92, 95%CI [1.18, 3.12]), but not for life-time psoriasis (OR 1.41, 95%CI [0.93, 2.15]). There was no evidence for two-way interaction between BMI and 25(OH)D.

CONCLUSIONS: This large population-based study found no significant relationship between 25(OH)D and psoriasis. The analysis may have been underpowered to detect a threshold effect in the lower 25(OH)D spectrum. Interaction analysis indicates that high BMI and vitamin D deficiency combined increase the odds of active psoriasis more than the sum of them, with an estimated 92% higher odds for active psoriasis in subjects with BMI>27.5 kg/m2 and 25(OH)D<25 nmol/L. Providing advice to prevent vitamin D deficiency may be considered in the follow-up of overweigth patients with psoriasis.