A small suberythemal ultraviolet B dose every second week is sufficient to maintain summer vitamin D levels: a randomized controlled trial

Background It is known that ultraviolet (UV) B radiation increases serum 25-hydroxyvitamin D-3 [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 similar to 88% body area once a week (n = 15 completed), every second week (n = 14 completed) or every fourth week (n = 12 completed). The controls (n = 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) (P < 0.0001), whereas UVB exposure every second week maintained 25(OH) D levels (P = 0.16). A significant decrease in mean 25(OH) D levels (from 56 4 to 47 8 nmol L-1) (P < 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) (P < 0.0001). The development in 25(OH) D levels during the 16-week study period were negatively correlated with baseline 25(OH) D (P < 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 (P < 0.0001). Conclusions Exposure to a UVB dose of 1 SED every second week to similar to 88% body area is sufficient for maintaining summer 25(OH) D levels during the winter.