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A. W. Norman: Sunlight, season, skin pigmentation, vitamin D, and 25-hydroxyvitamin D: integral components of the vitamin D endocrine system. In: The American Journal of Clinical Nutrition. Band67, Nr.6, 1. Juni 1998, S.1108–1110, doi:10.1093/ajcn/67.6.1108 (oup.com [abgerufen am 31. Mai 2020]).