Question: how is vitamin d made?
Answer: Vitamin D2 (ergocalciferol) is produced by invertebrates, some plants, and fungi. Biological production of D2 is stimulated by ultraviolet light. Vitamin D3 (cholecalciferol) is synthesized in the skin by the reaction of 7-dehydrocholesterol with UVB radiation, present in sunlight with an UV index of three or more.