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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) is an appropriate and feasible target at population level. We examined whether personal sunlight exposure levels can provide vitamin D sufficient (≥ 20 ng mL) and optimal status in the U.K. public. 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. Dietary vitamin D remained low in all seasons (median 3·27 μg 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; 28% optimal, zero deficient (<5 ng mL)], and lowest in February (18·3 ng mL; 7% optimal, 5% deficient). A February 25(OH)D level of 20 ng mL 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 in women and men, respectively, with 62% of variance explained by gender and September levels. 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. [ABSTRACT FROM AUTHOR]