Chapter

Vitamins and trace elements

J. Powell-Tuck and M. Eastwood

in Oxford Textbook of Medicine

Fifth edition

Published on behalf of Oxford University Press

ISBN: 9780199204854
Published online May 2012 | e-ISBN: 9780199570973 | DOI: http://dx.doi.org/10.1093/med/9780199204854.003.01102

Series: Oxford Textbooks

Vitamins and trace elements

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Vitamins

Vitamins are diverse, unrelated organic compounds that some higher animals, humans included, cannot synthesize and which play key roles in metabolism, underpinning the most crucial of biological reactions. They are required in small amounts and have diverse functions, e.g. as: ◆ activated carriers of biochemical groups—coenzymes (B vitamin derivatives) ◆ antioxidants (vitamins A, C and E, carotenoids) ◆ precursors of visual pigments (vitamin A) ◆ endocrine mediators, especially in calcium and phosphorus metabolism (vitamin D) ◆ facilitators of blood clotting (vitamin K)

The clinical importance of vitamins lies not only in overt deficiency syndromes, which develop because of persistent inadequate intake or absorption over different periods of time depending upon body storage, but also in the need for optimal intakes to maintain health. This latter role can be difficult to assess for individual nutrients, but is inferred from knowledge of the beneficial and harmful effects of diets containing particular foods.

The traditional classification of vitamins into water and lipid soluble, and by their associated deficiency conditions, becomes less useful as their biochemical roles are better understood. An inadequate dietary vitamin intake will result in specific cellular failure and even death.

Vitamins as biochemical cofactors

Oxygen is the final electron acceptor when food is oxidized, but the transfer from energy substrate to oxygen is not direct; it occurs via intermediaries, important among which are molecules such as nicotinamide adenine dinucleotide (NAD) or flavin adenine dinucleotide (FAD) derived from the B vitamins niacin and riboflavin respectively.

Other metabolically crucial fragments require activated carriage, e.g. thiamine is a carrier for aldehyde groups, folate for one-carbon groups, and coenzyme A (pantothenate) carries two-carbon (acetyl) fragments into the tricarboxylic acid cycle. Thus a small group of molecules derived from the B vitamins is responsible for diverse biochemical interchanges. The stability of these carrier molecules in the absence of catalysts enables enzymes to control the flow of free energy and reducing power.

Vitamins as antioxidants

Although molecular oxygen is an ideal final electron acceptor, ‘danger lurks in the reduction of O2’. Partial reduction, particularly the transfer of single electrons to form superoxide or two electrons to form peroxide, yields potentially damaging products or reactive oxygen species. White cells use this process to kill pathogens, and most cells are protected from it by antioxidants, particularly the enzyme superoxide dismutase—an enzyme that contains manganese in the enzyme’s mitochondrial form and copper and zinc in the cytoplasmic forms. The antioxidant vitamins C and E are also important in this process, with fat-soluble vitamin E functioning particularly to protect membranes from lipid peroxidation. NADPH generated by glucose-6-phosphate dehydrogenase maintains levels of reduced glutathione.

Intake of vitamins

There is a dose–response effect of vitamin intake, ranging from the physiological through the pharmacological to the toxic. Recommendations for vitamin intake for different ages, needs, and communities are based on dietary intake, bioavailability, steady-state concentrations in plasma and tissue at defined intakes, urine excretion, adverse effects, biochemical and molecular function, and freedom from deficiency. With increasing intake, either orally or as an infusion, a vitamin is distributed through the body fluids and tissues until the saturation point is exceeded.

Some artificial enteral supplements and feeds and most parenteral feeds are deficient in vitamins unless appropriately supplemented: it is the responsibility of the prescriber to ensure that appropriate vitamin intake is maintained during artificial feeding. The prescription of vitamins parenterally, bypassing the absorptive processes, also has dosage implications.

Trace elements

Trace elements (e.g. magnesium, iron, zinc, copper, manganese, fluoride, selenium, molybdenum, chromium, iodine) are essential nutrients that act as cofactors in enzyme oxidation–reduction reactions. They maintain the specific configuration of proteins, are incorporated into the structure of hormones, and play a structural and catalytic role in gene expression and transcriptional regulation.

Deficiency of trace elements can cause a very wide range of clinical problems, including anaemia from iron deficiency and goitre resulting from iodine deficiency, which is endemic in mountainous areas.

Chapter.  16191 words. 

Subjects: Dietetics

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