Benders'dictionary of nutrition and food technology

494consumers that enhance physical or mental quality of life andmay increase health status.vitamers Chemical compounds structurally related to a vitamin,and converted to the same active metabolites in the body. Theythus possess the same kind of biological activity, although sometimeswith lower potency.When there are several vitamers, the group of compoundsexhibiting the biological activity of the vitamin is given a genericdescriptor (e.g. vitamin a is the generic descriptor for retinoland its derivatives as well as several carotenoids).vitamin There are 13 organic compounds (thus excluding traceminerals) essential to human life in very small amounts. Elevenof these must be supplied in the diet (vitamins A, B 1 ,B 2 ,B 6 ,B 12 ,C, E, K, folic acid, biotin and pantothenic acid); two (niacinand vitamin d) can be made in the body if there is sufficient ofthe amino acid, tryptophan, and sunlight, respectively. The wordmay be pronounced either veitamin or vittamin.Vitamins A, D, E and K are grouped together as fat-solublevitamins, because they are soluble in lipids, but not inwater. Vitamin C and the B vitamins (including pantothenicacid, biotin and folic acid) are grouped together as the watersolublevitamins since they are all soluble in water, but notlipids.vitamin A (see p. 495) Fat-soluble vitamin, occurring either as thepreformed vitamin (retinol) found in animal foods or as a precursor(carotenes) found in plant foods. Required for control ofgrowth, cell turnover and fetal development, maintenance of fertilityand maintenance of the normal moist condition of epithelialtissues lining the mouth and respiratory and urinary tracts;essential in vision.The main active metabolites in the body areretinaldehyde, all-trans- and 9-cis-retinoic acids.Deficiency leads to slow adaptation to see in dim light (poordark adaptation), later to night blindness; then drying of thetear ducts (xerophthalmia) and ulceration of the cornea (keratomalacia)resulting in blindness.The vitamin A content of foods is expressed as retinolequivalents, i.e. retinol plus carotene; 1 µg retinol = 6µg β-carotene = 12µg other active carotenoids = 3.33 internationalunits.See also conjunctival impression cytology; relative doseresponse test; retinol binding protein; vision.vitamin A toxicity Retinol in excess of requirements is stored inthe liver, bound to proteins, and is a cumulative poison. Whenthe storage capacity is exceeded, free retinol causes damage tocell membranes. carotene is not toxic in excess, since there isonly a limited capacity to form retinol from carotene.