... Provitamins are vitamin precursors, that is, naturally occurringsubstances which are not vitamins themselves, but which can beconverted into vitamins by normal body metabolism.Most of the vitamins ... required to form 1 mg retinol) [150]. Vitamins in Foods: Analysis, Bioavailability, and Stability 73ContentsPart I Properties of Vitamins Chapter 1. Nutritional Aspects of Vitamins 1.1 Definition and ... distributed in natural food sources. Thirteen vitamins are recog-nized in human nutrition and these may be conveniently classified intotwo groups according to their solubility. The fat-soluble vitamins...