Editing Vitamin (section)

===Sources===
For the most part, vitamins are obtained from the diet, but some are acquired by other means: for example, microorganisms in the [[gut flora]] produce vitamin K and biotin; and one form of vitamin D is synthesized in skin cells when they are exposed to a certain wavelength of ultraviolet light present in [[sunlight]]. Humans can produce some vitamins from precursors they consume: for example, vitamin A is synthesized from [[beta carotene]]; and [[Niacin (nutrient)|niacin]] is synthesized from the [[amino acid]] [[tryptophan]].<ref name="DRI-Niacin">{{cite book|title=Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline|last1=Institute of Medicine|publisher=The National Academies Press|year=1998|isbn=978-0-309-06554-2|location=Washington, DC|pages=123–149|chapter=Niacin|access-date=29 August 2018|chapter-url=https://www.nap.edu/read/6015/chapter/8|author1-link=Institute of Medicine}}</ref> Vitamin C can be synthesized by some species but not by others. [[Vitamin B12|Vitamin B<sub>12</sub>]] is the only vitamin or nutrient not available from plant sources. The Food Fortification Initiative lists countries which have mandatory fortification programs for vitamins folic acid, niacin, vitamin A and vitamins B<sub>1</sub>, B<sub>2</sub> and B<sub>12</sub>.<ref name="FoodFortif" />