Vitamins and minerals: metabolic functions and deficiency
THE CONTEXT
Vitamins are organic nutrients with essential metabolic functions, required in small dietary amounts because the body cannot synthesize them. Mineral elements with bodily functions must also be supplied in the diet.
Lipid-soluble vitamins
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Vitamins A, D, E, and K are hydrophobic molecules whose absorption requires normal fat absorption.
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Vitamin A (retinol) from meat and its provitamin β-carotene from plants form retinaldehyde for vision and retinoic acid for control of gene expression.
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Vitamin D is a steroid prohormone that yields calcitriol. Calcitriol regulates calcium and phosphate metabolism and has roles in controlling cell differentiation and insulin secretion. Vitamin D deficiency causes rickets and osteomalacia.
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Vitamin E (tocopherol)* is the body's most important lipid-soluble antioxidant. It acts in the lipid phase of membranes to protect against free radicals.
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Vitamin K is a cofactor for a carboxylase acting on glutamate residues in precursor proteins of clotting factors, bone proteins, and other proteins, enabling them to chelate calcium.
Water-soluble vitamins and coenzyme functions
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Thiamin is a cofactor for oxidative decarboxylation of α-keto acids and for transketolase in the pentose phosphate pathway.
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Riboflavin and niacin are cofactors in oxidoreduction reactions. Riboflavin is present in flavoprotein enzymes, whereas niacin is present in NAD and NADP.
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Pantothenic acid is present in coenzyme A and acyl carrier protein, which carry acyl groups in metabolic reactions.
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Vitamin B6, as pyridoxal phosphate, is a coenzyme for several enzymes of amino acid metabolism, including transaminases, and for glycogen phosphorylase.
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Biotin is a coenzyme for several carboxylase enzymes.
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Vitamin B12 and folate provide one-carbon residues for DNA synthesis and other reactions; deficiency results in megaloblastic anemia.
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Vitamin C is a water-soluble antioxidant that maintains vitamin E and many metal cofactors in their reduced state.
Dietary mineral balance
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Inorganic mineral elements with functions in the body must be provided in the diet. Insufficient intake may cause deficiency, whereas excessive intake may be toxic.