Harpers

VITAMINS & MINERALS / 497amounts required vary from the order of grams per dayfor sodium and calcium, through milligrams per day(eg, iron) to micrograms per day for the trace elements.In general, mineral deficiencies are encountered whenfoods come from one region, where the soil may be deficientin some minerals, eg, iodine deficiency. Wherethe diet comes from a variety of different regions, mineraldeficiencies are unlikely. However, iron deficiencyis a general problem because if iron losses from thebody are relatively high (eg, from heavy menstrualblood loss), it is difficult to achieve an adequate intaketo replace the losses. Foods from soils containing highlevels of selenium cause toxicity, and increased intakesof common salt (sodium chloride) cause hypertensionin susceptible individuals.SUMMARY• Vitamins are organic nutrients with essential metabolicfunctions, generally required in small amountsin the diet because they cannot be synthesized by thebody. The lipid-soluble vitamins (A, D, E, and K)are hydrophobic molecules requiring normal fat absorptionfor their efficient absorption and the avoidanceof deficiency symptoms.• Vitamin A (retinol), present in carnivorous diets, andthe provitamin (β-carotene), found in plants, formretinaldehyde, utilized in vision, and retinoic acid,which acts in the control of gene expression. VitaminD is a steroid prohormone yielding the active hormonederivative calcitriol, which regulates calciumand phosphate metabolism. Vitamin D deficiencyleads to rickets and osteomalacia.• Vitamin E (tocopherol) is the most important antioxidantin the body, acting in the lipid phase ofmembranes and protecting against the effects of freeradicals. Vitamin K functions as cofactor to a carboxylasethat acts on glutamate residues of clottingfactor precursor proteins to enable them to chelatecalcium.• The water-soluble vitamins of the B complex act asenzyme cofactors. Thiamin is a cofactor in oxidativedecarboxylation of α-keto acids and of transketolasein the pentose phosphate pathway. Riboflavin andniacin are important cofactors in oxidoreduction reactions,respectively present in flavoprotein enzymesand in NAD and NADP.• Pantothenic acid is present in coenzyme A and acylcarrier protein, which act as carriers for acyl groupsin metabolic reactions. Pyridoxine, as pyridoxalphosphate, is the coenzyme for several enzymes ofamino acid metabolism, including the aminotransferases,and of glycogen phosphorylase. Biotin is thecoenzyme for several carboxylase enzymes.• Besides other functions, vitamin B 12 and folic acidtake part in providing one-carbon residues for DNAsynthesis, deficiency resulting in megaloblastic anemia.Vitamin C is a water-soluble antioxidant thatmaintains vitamin E and many metal cofactors in thereduced state.• Inorganic mineral elements that have a function inthe body must be provided in the diet. When insufficient,deficiency symptoms may arise, and if presentin excess they may be toxic.REFERENCESBender DA, Bender AE: Nutrition: A Reference Handbook. OxfordUniv Press, 1997.Bender DA: Nutritional Biochemistry of the Vitamins. 2nd ed. CambridgeUniv Press, 2003.Garrow JS, James WPT, Ralph A: Human Nutrition and Dietetics,10th ed. Churchill-Livingstone, 2000.Halliwell B, Chirico S: Lipid peroxidation: its mechanism, measurement,and significance. Am J Clin Nutr 1993;57(5Suppl):715S.Krinsky NI: Actions of carotenoids in biological systems. Annu RevNutr 1993;13:561.Padh H: Vitamin C: newer insights into its biochemical functions.Nutr Rev 1991;49:65.Shane B: Folylpolyglutamate synthesis and role in the regulation ofone-carbon metabolism. Vitam Horm 1989;45:263.Wiseman H, Halliwell B: Damage to DNA by reactive oxygen andnitrogen species: role in inflammatory disease and progressionto cancer. Biochem J 1996;313:17.