Vitamin A

VitaminA•DeficiencyofvitaminAisthemostcommoncauseofnon-accidentalblindness,worldwide•Preformed–Retinoids(retinal,retinol,retinoicacid)–Foundinanimalproducts•ProvitaminA–Carotenoids–Mustbeconvertedtoretinoidform–Intestinalcellscansplitcaroteneintwo(moleculesofretinoids)–FoundinplantproductsTerminalEndsofRetinoidsConversionofCarotenoidstoRetinoids•Enzymaticconversionofcarotenoidsoccursinliverorintestinalcells,formingretinalandretinoicacid•ProvitaminAcarotenoids–Beta-carotene–Alphacarotene–Beta-cryptoxanthin•Othercarotenoids–Lutein–Lycopene–ZeaxanthinAbsorptionofVitaminA•Retinoids–Retinylestersbrokendowntofreeretinolinsmallintestine-requiresbile,digestiveenzymes,integrationintomicelles–Onceabsorbed,retinylestersreformedinintestinalcells–90%ofretinoidscanbeabsorbed•Carotenoids–Absorbedintact,absorptionratemuchlower–IntestinalcellscanconvertcarotenoidstoretinoidsTransportandStorageofVitaminA•Liverstores90%ofvitaminAinthebody•Reserveisadequateforseveralmonths•Transportedviachylomicronsfromintestinalcellstotheliver•Transportedfromthelivertotargettissueasretinolviaretinol-bindingprotein,whichisboundtotransthyretinRetinoidBindingProteins•Targetcellscontaincellularretinoidbindingproteins–Directretinoidstofunctionalsiteswithincells–Protectretinoidsfromdegradation•RAR,RXRreceptorsonthenucleus–Retinoid-receptorcomplexbindstoDNA–DirectsgeneexpressionExcretionofVitaminA•Notreadilyexcreted•Somelostinurine•KidneydiseaseandagingincreaseriskoftoxicitybecauseexcretionisimpairedFunctionsofVitaminA:Vision•Retinalturnsvisuallightintonervesignalsinretinaofeye•Retinoicacidrequiredforstructuralcomponentsofeye–Conesintheretina•Responsibleforvisionunderbrightlights•Translateobjectstocolorvision–Rodsintheretina•Responsibleforvisionindimlights•TranslateobjectstoblackandwhitevisionTheVisualCycleFunctionsofVitaminA:GrowthandDifferentiationofCells•Retinoicacidisnecessaryforcellulardifferentiation•Importantforembryodevelopment,geneexpression•Retinoicacidinfluencesproduction,structure,andfunctionofepithelialcellsthatlinetheoutside(skin)andexternalpassages(mucusformingcells)withinthebodyFunctionsofVitaminA:Immunity•Deficiencyleadstodecreasedresistancetoinfections•SupplementationmaydecreaseseverityofinfectionsindeficientpersonVitaminAAnalogsforAcne•Topicaltreatment(Retin-A)–Causesirritation,followedbypeelingofskin–Antibacterialeffects•Oraltreatment–Regulatesdevelopmentofskincells–CautionregardingbirthdefectsPossibleCarotenoidFunctions•Preventionofcardiovasculardisease–Antioxidantcapabilities–≥5servings/dayoffruitsandvegetables•Cancerprevention–Antioxidantcapabilities–Lung,oral,andprostatecancers–StudiesindicatethatvitaminA-containingfoodsaremoreprotectivethansupplements•Age-relatedmaculardegeneration•Cataracts•Ingeneral,foodsrichinvitaminAandotherphytochemicalsareadvisedratherthansupplementsVitaminAinFoods•Preformed–Liver,fishoils,fortifiedmilk,eggs,otherfortifiedfoods–Contributes~70%ofvitaminAintakeforAmericans•ProvitaminAcarotenoids–Darkleafygreen,yellow-orangevegetables/fruitsRDAforVitaminAforAdults•900RAEformen•700RAEforwomen•AverageintakemeetsRDA•Muchstoredintheliver•VitaminAsupplementsareunnecessary•NoseparateRDAforcarotenoidsDeficiencyofVitaminA•Mostsusceptiblepopulations:–PreschoolchildrenwithlowF&Vintake–Urbanpoor–Olderadults–Alcoholism–Liverdisease(limitsstorage)–Fatmalabsorption•Consequences:–Nightblindness–Decreasedmucusproduction–Decreasedimmunity–Bacterialinvasionoftheeye–Conjunctivalxerosis–Bitot’sspots–Xerophthalmia–Irreversibleblindness–Follicularhyperkeratosis–PoorgrowthUpperLevelforVitaminA•3000μgretinol•HypervitaminosisAresultsfromlong-termsupplementuse(2–4xRDA)•Toxicity•Fataldose(12g)ToxicityofVitaminA–Acute–short-termmegadose(100xRDA);symptomsdisappearwhenintakestops•GIeffects•Headaches•Blurredvision•PoormusclecoordinationToxicityofVitaminA–Chronic–long-termmegadose;possiblepermanentdamage•Boneandmusclepain•Lossofappetite•Skindisorders•Headache•Dryskin•Hairloss•Increasedliversize•VomitingToxicityofVitaminA•Teratogenic(mayoccurwithaslittleas3xRDAofpreformedvitaminA)–TendstoproducephysicaldefectondevelopingfetusasaresultofexcessvitaminAintake–Spontaneousabortion–BirthdefectsHealthEffectsofVitaminAToxicityofCarotenoids•Notlikely,asrateofconversionofcarotenoidstoretinoicacidbyliverisslowandefficiencyofabsorptionofcarotenoidsdecreasesasintakeincreases•Hypercarotenemia–Highamountsofcarotenoidsinthebloodstream–Excessiveconsumptionofcarrots/squash/beta-carotenesupplements–Skinturnsayellow-orangecolor•WhatarethefunctionsofvitaminA?•WhatarethetwoformsofvitaminAandinwhatfoodscantheybefound?•HowdoesvitaminAhelpwithnightvision?•WhataretheeffectsofadietthatisdeficientinvitaminA?•WhataretheeffectsofadietthatistoxicinvitaminA?ContentReview