The major finding of this study is that vitamin D deficiency is very frequent in patients with chronic plaque psoriasis and in those with PsA. This finding was more common (i.e. 80% of cases) in winter, but it was found also in summer in approximately 50% of patients. The association between vitamin D insufficiency and psoriasis was confirmed independently of age, sex, BMI, PASI score, PTH and the season in which the serum sample was taken. We found also a higher prevalence of vitamin D deficiency in patients with RA, as already reported.
The finding of vitamin D deficiency in patients with psoriasis could be relevant for several reasons. Firstly, it has been clearly established that vitamin D deficiency is a risk factor for osteoporosis and increases the risk of falling in the elderly. An increased risk of male patients with psoriasis developing osteoporosis has been addressed in a large population-based case–control study conducted in Israel. Secondly, observational studies in large cohorts have shown significant associations between low levels of 25(OH)D and increased risk of diabetes mellitus, metabolic syndrome and cardiovascular mortality.[18,19] Similarly, psoriasis is frequently associated with cardiometabolic comorbidities and with an increased cardiovascular mortality.[20–22] Moreover, low levels of vitamin D may also have important implications in the pathogenesis of psoriasis. Vitamin D3 acts mainly on the vitamin D receptor to regulate keratinocyte growth and differentiation, but also has an influence on immune functions of dendritic cells and T lymphocytes.[23,24] Vitamin D3 inhibits production of interleukin (IL)-2 and IL-6, blocks transcription of interferon-γ and granulocyte-macrophage colony-stimulating factor mRNA, and inhibits cytotoxic T cells and natural killer cell activity. Topical vitamin D derivatives, including calcipotriol (calcipotriene) and calcitriol, have immunomodulatory effects on monocytes, macrophages, T cells and dendritic cells. Indeed, topical vitamin D derivatives are extensively used as monotherapy or in combination with steroids for the topical treatment of psoriasis. Moreover, phototherapy increases the levels of serum 25(OH)D in patients with psoriasis and it has been proposed that narrowband ultraviolet (UV) B radiation may mediate its beneficial effect on psoriasis also by increasing endogenous vitamin D levels. That oral supplementation with vitamin D could be effective in the treatment of psoriasis was suggested some years ago.[29,30] Furthermore, the recent finding of a resolution of adalimumab-induced psoriasis in a woman with RA after the use of high vitamin D doses for the treatment of vitamin D deficiency raises the interesting question on the possible use of vitamin D in the treatment of psoriasis. However, it is clear that more definitive evidence is required to demonstrate that a serum level of 25(OH)D < 20 ng mL−1 in psoriasis is pathologically low, and that clinical benefit would be gained from vitamin D supplementation.
Several conditions may contribute to low serum levels of vitamin D in the general population, including poor dietary intake of vitamin D; sun avoidance and/or negligible sun exposure, possibly related also to impaired quality of life; malabsorption due to inflammatory bowel disease, gluten enteropathy, gastric surgery, biliary disease, or intestinal bacteria overgrowth; use of antiseizure medications (e.g. phenobarbital or phenytoin) and long-term use of glucocorticoids. The reason for the higher prevalence of vitamin D deficiency in patients with psoriasis is not clear. However, we can exclude the possibility that this difference was related to different sun exposure between groups. Vitamin D deficiency has been already reported in other chronic immune-mediated inflammatory skin diseases including atopic dermatitis, vitiligo and chronic urticaria.[31–33] A possible role of vitamin D deficiency in the development of these conditions has been also proposed.[33–35]
Once detected, vitamin D deficiency could be corrected, although no evidence of the possible benefits of vitamin D supplementation in reducing inflammation and/or the risk of other incident autoimmune diseases has yet been proven. Moreover, optimal dosage regimens for vitamin D remain uncertain. In general, for every 100 IU of vitamin D taken in, there is an increase of roughly 1 ng mL−1 (3 nmol L−1) in the serum level of 25(OH)D. Most trials assessing the effectiveness of the supplementation of 25(OH)D levels and the risk of fractures and falling have used daily doses of vitamin D between 400 and 1000 IU.[37,38] Toxicity from vitamin D supplementation is very rare and consists principally of acute hypercalcaemia, which usually results from doses that exceed 10 000 IU per day. The tolerable upper level of daily vitamin D intake recently set by the Institute of Medicine is 4000 IU.
This study has some limitations. Firstly, the cross-sectional study design does not allow a causal or temporal relationship between vitamin D insufficiency and psoriasis to be established. Prospective studies will be required to resolve these issues. Moreover, we did not assess daily vitamin D intake in foods; however, we excluded from the study patients receiving oral supplementation of vitamin D or drugs that interfere with calcium metabolism. Furthermore, we decided to choose the partners of patients with psoriasis as a control group in order to minimize differences due to different dietary intake of vitamin D. We chose patients with RA who were not matched for sex because the prevalence of RA is significantly higher in women, whereas psoriasis has a similar prevalence in men and women. If we had matched for sex we could have added a selection bias in the study.
In conclusion, vitamin D deficiency may be common in patients with psoriasis, especially in winter time. Therefore, patients could be routinely screened for vitamin D insufficiency for a more comprehensive management.
This work was supported by the Ministero della Salute, and the Ministero dell'Istruzione, Università e Ricerca Scientifica (Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale, PRIN).
Conflicts of interest
The British Journal of Dermatology. 2012;166(3):505-510. © 2012 Blackwell Publishing