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OBJECTIVES
After studying this chapter, you should be able to:
Describe how reference intakes for vitamins and minerals are determined and explain why reference intakes published by different national and international authorities differ.
Define a vitamin and describe the metabolism, principal functions, deficiency diseases associated with inadequate intake, and the toxicity of excessive intakes of the vitamins.
Explain why mineral salts are required in the diet.
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Vitamins are a group of organic nutrients, required in small quantities for a variety of biochemical functions that, generally, cannot be synthesized by the body and must therefore be supplied in the diet.
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The lipid-soluble vitamins are hydrophobic compounds that can be absorbed efficiently only when there is normal fat absorption. Like other lipids, they are transported in the blood in lipoproteins or attached to specific binding proteins. They have diverse functions—for example, vitamin A, vision and cell differentiation; vitamin D, calcium and phosphate metabolism, and cell differentiation; vitamin E, antioxidant; and vitamin K, blood clotting. As well as dietary inadequacy, conditions affecting the digestion and absorption of the lipid-soluble vitamins, such as a very low fat diet, steatorrhea and disorders of the biliary system, can all lead to deficiency syndromes, including night blindness and xerophthalmia (vitamin A); rickets in young children and osteomalacia in adults (vitamin D); neurological disorders and hemolytic anemia of the newborn (vitamin E); and hemorrhagic disease of the newborn (vitamin K). Toxicity can result from excessive intake of vitamins A and D. Vitamin A and the carotenes (many of which are precursors of vitamin A), and vitamin E are antioxidants (see Chapter 45) and have possible roles in prevention of atherosclerosis and cancer, although in excess they may also act as damaging pro-oxidants.
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The water-soluble vitamins are vitamins B and C, folic acid, biotin and pantothenic acid; they function mainly as enzyme cofactors. Folic acid acts as a carrier of one-carbon units. Deficiency of a single vitamin of the B complex is rare since poor diets are most often associated with multiple deficiency states. Nevertheless, specific syndromes are characteristic of deficiencies of individual vitamins, eg, beriberi (thiamin); cheilosis, glossitis, seborrhea (riboflavin); pellagra (niacin); megaloblastic anemia, methylmalonic aciduria, and pernicious anemia (vitamin B12); megaloblastic anemia (folic acid); and scurvy (vitamin C).
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Inorganic mineral elements that have a function in the body must be provided in the diet. When the intake is insufficient, deficiency signs may arise, eg, anemia (iron), and cretinism and goiter (iodine). Excessive intakes may be toxic.
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The Determination of Micronutrient Requirements Depends on the Criteria of Adequacy Chosen
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For any nutrient, there is a range of intakes between that which is clearly inadequate, leading to clinical deficiency disease, and that which is so much in excess of the body’s metabolic capacity that there may be signs of toxicity. Between ...