VITAMINS

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. 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 lipidsoluble 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 and have possible roles in prevention of atherosclerosis and cancer, although in excess they may also act as damaging pro-oxidants. 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). 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.

Vitamins are organic nutrients with essential metabolic functions that are required in small amounts in the diet because they cannot be synthesized by the body. The lipidsoluble vitamins (A, D, E, and K) are hydrophobic molecules requiring normal fat absorption for their absorption and the avoidance of deficiency.

■ Vitamin A (retinol), present in meat, and the provitamin (β-carotene), found in plants, form retinaldehyde, utilized in vision, and retinoic acid, which acts in the control of gene expression.

■ Vitamin D is a steroid prohormone yielding the active hormone calcitriol, which regulates calcium and phosphate metabolism; deficiency leads to rickets and osteomalacia. It has a role in controlling cell differentiation and insulin secretion.

■ Vitamin E (tocopherol) is the most important lipid-soluble antioxidant in the body, acting in the lipid phase of membranes protecting against the effects of free radicals.

■ Vitamin K functions acts as the cofactor of a carboxylase that acts on glutamate residues of precursor proteins of clotting factors and bone and other proteins to enable them to chelate calcium.

The water-soluble vitamins act as enzyme cofactors. Thiamin is a cofactor in oxidative decarboxylation of α-keto acids and of transketolase in the pentose phosphate pathway. Riboflavin and niacin are important cofactors in oxidoreduction reactions, present in flavoprotein enzymes and in NAD and NADP, respectively.
Pantothenic acid is present in coenzyme A and acyl carrier protein, which act as carriers of acyl groups in metabolic reactions.