Vitamin A

The recommended upper limits of habitual daily intake of retinol are about 12.5 x reference intake for adults, but only 2.5 x reference intake for infants. Retinol is also teratogenic in excess, and for pregnant women the recommended upper limit of daily intake is 3000-3300 |g. vitamin A2 Old name for dehydroretinol, the form found in livers of freshwater fish; has 40% of the biological activity of retinol. vitamin B complex Old-fashioned term for the various B vitamins: vitamin b1 (thiamin), vitamin b2 (riboflavin), niacin, vitamin b6, vitamin b12, folic acid, biotin and pantothenic acid. These vitamins occur together in cereal germ, liver and yeast; function as coenzymes; and historically were discovered by separation from what was known originally as 'vitamin B'; hence, they are grouped together as the B complex. vitamin Bx Thiamin. Thiamin diphosphate is a coenzyme in metabolism of glucose, and in the citric acid cycle. Thiamin triphosphate has a role in nerve conduction, by activating a chloride channel. Deficiency, especially when associated with a carbohydrate-rich diet, results in the disease beriberi, degeneration of the sensory nerves in the hands and feet, spreading through the limbs, with fluid retention and heart failure. Relatively acute deficiency, especially associated with alcohol abuse, results in central nervous system damage, the wernicke-korsakoff syndrome. See also thiochrome; transketolase.

results in central nervous system damage, the wernicke-korsakoff syndrome. See also thiochrome; transketolase.

VITAMIN B,

vitamin Ex dependency syndromes A very small number of children have been reported with a variant form of maple syrup urine disease in which the defect is in the binding of thiamin diphosphate to the branched chain keto acid dehydrogenase (EC 1.2.4.4). These children respond well to supplements of large amounts of vitamin B1, without the need for strict control of their intake of the amino acids. vitamin E2 Riboflavin. Coenzyme in a wide range of oxidation reactions of fats, carbohydrates and amino acids, as riboflavin phosphate (flavin mononucleotide), flavin adenine dinucleotide or covalently bound riboflavin at the active site of the enzyme. Riboflavin-dependent enzymes are collectively known as flavoproteins.

Deficiency impairs energy-yielding metabolism and results in a group of symptoms known as ariboflavinosis, including cracking of the skin at the corners of the mouth (angular stomatitis), fissuring of the lips (cheilosis) and tongue changes (glossitis); seborrhoeic accumulations appear around the nose and eyes. Not fatal because there is very efficient recycling of riboflavin in deficiency.

See also glutathione reductase; lumichrome; lumiflavin.

VITAMIN B2

vitamin B3 Term once used for pantothenic acid and sometimes, incorrectly, used for niacin. vitamin B4 Name given to what was later identified as a mixture of the amino acids arginine, glycine and cystine. vitamin B5 Name given to a substance later presumed to be identical with vitamin B6 or possibly nicotinic acid: also sometimes used for pantothenic acid. vitamin B6 Generic descriptor for three compounds (chemically derivatives of 2-methylpyridine): the hydroxyl (alcohol) compound, pyridoxine (previously known as adermin and pyridoxol); the aldehyde, pyridoxal; and the amine, pyridoxamine; and their phosphates. All are equally active biologically. The active metabolite is pyridoxal 5'-phosphate, which acts as a coenzyme in decarboxylation and transamination of amino acids, and in glycogen phosphorylase (EC 2.4.1.1), it also has a role in terminating the actions of steroid hormones.

Deficiency causes abnormalities in the metabolism of the amino acids tryptophan and methionine; in rats convulsions and skin lesions (acrodynia) and in dairy cows and dogs, anaemia with abnormal red blood cells. Dietary deficiency leading to clinical signs is not known in human beings, apart from a single outbreak in babies fed a severely overheated preparation of formula milk in the 1950s; they showed abnormalities of amino acid metabolism and convulsions resembling epileptic seizures, which responded to supplements of the vitamin.

See also methionine load test; transaminase; tryptophan load test.

pyridoxamine pyridoxal pyridoxin?

VITAMIN B6

VITAMIN B6

vitamin B6 dependency syndromes A very small number of children suffer from genetic diseases affecting the binding of pyridoxal phosphate to just one of the pyridoxal phosphate-

dependent enzymes. The abnormality is corrected by the administration of large supplements of vitamin B6. vitamin B6 toxicity High intakes of supplements of vitamin B6, in excess of 200-1000 mg/day (far in excess of what could be obtained from foods) cause peripheral sensory neuropathy. vitamin B7, B8 and B9 In the early days of nutrition research, when a new factor was discovered that was claimed to be essential for chick growth and feathering, the claimant stated that since nine factors were known the new factors should be called vitamins B10 and B11. In fact, the B vitamins had been numbered only up to B6, hence B7, B8 and B9 have never existed. B9 is sometimes (incorrectly) used for folic acid. vitamin B10 and B11 The names given to two factors claimed to be essential for chick growth and feathering; they were later shown to be a mixture of vitamin B1 and folic acid. vitamin B12 (see p. 499) Cobalamin; coenzyme for methionine synthetase (EC 2.1.1.13, important in metabolism of folic acid), methylmalonyl CoA mutase (EC 5.4.99.2) and leucine aminomu-tase (EC 5.4.3.7).

Deficiency leads to pernicious anaemia when immature red blood cells are released into the bloodstream, and there is degeneration of the spinal cord. The anaemia is the same as seen in folate deficiency, and is due to impairment of folate metabolism. There is also urinary excretion of methylmalonic acid.

Absorption of vitamin B12 requires intrinsic factor, a protein secreted in the gastric juice. Failure of absorption, rather than dietary deficiency, is the main cause of pernicious anaemia. However, B12 is found only in animal foods so strict vegetarians are at risk.

See also dump suppression test; methyl folate trap; schilling test.

vitamin B13 Orotic acid, an intermediate in pyrimidine synthesis;

no evidence that it is a dietary essential; not a vitamin. vitamin B14 Not an established vitamin; name originally given to a compound found in human urine that increases the rate of cell proliferation in bone marrow culture. vitamin B15 pangamic acid; no evidence that it has any physiological function in the body; not a vitamin. vitamin B16 This term has never been used. vitamin B17 amygdalin (laetrile); no evidence that it has any physiological function in the body; not a vitamin. vitamin BC Obsolete name for folic acid. vitamin BD Called the antiperosis factor for chicks, but can be replaced by manganese and choline (not a dietary essential for human beings).

Ariboflavinosis Pictures
VITAMIN Bi;

vitamin Bj carnitine; an essential dietary factor for the mealworm Tenebrio molitor, and certain related species, but not a dietary essential for human beings. vitamin BW Or factor W; probably identical to biotin. vitamin BX Non-existent; has been used in the past for both pantothenic acid and pa«a-amino benzoic acid. vitamin C ascorbic acid. For formula, see p. 39. It functions as a cofactor for a group of hydroxylases that also catalyse the decarboxylation of 2-oxoglutarate (including the hydroxylation of lysine and proline in the synthesis of collagen, and two hydroxylases in the synthesis of carnitine); in these reactions it is consumed, but not stoichiometrically with substrates. It is also the coenzyme for dopamine P-hydroxylase (EC 1.14.17.1) in the synthesis of noradrenaline, and peptidyl glycine hydroxylase (EC 1.14.17.3) in the post-synthetic modification of a number of peptide hormones. It is a general (non-enzymic) antioxidant, including the reduction of oxidised vitamin e in cell membranes.

Deficiency results in scurvy: seepage of blood from capillaries, subcutaneous bleeding, weakness of muscles, soft, spongy gums and loss of dental cement, leading to loss of teeth and in advanced cases deep bone pain. A lesser degree of deficiency results in impaired healing of wounds.

The requirement to prevent scurvy is less than 10mg/day; reference intakes range between 30 and 85 mg/day, depending on the criteria of adequacy adopted and the assumptions made in the interpretation of experimental data. At intakes above 100mg/day the vitamin is excreted in the urine; there is no evidence of any adverse effects at intakes up to 4000 mg/day.

Fruits and vegetables are rich sources; also used in curing ham, and as an antioxidant and bread improver.

See also dichlorophenol indophenol; erythorbic acid; iron; o-phenylene diamine; oxalic acid. vitamin D (see p. 501) Vitamin D3 is calciol or cholecalciferol; formed in the skin by the action of ultraviolet light on 7-dehy-drocholesterol, hence not strictly a vitamin. However, in northern latitudes sunlight exposure may not be adequate to meet requirements, and a dietary source becomes essential.

Vitamin D2 (ercalciol or ergocalciferol) is a synthetic vitamer produced by irradiation of ergosterol. The name vitamin D1 was given originally to an impure mixture and is not used now.

The main storage form of the vitamin is the 25-hydroxy derivative, calcidiol, in plasma; the active metabolite is the 1,25-dihydroxy derivative, calcitriol. Formation of calcitriol is regulated by the state of calcium balance.

The function of calcitriol is mainly in regulation of calcium metabolism; it acts via nuclear receptors, like a steroid hormone, and also via cell-surface receptors. Stimulates absorption of dietary calcium from the small intestine and calcium turnover in bone, by activating osteoblasts to mobilise calcium, then later recruiting and stimulating differentiation of osteoblast precursors for bone formation. Acting to regulate intracellular calcium concentrations, it is important in control of the secretion of insulin and other hormones. It also has a role (together with vitamin a) in regulation of cell differentiation and replication, and control of the cell cycle.

Deficiency causes rickets in young children, osteomalacia in adults.

Not widely distributed in foods; egg yolk, butter, oily fish and enriched margarine are the only significant sources. Reference

intakes are 10-15 ^g/day for adults, amounts that are unlikely to be obtained from unsupplemented diets.

The obsolete international unit of vitamin D = 25 ng calciol; 1mg calciol = 40IU. vitamin D resistant rickets See rickets.

vitamin D toxicity Excessive intake of vitamin D results in disturbance of calcium metabolism, resulting in hypercalcaemia, dangerously raised blood calcium concentrations, leading to raised blood pressure, and calcinosis, inappropriate deposition of calcium in soft tissues, leading to brain and kidney damage. Excessive exposure to sunlight does not lead to excessive formation of vitamin D because previtamin D undergoes further light-catalysed reactions to inactive compounds, and there is only limited availability of 7-dehydrocholesterol in the skin. vitamin E (see p. 502) Two main groups of compounds have vitamin E activity: the tocopherols and the tocotrienols; there are four isomers of each: a-, P-, y- and 8-tocopherols and a-, P-, y-and 8-tocotrienols, with differing potencies.

Deficiency symptoms vary considerably in different animal species; sterility in mouse, rat, rabbit, sheep and turkey; muscular dystrophy in several species; capillary permeability in chick and turkey; anaemia in monkey. Human dietary deficiency is unknown, but hereditary lack of P-lipoprotein leads to functional deficiency, with severe neurological damage. Premature infants may show haemolytic anaemia as a result of vitamin E deficiency.

Functions as an antioxidant in cell membranes, protecting unsaturated fatty acids from oxidative damage. It also has membrane-specific functions, and a role in cell signalling and modulation of gene expression.

The vitamin E content of foods is expressed as milligrams a-tocopherol equivalent (based on the different potency of the different vitamers). The obsolete international unit of vitamin E

CH;

HO J^ ^ B-tocoDherol

HO J^ ^ B-tocoDherol

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