chapter 6 脂肪 英文版 营养学Word文档格式.docx
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chapter 6 脂肪 英文版 营养学Word文档格式.docx
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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'
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
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