What are vitamins? Solution Vitamins: Vitamins are organic nitrogenous compounds that are required in very small quantities for carrying out various biochemical reactions. They are not synthesized in the human body and hence should be supplemented in diet by including different varieties of fruits, vegetables and food in diet. Their deficiency in diet leads to characteristic deficiency disorders. Classification of Vitamins: There are about 15 vitamins are essential for humans. They are classified as.. A. Fat soluble vitamins: Vitamin A, D, E and K. B. Water soluble vitamins: It again divides into 2 groups. 1. Non B-Complex: vitamin C 2. B-Complex: Vitamin B1, B2, B3, B5, B6, B7, B9, B12..