This Publication Is Searchable The Merck Manual of Diagnosis and Therapy Section 1. Nutritional Disorders Chapter 3.
Milk is fortified with both forms.
Synthesis in the skin is normally the major source. Vitamin D is a prohormone with several active metabolites that act as hormones. It passes through the enterohepatic circulation and is reabsorbed from the gut. The main function of vitamin D hormone is to increase calcium absorption from the intestine and promote normal bone formation and mineralization. These functions are mediated by a vitamin D receptor that is a transcription factor, which is instrumental in turning on a panoply of genes that express the biologic activity of vitamin D hormone.