Vitamin D levels in USA peak in August, study finds

USA Today:http://www.usatoday.com/story/news/nation/2013/06/29/vitamin-d-levels-seasonality/2453955/

PLoS:ONE: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0065785

NIH Office of Dietary Supplements: http://ods.od.nih.gov/factsheets/VitaminD-QuickFacts/