Winter sunlight makes no vitamin D
In a classic study, cloudless winter sunlight in Boston (42.2°N) from November to February produced no previtamin D3 in skin at all. A survey of 55,844 Europeans found 13.0% had a year-round average serum 25(OH)D below 30 nmol/L. Moscow and St Petersburg sit further north than Boston.
Sources
- • Webb et al., 1988, J Clin Endocrinol Metab (PMID 2839537)
- • Cashman et al., 2016, American Journal of Clinical Nutrition (PMID 26864360)