chapter 6 脂肪 英文版 营养学.docx

chapter 6 脂肪 英文版 营养学.docx

《chapter 6 脂肪 英文版 营养学.docx》由会员分享，可在线阅读，更多相关《chapter 6 脂肪 英文版 营养学.docx（26页珍藏版）》请在冰豆网上搜索。

chapter6脂肪英文版营养学

LipidBasics

Idon'thaveanycleversongparodiesthisweek.:

（Iknow,it'sdisappointing.Maybeyoucanmakeoneupandpostitintheforums?

Ourfabulousandexcitingtopicthisweekislipids!

Sotasty,andsoabused.Let'sstartwiththebasics,asusual.

Technicallyspeaking,therearethreetypesofmoleculesthatfallunderthecategoryof"lipids."Theseare triglycerides, phospholipids,and sterols.It'sprettyeasytoseethatbothtriglyceridesandphospholipidshavefattyacidandglycerolcomponents...butwhataboutsterols?

Theirchemicalstructurelookscompletelydifferent.Evenso,theonethingthatallthreehaveincommonistheirhydrophobicnature,andthisisthebasisoftheircategorizationaslipids.

AsIjustmentioned,bothphospholipidsandtriglycerideshavefattyacidcomponents,solet'stakeacloserlookattheseforamoment. Each fattyacid hasamethylgroup ononeendanda carboxylgroup ontheother.Theseareconnectedbya"carbonbackbone."Thisbackbonecanbejustafewormanycarbonsinlength.Thefattyacidsinfoodsmostcommonlycontainanevennumberofcarbonsintheirchains,andthisisimportantbecauseit'sthecarbonbackbonethatultimatelydeterminestheconsistencyandthetasteofthefat.

Therearetwofeaturesofthecarbonbackbonethatmostinfluenceit'sconsistency.Thefirstisthe length ofthecarbonbackbone.Ifthefattyacidhas2-4carbons,it'sconsideredtobea"short-chain"fattyacid.Ifithas6-10,it'sa"medium-chain"andifithas12ormorecarbonsit'sa"long-chain"fattyacid.Theshorterthefattyacid,themorelikelyitistobeliquidatroomtemperature;conversely,thelongerthefattyacid,themorelikelyitistobesolidatroomtemperature.

Thesecondmajorfactoraffectingconsistencyisthesaturationofthecarbonchain.Rememberfromchemistryclassthatcarboniscapableofmakingfourbonds.Whenallthecarbonsinafattyacidchainareallmakingfourbondseach,thefattyacidissaturated.Ifanyofthecarbonsinthechainaremakingadoublebonds,though,thenthatfattyacidisunsaturated.Ifthere'sjustonedoublebond,it'sa monounsaturated fat,whiletwoormoredoublebondsmakeafattyacid polyunsaturated.Asfarasconsistencygoes,themoresaturatedafattyacidis,themorelikelyitistobesolidatroomtemperature.Fattyacidsthatarepolyunsaturatedaremorelikelytobeliquid.Attheendoftheday,boththelengthandsaturationofthechaininfluencehowliquidorsolidthefatis.Scientistsgobackandforthrepeatedlybetweenmono-versuspolyunsaturatedasfaraswhichtypeoffatisthe"healthiest"toincludeinourdiets.Currentthinkingisthatoncewefulfilourminimumneedforpolyunsaturatedfat（discussedinthenextsection）it'sbesttogettherestofourfatfrommonounsaturatedsources.Weshouldminimizetheamountofsaturatedfatsinourdietasmuchaspossible.

It'sonlyunsaturatedfatsthatcanbe essential.Saturatedfats,whileextemelyimportant,arenotessentialbyournutritiondefinition-yourbodyhastheabilitytomakeallitneedssothere'snoneedforyoutogettheminyourdiet.Ifafatisunsaturated,wecanlookat thepositionofthefirstdoublebond inordertofigureoutwhetherit'sanessentialfattyacidornot.Whenmakingthedetermination,wenumberthemethylcarbonas"carbon#1."Iknow,Iknow,thisdoesnotfollowtherulesyouweretaughtinorganicchemistry.Idon'tknowwhyit'sdonethiswayforfattyacids,butitis.Ifitmakesyoufeelbetteratall,themethylcarbonisalsocalledthe"omegacarbon."Fromcarbon#1wecountdownthechaintothefirstcarboninvolvedinadoublebond.Ifthisendsupbeingbeforetheninthcarboninthechain,it'sanessentialfattyacid（a.k.a.essentialfat）.

Checkoutthepolyunsaturatedfattyacidinthepicture,above.Noticethatthefirstdoublebondoccursatcarbonnumbersix.Thisisan omega-6 fattyacid!

Themostcommonexampleofanomega-6fattyacidislinoleicacid.Ontheotherhand,ifthefirstdoublebondwereatcarbonnumberthree,itwouldbean omega-3 fattyacid.Theomega-3'sthatyou'llhearthemostaboutarealpha-linolenicacid,docosahexaenoicacid（DHA）andeicosapentaenoicacid（EPA）.Now,althoughhumansdoneedtoobtaintheseessentialfattyacidsinourdiet,itdoesn'ttakemuch-theaverageadultneedsonly2-4tablespoonsofplantoilseachdaytofulfilltherequirement.Fattyfishlikesalmon,halibut,whitefish,etc.arealsoagreatsource-aslongasthey'rebakedorbroiledandnotdeep-fried.

Theomega-3andomega-6fattyacidshavesomevitalbiologicalroles.Mostimportantly,theyaretheprecursorstomoleculescalled eicosanoids.Iknowyourtextbookgoesintosomeprettygorydetailaboutthevariouseicosanoidsandtheirfunctions.However,forourpurposes,Iwouldbethrilled