Vitamin D serum level changes in psoriatic patients treated with narrowband ultraviolet B phototherapy are related to the season of the irradiation

Background: Vitamin D is produced in the human skin by short wavelength (290-315 nm) ultraviolet (UV) radiation. Purpose: The aim of the study was to investigate how outdoor conditions may influence the serum levels of 25(OH) vitamin D in psoriasis patients under narrowband ultraviolet B (UVB) phototherapy. Methods: The winter and summer groups of patients received almost the same narrowband UV (nUVB) doses during whole-body phototherapy. The 25(OH) D serum concentration was measured before and after two series of 10 exposures. The cabinet doses were compared with potentially available cumulative solar doses. The solar doses (unweighted UVB and vitamin D effective dose) and duration of solar intensity sufficient to produce vitamin D were calculated using a model based on local atmospheric data. Results: After an initial 10 nUVB treatments, 25(OH) D serum concentration increased by 68% for winter patients in relation to the level before therapy, whereas a 20% increase was found for the summer patients. The next 10 treatments caused a much lower increase in 25(OH) D concentration: 5% and 3.5% for the winter and summer patients, respectively. No statistically significant relationship was observed between post-therapy 25(OH) D serum concentration and solar radiation variability. Conclusions: The different baseline values of 25(OH) D serum levels in winter and summer patients result from seasonal variability in solar irradiance. Thus, outdoor solar radiation affects the patients over a much longer period, and artificial UV light is the main factor responsible for increase in 25(OH) D serum level over a 30-day period of cabinet therapy.