Reference(s)

1. National Institute of Health, Office of Dietary Supplements. Vitamin A Health Professional Fact Sheet. Downloaded from:
www.ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/

2. Wintergerst ES, et al. Contribution of selected vitamins and trace elements to immune function.
Ann Nutr Metab 2007;51:301-23.