SummaryBackground 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.