VITAMIN A EXCESS

also called Hypervitaminosis A, toxic condition most frequently produced by a high intake of vitamin A, generally 150 mg daily over a period of several months. Unlike the water-soluble vitamins (e.g., ascorbic acid, thiamine, riboflavin), vitamin A is soluble in fat, and a surplus in the body is not eliminated in the urine but rather is stored in the liver, where it may eventually reach toxic levels. General signs of toxicity include nausea, coarsening and loss of hair, drying and scaling of the skin, bone pain, fatigue, and drowsiness. There may also be blurred vision and headache in adults and growth failure, enlargement of the liver, and nervous irritability in infants. Prognosis is good when vitamin A intake is reduced. Although hypervitaminosis A is most often caused by an excessive, regular intake of vitamin A concentrate, it has also been known to occur among explorers and inhabitants of the Arctic, following large meals of the vitamin A-rich polar bear liver. Carotene, the yellowish pigment (found in such foods as carrots, sweet potatoes, and egg yolks) that is converted to vitamin A in the body, is not toxic per se, but the blood plasma may contain a high enough concentration of the pigment to impart a yellowish colour to the skin, a condition known as carotenemia. Additional reading Comprehensive works are Henry R. Mahler and Eugene H. Cordes, Basic Biological Chemistry (1968); Abraham White et al., Principles of Biochemistry, 6th ed. (1978); Albert L. Lehninger, David L. Nelson, and Michael L. Cox, Principles of Biochemistry, 2nd ed. (1993); Thomas Briggs and Albert M. Chandler (eds.), Biochemistry, 2nd ed. (1992); John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Chemistry and Life, 4th ed. (1993); Lubert Stryer, Biochemistry, 3rd ed. (1988); Donald Voet and Judith G. Voet, Biochemistry (1990); Geoffrey Zubay, Biochemistry, 3rd ed. (1993); and Laurence A. Moran et al., Biochemistry, 2nd ed. (1994). David J. Holme and Hazel Peck, Analytical Biochemistry, 2nd ed. (1993), covers newer methods of analysis. The Editors of the Encyclopdia BritannicaAnnette Natow and Jo-Ann Heslin, Complete Book of Vitamins & Minerals (1988); The Complete Book of Vitamins, new ed. (1984); Kenneth J. Carpenter, The History of Scurvy and Vitamin C (1986); and John Marks, The Vitamins in Health and Disease (1968), are among various popular works on vitamins. The following references are more advanced and require some background knowledge of biochemistry: George H. Beaton and Earle Willard McHenry (eds.), Nutrition: A Comprehensive Treatise, vol. 2, Vitamins, Nutrient Requirements and Food Selection (1964), a discussion of the requirements and metabolism of vitamins and of deficiency diseases; Stewart A. Koser, Vitamin Requirements of Bacteria and Yeasts (1968), on the requirements and metabolism of vitamins in microorganisms; F.A. Robinson, The Vitamin Co-Factors of Enzyme Systems (1966), on the coenzyme function of vitamins; W.H. Sebrell, Jr., and Robert S. Harris (eds.), The Vitamins, 2nd ed. (1967), on the chemistry, biochemistry, and metabolism of vitamins, especially vol. 6 and 7 for a discussion of the methodology used in vitamin research; Arthur F. Wagner and Karl Folkers, Vitamins and Coenzymes (1964, reprinted 1975), a text with emphasis on molecular structure, organic synthesis and biosynthesis, chemical reactions, and metabolic roles of vitamins; Roman J. Kutsky, Handbook of Vitamins, Minerals, and Hormones, 2nd ed. (1981); and Lawrence J. Machlin (ed.), Handbook of Vitamins, 2nd ed. rev. and expanded (1991). Margaret J. Baigent The Editors of the Encyclopdia Britannica