The role of sunlight exposure in determining the vitamin D status of the U.K. white adult population

Background Vitamin D is necessary for bone health and is potentially protective against a range of malignancies. Opinions are divided on whether the proposed optimal circulating 25-hydroxyvitamin D [25(OH) D] level (>= 32 ng mL(-1)) is an appropriate and feasible target at population level. Objectives We examined whether personal sunlight exposure levels can provide vitamin D sufficient (>= 20 ng mL(-1)) and optimal status in the U. K. public. Methods This prospective cohort study measured circulating 25(OH) D monthly for 12 months in 125 white adults aged 20-60 years in Greater Manchester. Dietary vitamin D and personal ultraviolet radiation (UVR) exposure were assessed over 1-2 weeks in each season. The primary analysis determined the post-summer peak 25(OH) D required to maintain sufficiency in wintertime. Results Dietary vitamin D remained low in all seasons (median 3 27 mu g daily, range 2.76-4.15) while personal UVR exposure levels were high in spring and summer, low in autumn and negligible in winter. Mean 25(OH) D levels were highest in September [28.4 ng mL(-1); 28% optimal, zero deficient (<5 ng mL(-1))], and lowest in February (18 3 ng mL(-1); 7% optimal, 5% deficient). A February 25(OH) D level of 20 ng mL(-1) was achieved following a mean (95% confidence interval) late summer level of 30.4 (25.6-35.2) and 34.9 (27.9-41.9) ng mL(-1) in women and men, respectively, with 62% of variance explained by gender and September levels. Conclusions Late summer 25(OH) D levels approximating the optimal range are required to retain sufficiency throughout the U. K. winter. Currently the majority of the population fails to reach this post-summer level and becomes vitamin D insufficient during the winter.