Seasonality of UV-radiation and vitamin D status at 69 degrees north

Abstract

The main purpose with this study was to assess the seasonal variation in measured UV-radiation and its impact on vitamin D status throughout one year in subjects living at high latitude. Blood samples drawn from 60 volunteers (44 women, 16 men) living at Andenes (69° N), Norway, were collected throughout one year, at two-month intervals. The blood samples were analysed for 25-hydroxy vitamin D [25(OH)D]. Data on dietary intakes of vitamin D, time spent in daylight, use of sun beds and sun seeking holidays were collected by using questionnaires. The ambient vitamin D effective UV-radiation was measured at a site near by Andenes, and the number of hours spent outdoors with sufficient radiation for cutaneous vitamin D production (UV-hours) was estimated for each day. The mean 25(OH)D values were significantly higher at the end of the summer and in December, 2004 and varied from 42.0 nmol L⁻¹ in October, 2004 and April, 2005 to around 47 nmol L⁻¹ in December, 2004 and September, 2005. For the whole group, a positive relationship between UV-hours and 25(OH)D was found at UV-hours ≥ 3.5. However, for subjects with lower 25(OH)D levels i.e. at least one blood measurement with 25(OH)D < 37.5 nmol L⁻¹, the positive relationship were found at around 1.5 UV-hours and more, whereas for the group of subjects that had all their vitamin D values above 37.5 nmol L⁻¹, positive relationship was found at UV-hours ≥ 4.0, when adjusting for vitamin D intake, sun bed use and sun seeking holidays. The generally high dietary intakes of vitamin D, especially in winter, mask largely the effect of seasonal variation in UV-exposure, causing an atypical seasonal variation in vitamin D status. The UV-hour variable significantly predicted 25(OH)D levels in blood when adjusted for intakes and artificial UV-radiation exposure and sun holidays abroad.