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Summary

Background  It is known that ultraviolet (UV) B radiation increases serum 25-hydroxyvitamin D3 [25(OH)D] level. However, there is uncertainty about the relationship between the maintenance of vitamin D status and UVB.

Objectives  To define the frequency of UVB exposure necessary for maintaining summer 25(OH)D levels during the winter.

Methods  In total, 60 participants were included from October 2008 to February 2009 (16 weeks) and randomized for UVB exposure of 1 standard erythema dose (SED) to ∼88% body area once a week (= 15 completed), every second week (= 14 completed) or every fourth week (= 12 completed). The controls (= 14 completed) had no intervention. Vitamin D was measured at baseline, every fourth week before exposure, and 2 days after the last UVB exposure.

Results  The 25(OH)D levels (mean) after UVB exposure once a week increased significantly (from 71·9 to 84·5 nmol L−1) (< 0·0001), whereas UVB exposure every second week maintained 25(OH)D levels (= 0·16). A significant decrease in mean 25(OH)D levels (from 56·4 to 47·8 nmol L−1) (< 0·0001) was found after UVB exposure once every fourth week and for the control group (from 64·8 to 40·1 nmol L−1) (< 0·0001). The development in 25(OH)D levels during the 16-week study period were negatively correlated with baseline 25(OH)D (< 0·0001). Further, the increase in 25(OH)D after the last UVB exposure was negatively correlated with the 25(OH)D level just before the last UVB exposure (< 0·0001).

Conclusions  Exposure to a UVB dose of 1 SED every second week to ∼88% body area is sufficient for maintaining summer 25(OH)D levels during the winter.