Vitamin. An organic substance that is produced in plant and animal tissues and is required, usually in minute amounts, for normal metabolic and such physiological reactions as participation in enzymatic activity, tissue regeneration, growth promotion, and the prevention of specific pathological conditions. The many vitamins found in nature are generally classified according to their solubility characteristics into water-soluble vitamins that include ascorbic acid (q.v.) (vitamin C) and the vitamins of the B-complex (q.v.), and the fat-soluble vitamins that include vitamins A, D, E, and K (q.v.). Vitamins present in wheat (q.v.) and its flour (q.v.) include those of the B-complex and vitamin E (q.v.).