Tài liệu Tự điển Food Science, Technology And Nutrition - Vần U,V,W ppt

Eleven of these must be supplied in the diet vitamins A, B1, B2, B6, B12, C, E, K, folic acid, biotin and pantothenic acid; two niacinand vitamin d can be made in the body if there is su

tion, 2 = toxin produced in host without adherence, 3 = toxin duced in host with adherence to cells, 4 = toxin produced by inva-sive bacteria, 5 = toxin produced by bacteria causing systemicinfection; type of toxin: 1 = enterotoxin, 2 = neurotoxin, 3 = non-protein toxin; target or mechanism of action; individual toxinnumber Shown as TX x.x.x.x.

pro-Tyndall effect Dispersion of light by a colloidal suspension (see

colloid), commonly determined as turbidity by measuring thelight emitted at 90 ° to the direction of incident light

typhoid Gastrointestinal infection caused by Salmonella

typhi, transmitted by food or water contaminated by faeces of

patients or asymptomatic carriers Paratyphoid is due to S.

paratyphi

tyramine The amine formed by decarboxylation of the amino

acid tyrosine; chemically p-hydroxyphenylethylamine.

tyrosinase See phenol oxidases.

tyrosine A non-essential amino acid, abbr Tyr (Y), Mr181.2, pKa

2.43, 9.11, 10.13 (—OH), codons UAPy Can be formed from theessential amino acid phenylalanine, hence it has some sparingaction on phenylalanine In addition to its role in proteins,tyrosine is the precursor for the synthesis of melanin (the blackand brown pigment of skin and hair), and for adrenaline andnoradrenaline

tyrosinosis genetic disease due to lack of

p-hydroxyphenylpyru-vate oxidase (EC 1.13.11.27), affecting the metabolism of

tyro-sine and leading to excretion of p-hydroxyphenylpyruvate in the

urine Treatment is by restriction of dietary intake of lalanine and tyrosine

pheny-tzatziki Greek; grated cucumber in yogurt, flavoured with garlic,olive oil and vinegar

U ubichromenol Cyclised derivatives of ubiquinones

ubiquinones Coenzymes in the respiratory (electron transport)chain in mitochondria, also known as coenzyme Q or mito-quinones; widely distributed in nature Chemically, derivatives ofbenzoquinone with isoprene side chains There is no evidencethat they are dietary essentials; they may have antioxidantactivity

ucuhuba butter A yellow solid fat obtained from ucuhuba nuts,

the fruit of Myristica surinamensis 90% saturated, 7%

mono-unsaturated, 3% polymono-unsaturated, vitamin E 0.6 mg/100 mL

udon Japanese; fine transparent noodles made from wheat

UFA Unesterified fatty acids, see fatty acids, non-esterified.

485

ugli citrus fruit; cross between grapefruit and tangerine, alsocalled tangelo (USA); first produced in Jamaica in 1930.

UHT See ultra-high-temperature sterilisation.

UL Tolerable upper intake level of a nutrient; maximum intake(from supplements and enriched foods) that is unlikely to pose

a risk of adverse effects on health

ulcer A crater-like lesion of the skin or a mucous membrane ing from tissue death associated with inflammatory disease, infec-tion or cancer.Peptic ulcers affect regions of the gastrointestinaltract exposed to gastric juices containing acid and pepsin: gastriculcer in the stomach and duodenal ulcer in the duodenum

result-Treatment was formerly conservative, with a bland diet, lowed if necessary by surgery Now treated by inhibition ofgastric acid secretion using histamine receptor antagonists orinhibitors of the proton pump May be caused or exacerbated by

fol-infection with Helicobacter pylori.

ulcerative colitis See colitis.

ullage Air space left in cask or bottle after some liquid has beenremoved

ultracentrifuge See centrifuge.

ultrafiltration Procedure for removal of low molecular weightcompounds from plasma, protein solutions, etc., using a semipermeable membrane and either hydrostatic pressure orcentrifugation

ultra-high-temperature sterilisation (UHT) Sterilisation at highertemperatures and for shorter times, than high-temperatureshort-time sterilisation

ultrasound Sound above the normal range of human hearing,commonly above 20 kHz

ultraviolet (UV) irradiation Light of wavelength below thevisible range Wavelength for maximal germicidal action is

260 nm; poor penetrating power and of value only for surfacesterilisation or sterilising air and water Also used for tenderis-ing and ageing of meat, curing cheese, and prevention of mouldgrowth on the surface of bakery products Ultraviolet from sun-light is responsible for skin tanning, and the formation of vitamin

d from 7-dehydrocholesterol in the skin

umami Name given to the special taste of monosodium mate, some other amino acids, protein and the ribonucleotides(inosinate and guanylate) The Japanese name for a savouryflavour, now considered one of the five basic senses of taste

gluta-umbles Edible entrails of any animal (especially deer) which used

to be made into pie, umble pie or humble pie

uncoupling proteins Proteins in mitochondria that act to ple the processes of electron transport and oxidative phospho-486

uncou-rylation, so permitting more or less uncontrolled oxidation ofmetabolic fuels, with production of heat An important part ofmaintenance of body temperature by non-shivering thermogen-esis, and maintenance of energy balance; they are stimulated byleptin.

UCP-1 (thermogenin) is the best studied It occurs in brown

adipose tissue (see adipose tissue, brown), and is activated by

free fatty acids produced in response to β-adrenergic stimulation.UCP-2 occurs in a variety of tissues, including skeletal muscleand lung; UCP-3 occurs only in skeletal muscle

uncrystallisable syrup See syrup.

unesterified fatty acids (UFA) See fatty acids, non-esterified.

UNICEF United Nations Children’s fund; web sitehttp://www.unicef.org/

universal product codes (UPC) Standard multidigit numbers thatrepresent product, size, manufacturer and nature of contents, onfood and other labels as machine-readable bar codes

unsaponifiable See non-saponified.

unsaturated fatty acids See fatty acids.

UNU United Nations University; web site http://www.unu.edu/

UPC See universal product codes.

uperisation A method of sterilising milk by injecting steam underpressure to raise the temperature to 150 °C The added water isevaporated off

uracil A pyrimidine; see nucleic acids.

urataemia High blood concentration of uric acid and its salts, as

in gout

uraturia Urinary excretion of high concentrations of uric acidand its salts

urd bean See grams, indian.

urea CO(NH2)2, the end-product of nitrogen metabolism in mostmammals, excreted in the urine Synthesised in the liver fromammonia (arising from the deamination of amino acids) and theamino acid aspartic acid It is the major nitrogenous compound

in urine, and the major component of the non-protein nitrogen

in blood plasma

urease Intestinal bacterial enzyme (EC 3.5.1.5) that hydrolysesurea to ammonia and carbon dioxide Important in the entero-hepatic cycling of urea Also found in some beans

urethane Ethyl carbamate, used as intermediate in organic theses, as a solubiliser and as the precursor for polyurethanefoam Found in small amounts in liqueurs made from stone fruits,wines and some distilled spirits where it is formed by reactionbetween alcohol and nitrogenous compounds; cause for concernsince it is genotoxic, and hence a potential carcinogen

syn-487

uric acid The end-product of purine metabolism in human beingsand other apes; most other mammals have the enzyme uricase(EC 1.7.3.3), which oxidises uric acid to allantoin, which is moresoluble in water gout is the result of excessive formation of uricacid, and/or impaired excretion; it is only slightly soluble inwater, and in excess it crystallises in joints, as gouty nodules(tophi) under the skin and sometimes in the kidney.

urobilinogen Pigment in urine derived from the bile pigments,which, in turn, are formed from haemoglobin When urine is left

to stand, the urobilinogen is oxidised in air to urobilin

urogastrone Name given to a peptide found in urine that inhibits gastric secretion, (nearly) identical to epidermal growthfactor

urwaga See orubisi.

USDA US Department of Agriculture, created as an independentdepartment in 1862; web site http://www.usda.gov/

USRDA reference intakes used for nutritional labelling offoods in the USA before the introduction of daily values

uszka Polish; type of ravioli, egg-flour dough stuffed with mushrooms

UV See ultraviolet.

V vacherin (1) Circular cakes of meringue and cream

(2) French mild cheeses made from cow’s milk; traditionallymoulded in flat circles and wrapped in a border of bark

vac-ice process Alternative name for freeze drying

vacreation deodorisation of cream by steam distillation underreduced pressure; developed in New Zealand

vacuum contact drying Or vacuum contact plate process, amethod of drying food in a vacuum oven in which the material

is heated by hot plates both above and below As the materialshrinks due to water loss, continuous contact is maintained byclosing the plates; heats the food more effectively than a simplevacuum oven

vada Indian; spiced, deep fried balls of legume flour that has beenleft overnight to undergo a lactic acid bacterial fermentation,

together with Leuconostoc mesenteroides, which produces

carbon dioxide as a leavening agent

vagotomy Surgical cutting of part of the vagus (10th cranial)nerve, usually to reduce secretion of acid and pepsin by thegastric mucosa

valerian Extracts and the essential oil of the herbaceous

peren-nial Valeriana officianalis are used as flavouring in many foods.

488

The root has traditionally been used as a sedative and liser, with evidence of efficacy.

tranquil-valgus Any deformity that displaces the hand or foot away fromthe mid-line of the body; e.g genu valgus is knock knees, as seen

in rickets

See also varus.

validity Of an assay, the extent to which a method measures what

it purports to measure

See also accuracy; precision; sensitivity; specificity.

valine An essential amino acid, abbr Val (V), Mr117.1, pKa2.29,9.74, codons GUNu; rarely, if ever, limiting in foods

valzin, valzol See dulcin.

vanadium A mineral known to be essential to experimentalanimals, although sufficiently widespread for human dietary defi-ciency to be unknown Its precise function is unknown, although

it acts as an activator of a number of enzymes

vanaspati Indian; purified hydrogenated vegetable oil; similar tomargarine and usually fortified with vitamins A and D Alsoused to prepare ghee (vanaspati ghee)

vanilla Extract of the vanilla bean, fruit of the tropical orchid

Aracus (or Vanilla) aromaticus and related species Discovered

in Mexico in 1571 and could not be grown elsewhere, becausepollination could be effected only by a small Mexican bee, untilartificial pollination was introduced in 1820 Main growingregions now Madagascar and Tahiti

The major flavouring principle is vanillin (chemically methylprotocatechuic aldehyde), but other substances present aid theflavour Ethyl vanillin is a synthetic substance which does notoccur in the vanilla bean; 3.5 times as strong in flavour, and morestable to store than vanillin, but does not have the true flavour

vanillin See vanilla.

VaporPrintTM imaging A graphical representation of the flavourprofile obtained using a znoseTM‘electronic nose’

variety meat American name for offal

varus Any deformity that displaces the hand or foot towards themid-line of the body; e.g genu varus is bow legs, as seen inrickets

See also valgus.

vasoactive intestinal peptide (VIP) Protein secreted by the pancreas; over-secretion can cause severe diarrhoea

vasoconstriction Constriction of the blood vessels; the reverse ofvasodilatation

vasodilatation (vasodilation) Dilation of the blood vessels; thereverse of vasoconstriction Caused by a rise in body tempera-ture; serves to lose heat from the body

489

vasopressin Antidiuretic hormone secreted by the pituitary; acts

to increase resorption of water in the kidneys and to constrictblood vessels

VCD See vacuum contact drying.

VDP Volatile decomposition products

veal Meat of young calf (Bos taurus) 21/2–3 months old

Composition/100 g: water 76 g, 456 kJ (109 kcal), protein 20.3 g,fat 2.5 g (of which 42% saturated, 42% mono-unsaturated, 16%polyunsaturated), cholesterol 84 mg, carbohydrate 0 g, ash 1.1 g,

Ca 17 mg, Fe 0.9 mg, Mg 25 mg, P 213 mg, K 331 mg, Na 83 mg,

Zn 3.5 mg, Cu 0.1 mg, Se 8.8µg, vitamin E 0.3mg, B1 0.09 mg,

B2 0.29 mg, niacin 7.4 mg, B6 0.45 mg, folate 13µg, B12 1.5µg,pantothenate 1.3 mg A 100 g serving is a source of Se, vitamin

B2, a good source of P, Zn, vitamin B6, pantothenate, a rich source

of niacin, vitamin B12

vegans Those who consume no foods of animal origin See

vegetarians

VegemiteTM Australian; yeast extract

vegetable See fruit.

vegetable butters See cocoa butter equivalents; cocoa butter

substitutes

vegetable oyster See salsify.

vegetable pepsin See papain.

vegetable protein products General term to include textured soya

and other bean products, often made to simulate meat (see

tex-tured vegetable protein) The basic material is termed flourwhen the protein content is not less than 50%; concentrate, notless than 65%; isolate, not less than 90% protein

vegetable spaghetti See spaghetti squash.

vegetarians Those who do not eat meat or fish, either forethical/religious reasons or because they believe that a meat-freediet confers health benefits Apart from a risk of vitamin b12defi-ciency, there are no adverse effects of a wholly meat-free diet,although vegetarian women are more at risk of iron deficiencythan those who eat meat Vitamin B12is found only in meat andmeat products, but supplements prepared by bacterial fermenta-tion (and hence ethically acceptable to the strictest of vegetari-ans) are available

The strictest vegetarians are vegans, who consume no products

of animal origin at all Those who consume milk and milk ucts are termed lacto-vegetarians; those who also eat eggs, ovo-lacto-vegetarians Some vegetarians (pescetarians) will eat fish,but not meat; demi-vegetarians eat little or no meat, or eatpoultry but not red meat

prod-veitchberry Variety of loganberry

veltol See maltol.

490

venison Meat of deer (Odocoileus spp.); traditionally game, but

now mainly farmed

Composition/100 g: water 74 g, 502 kJ (120 kcal), protein 23 g,fat 2.4 g (of which 43% saturated, 33% mono-unsaturated, 24%polyunsaturated), cholesterol 85 mg, carbohydrate 0 g, ash 1.2 g,

galactosyl-verbena A lemon flavoured herb, the leaves of Lippia citroidora.

verdoflavin Name given to a substance isolated from grass, latershown to be riboflavin (vitamin b2)

verjuice Literally green juice; sour juice of crab apples (andsometimes unripe grapes) formerly used in cooking meat, fishand game dishes Now normally replaced by lemon juice

vermicelli See pasta.

vermicide Any drug used to kill or expel intestinal parasiticworms

vermouth Fortified wine (about 16% alcohol by volume)flavoured with herbs and quinine French vermouth is dry andcolourless; Italian may be red or white and is sweet Drunk as anapéritif, either with soda or with gin or vodka (when called a

martini) Name originally derived from German Wermut for

wormwood, a toxic ingredient that was included in early mouths (as in absinthe)

ver-Sweet or Italian vermouth, 15–17% alcohol (by volume),12–20% sugar (by weight) Dry or French type 18–20% alcohol,3–5% sugar

VerseneTM Ethylenediamine tetra-acetic acid, see edta.

VervTM Calcium stearyl-2-lactate, used to reduce baking tions in flour It produces a more extensible dough, more easilymachined, and gives a loaf with better keeping properties andmore uniform structure

varia-vervain Herb (Verbena officianilis) used to make herb tea.

very low-density lipoproteins (VLDL) See lipoproteins, plasma.

vetch Old term applied generally to legumes; originally Vicia

spp., also called tares

ve-tsin See monosodium glutamate.

Vibrio cholerae The causative agent of cholera, bacterium mitted especially through water; forms an enterotoxin after

trans-491

adhering to epithelial cells in gut Infective dose 108organisms,onset 2–5 days, duration 4–6 days, TX 3.1.2.2.

vichyssoise Leek and potato cream soup, served cold

vicilin Globulin protein in pea and lentil

vicine One of the toxins in broad beans, responsible for acutehaemolytic anaemia or favism

victory bread American; recipe for bread containing soya flour tospare wheat, in a circular published by the US Secretary of Agri-culture in 1918

Vienna flour Specially fine flour used to make strudel pastry,Vienna bread and cakes

Viennese coffee Ground coffee containing dried figs

viili Finnish; yogurt made using Streptococcus cremoris as the

main organism

villi, intestinal Small, finger-like processes covering the surface ofthe small intestine in large numbers (20–40/mm2), projectingsome 0.5–1 mm into the lumen They provide a surface area ofabout 300 m2 for the absorption of nutrients from the small intestine

See also gastrointestinal tract.

vinasses The residual liquors from sugar beet molasses; containappreciable quantities of betaine

vinegar A solution of acetic acid (not less than 4%); the product

of two fermentations, first with yeast to convert sugars intoalcohol; this liquor, called gyle (6–9% alcohol), is then fermented

with Acetobacter spp to form acetic acid In most countries

vinegar is made from grape juice (wine vinegar, may be from red,white or rosé wine)

vinegar, balsamic Made from grape juice that has been trated over a low flame and fermented slowly in a series

concen-of wooden barrels; traditionally made only around Modena,Italy

vinegar, cider Made from apple juice, and known simply asvinegar in the USA

vinegar, malt Made from malted barley and may be distilled to acolourless liquid with the same acetic acid content but a moremellow flavour

vinegar, non-brewed (or non-brewed condiment) A solution ofacetic acid, 4–8%, coloured with caramel

vinegar, rice Made from saké

vine leaves Leaves of the grape vine, Vitis vinifera, used in

Mediterranean cuisine

Composition/100 g: (edible portion 95%) water 73 g, 389 kJ (93 kcal), protein 5.6 g, fat 2.1 g (of which 20% saturated, 7%mono-unsaturated, 73% polyunsaturated), carbohydrate 17.3 g492

(6.3 g sugars), fibre 11 g, ash 1.6 g, Ca 363 mg, Fe 2.6 mg, Mg 95 mg,

viosterol Irradiated ergosterol; vitamin d2

VIP See vasoactive intestinal peptide.

Virginia date See persimmon.

VirolTM A vitamin preparation based on malt extract

virpa See sowans.

viscera The organs within a body cavity, used especially for theabdominal viscera, liver, spleen, gastrointestinal tract,kidneys, etc

viscoelastic Material such as cheese, dough or gelled food, that

has both viscous and elastic properties (see viscosity); when a

shear stress is removed it does not return to its original shape,but is deformed

viscogen Thickening agent for whipping cream Two parts of lime (calcium oxide) in six parts of water, added to five parts

of sugar in ten parts of water; used at the rate of 3–6 g/L of cream

viscometer Instrument for measuring the viscosity of liquids

viscosity Of a liquid or gas, its resistance to flow Decreases withincreasing temperature for liquids, but increases for gases.Dynamic viscosity is the ratio of shear stress : shear rate.Kinematic viscosity is dynamic viscosity/density

See also dilatant; plastic fluids; pseudoplastic; reynolds

number; rheopectic; thixotropic; viscoelastic

viscosity, dynamic (or absolute) The ratio of shear stress : shearrate for fluids that exhibit a linear relationship between shearstress and shear rate (Newtonian flow)

vision The process of vision is mediated by photosensitive ments formed by reaction between retinaldehyde (vitamin aaldehyde) and the protein opsin The pigments are known vari-ously as visual purple (because of its colour), rhodopsin (in therod cells of the retina) and iodopsin (in the cone cells, with sen-sitivity to different wavelengths of light in different cells) Expo-sure to light results in bleaching of the pigment, with loss of theretinaldehyde and a conformational change in the protein, whichleads to closure of a sodium channel in the retinal cell, and ini-tiation of a nerve impulse

pig-visual pigments, pig-visual purple See vision.

vitafoods Foods designed to meet the needs of health-conscious

493

consumers that enhance physical or mental quality of life andmay increase health status.

vitamers Chemical compounds structurally related to a vitamin,and converted to the same active metabolites in the body Theythus possess the same kind of biological activity, although some-times with lower potency

When there are several vitamers, the group of compoundsexhibiting the biological activity of the vitamin is given a genericdescriptor (e.g vitamin a is the generic descriptor for retinoland its derivatives as well as several carotenoids)

vitamin There are 13 organic compounds (thus excluding traceminerals) essential to human life in very small amounts Eleven

of these must be supplied in the diet (vitamins A, B1, B2, B6, B12,

C, E, K, folic acid, biotin and pantothenic acid); two (niacinand vitamin d) can be made in the body if there is sufficient ofthe amino acid, tryptophan, and sunlight, respectively The word

may be pronounced either veitamin or vittamin.

Vitamins A, D, E and K are grouped together as fat-solublevitamins, because they are soluble in lipids, but not in water Vitamin C and the B vitamins (including pantothenic acid, biotin and folic acid) are grouped together as the water-soluble vitamins since they are all soluble in water, but not lipids

vitamin A (see p 495) Fat-soluble vitamin, occurring either as the

preformed vitamin (retinol) found in animal foods or as a cursor (carotenes) found in plant foods Required for control ofgrowth, cell turnover and fetal development, maintenance of fer-tility and maintenance of the normal moist condition of epithe-lial tissues lining the mouth and respiratory and urinary tracts;essential in vision The main active metabolites in the body are

pre-retinaldehyde, all-trans- and 9-cis-retinoic acids.

Deficiency leads to slow adaptation to see in dim light (poordark adaptation), later to night blindness; then drying of thetear ducts (xerophthalmia) and ulceration of the cornea (ker-atomalacia) resulting in blindness

The vitamin A content of foods is expressed as retinol equivalents, i.e retinol plus carotene; 1µg retinol = 6µg β-carotene = 12µg other active carotenoids = 3.33 internationalunits

See also conjunctival impression cytology; relative dose

response test; retinol binding protein; vision

vitamin A toxicity Retinol in excess of requirements is stored inthe liver, bound to proteins, and is a cumulative poison Whenthe storage capacity is exceeded, free retinol causes damage tocell membranes carotene is not toxic in excess, since there isonly a limited capacity to form retinol from carotene

494

The recommended upper limits of habitual daily intake ofretinol are about 12.5 × reference intake for adults, but only 2.5

× reference intake for infants Retinol is also teratogenic inexcess, and for pregnant women the recommended upper limit

of daily intake is 3000–3300µg

vitamin A 2 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 sep-aration from what was known originally as ‘vitamin B’; hence,they are grouped together as the B complex

vitamin B 1 Thiamin Thiamin diphosphate is a coenzyme inmetabolism of glucose, and in the citric acid cycle Thiamintriphosphate has a role in nerve conduction, by activating a chlo-ride channel Deficiency, especially when associated with a carbohydrate-rich diet, results in the disease beriberi, degenera-tion of the sensory nerves in the hands and feet, spreadingthrough the limbs, with fluid retention and heart failure Rela-tively acute deficiency, especially associated with alcohol abuse,

495

VITAMIN A

results in central nervous system damage, the wernicke–korsakoff syndrome.

See also thiochrome; transketolase.

496

vitamin B 1 dependency syndromes A very small number of dren have been reported with a variant form of maple syrupurine disease in which the defect is in the binding of thiamindiphosphate to the branched chain keto acid dehydrogenase (EC1.2.4.4) These children respond well to supplements of largeamounts of vitamin B1, without the need for strict control of theirintake of the amino acids

chil-vitamin B 2 Riboflavin Coenzyme in a wide range of oxidationreactions of fats, carbohydrates and amino acids, as riboflavinphosphate (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 ing 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 Notfatal because there is very efficient recycling of riboflavin in deficiency

crack-See also glutathione reductase; lumichrome; lumiflavin.

VITAMIN B 1

B

vitamin B 3 Term once used for pantothenic acid and sometimes,incorrectly, used for niacin.

vitamin B 4 Name given to what was later identified as a mixture

of the amino acids arginine, glycine and cystine

vitamin B 5 Name given to a substance later presumed to be tical with vitamin B6or possibly nicotinic acid: also sometimesused for pantothenic acid

iden-vitamin B 6 Generic descriptor for three compounds (chemicallyderivatives of 2-methylpyridine): the hydroxyl (alcohol) com-pound, pyridoxine (previously known as adermin and pyridoxol);the aldehyde, pyridoxal; and the amine, pyridoxamine; and their phosphates All are equally active biologically The activemetabolite is pyridoxal 5′-phosphate, which acts as a coenzyme

in decarboxylation and transamination of amino acids, and inglycogen phosphorylase (EC 2.4.1.1), it also has a role in termi-nating the actions of steroid hormones

Deficiency causes abnormalities in the metabolism of theamino acids tryptophan and methionine; in rats convulsions andskin lesions (acrodynia) and in dairy cows and dogs, anaemiawith abnormal red blood cells Dietary deficiency leading to clin-ical signs is not known in human beings, apart from a single out-break in babies fed a severely overheated preparation of formulamilk in the 1950s; they showed abnormalities of amino acidmetabolism and convulsions resembling epileptic seizures, whichresponded to supplements of the vitamin

See also methionine load test; transaminase; tryptophan

load test

497

vitamin B 6 dependency syndromes A very small number of dren suffer from genetic diseases affecting the binding of pyridoxal phosphate to just one of the pyridoxal phosphate-

chil-VITAMIN B 6

dependent enzymes The abnormality is corrected by the istration of large supplements of vitamin B6.

admin-vitamin B 6 toxicity High intakes of supplements of vitamin B6,

in excess of 200–1000 mg/day (far in excess of what could beobtained from foods) cause peripheral sensory neuropathy

vitamin B 7 , B 8 and B 9 In the early days of nutrition research, when

a new factor was discovered that was claimed to be essential forchick growth and feathering, the claimant stated that since ninefactors were known the new factors should be called vitamins B10and 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 B 10 and B 11 The names given to two factors claimed to beessential for chick growth and feathering; they were later shown

to be a mixture of vitamin B1and folic acid

vitamin B 12 (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 redblood cells are released into the bloodstream, and there is degen-eration of the spinal cord The anaemia is the same as seen infolate deficiency, and is due to impairment of folate metabolism.There is also urinary excretion of methylmalonic acid

Absorption of vitamin B12requires intrinsic factor, a proteinsecreted in the gastric juice Failure of absorption, rather thandietary deficiency, is the main cause of pernicious anaemia.However, B12is found only in animal foods so strict vegetariansare at risk

See also dump suppression test; methyl folate trap;

schilling test

vitamin B 13 Orotic acid, an intermediate in pyrimidine synthesis;

no evidence that it is a dietary essential; not a vitamin

vitamin B 14 Not an established vitamin; name originally given to

a compound found in human urine that increases the rate of cellproliferation in bone marrow culture

vitamin B 15 pangamic acid; no evidence that it has any logical function in the body; not a vitamin

physio-vitamin B 16 This term has never been used

vitamin B 17 amygdalin (laetrile); no evidence that it has anyphysiological function in the body; not a vitamin

vitamin B C Obsolete name for folic acid

vitamin B D Called the antiperosis factor for chicks, but can bereplaced by manganese and choline (not a dietary essential forhuman beings)

498

vitamin B T carnitine; an essential dietary factor for the

meal-worm Tenebrio molitor, and certain related species, but not a

dietary essential for human beings

vitamin B W Or factor W; probably identical to biotin

vitamin B X Non-existent; has been used in the past for both pantothenic acid and PARA-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 thedecarboxylation of 2-oxoglutarate (including the hydroxylation

of lysine and proline in the synthesis of collagen, and twohydroxylases in the synthesis of carnitine); in these reactions it

is consumed, but not stoichiometrically with substrates It is alsothe coenzyme for dopamine β-hydroxylase (EC 1.14.17.1) in the

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VITAMIN B12

synthesis of noradrenaline, and peptidyl glycine hydroxylase(EC 1.14.17.3) in the post-synthetic modification of a number ofpeptide 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 capillar-ies, subcutaneous bleeding, weakness of muscles, soft, spongygums and loss of dental cement, leading to loss of teeth and inadvanced cases deep bone pain A lesser degree of deficiencyresults in impaired healing of wounds.

The requirement to prevent scurvy is less than 10 mg/day; erence intakes range between 30 and 85 mg/day, depending onthe criteria of adequacy adopted and the assumptions made inthe interpretation of experimental data At intakes above

ref-100 mg/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 north-ern latitudes sunlight exposure may not be adequate to meetrequirements, and a dietary source becomes essential

Vitamin D2(ercalciol or ergocalciferol) is a synthetic vitamerproduced by irradiation of ergosterol The name vitamin D1wasgiven 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 isregulated by the state of calcium balance

The function of calcitriol is mainly in regulation of calciummetabolism; it acts via nuclear receptors, like a steroid hormone,and also via cell-surface receptors Stimulates absorption ofdietary calcium from the small intestine and calcium turnover inbone, by activating osteoblasts to mobilise calcium, then laterrecruiting and stimulating differentiation of osteoblast precur-sors for bone formation Acting to regulate intracellular calciumconcentrations, it is important in control of the secretion ofinsulin and other hormones It also has a role (together withvitamin a) in regulation of cell differentiation and replication,and control of the cell cycle

Deficiency causes rickets in young children, osteomalacia inadults

Not widely distributed in foods; egg yolk, butter, oily fish andenriched margarine are the only significant sources Reference500