Introduction: Widespread vitamin D deficiency has been reported worldwide and evidence of its deleterious effects on health has been accumulating. There is insufficient data concerning vitamin D status among the Polish adult population. The aim of the study was to determine vitamin D status and factors influencing it — UVB exposure, supplementation, and diet — among adults in northern Poland following months of low and high natural UVB radiation.

Material and methods: Adults were recruited in an outpatient clinic. All were examined twice in 2012: in winter (in February, March and the first half of April), and autumn (between 25 September and 8 November). Questionnaire examinations were performed, and serum concentrations of 25-hydroxy-vitamin D (25(OH)D), calcium, phosphorus, parathyroid hormone (PTH), and alkaline phosphatase (ALP) were determined.

Results: 40 men and 69 women participated in the study (age 48.4 ± 15 years, body mass index (BMI) 25.9 ± 4.3 kg/m2, mean ± SD). Mean 25(OH)D serum level in winter was 13.3 ± 6.6 ng/mL, 81.1% of participants were vitamin D-deficient. Mean 25(OH)D concentration in autumn was 22.8 ± 7.9 ng/mL (42.2% of subjects were vitamin D-deficient). Median 25(OH)D and PTH concentrations between the two examination periods differed significantly (11.9 vs. 22.1 ng/mL, and 46.3 vs. 32.2 pg/mL, respectively). In autumn, negative correlations were found between: 25(OH)D and PTH serum levels, 25(OH)D and BMI values. When compared to respective counterparts, participants declaring vitamin D supplementation, and sunbed use had significantly higher median 25(OH)D concentrations.

Conclusions: UV exposure during the summer was insufficient to provide adequate vitamin D status for almost half of the participants by as soon as the early autumn. Our results suggest all-year-round vitamin D supplementation should be widely implemented.