chapter 6 脂肪 英文版 营养学Word文档格式.docx

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