Vitamin D status of middle-aged women at 65–71°N in relation to dietary intake and exposure to ultraviolet radiation

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Abstract
To determine the vitamin D status of middle-aged women living in the Norwegian arctic and its relationship with vitamin D intake and exposure to ultraviolet (UV) radiation. Cross-sectional study. This study is based on measurements of 25-hydroxyvitamin D (25(OH)D) levels in a sub-sample of the Norwegian component of the EPIC biological bank, which consists of blood samples from a random selection of participants in the Norwegian Women and Cancer Study. From November 2001 until June 2002, 309 blood samples were collected from a total of 443 invited middle-aged women (44-59 years) in northern Norway (65-71 degrees N) (crude response rate, 69.8%). Questionnaire data provided information on dietary sources of vitamin D and UV exposure. Median plasma 25(OH)D concentration for the whole group was 55.0 nmol l(-1) (range 8.1-142.8 nmol l(-1)). Vitamin D intake was a significant predictor of 25(OH)D status (P=0.0003). The time of the year when the blood sample was collected significantly predicted plasma 25(OH)D level (P=0.005). Levels of 25(OH)D were positively associated (P=0.0002) with estimated hours per day of exposure to UV-B radiation. Residing in northern Norway during the summer prior to blood sampling was negatively associated with 25(OH)D concentration (P=0.001). The prevalence of moderate hypovitaminosis D was highest in January-February, when a quarter of the participants had 25(OH)D concentrations < or =37.5 nmol l(-1). Increased ingestion of marine food items that provide vitamin D should be promoted and further studies should be carried out to investigate vitamin D status in arctic populations in relation to both UV exposure and traditional food sources.