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1、09年6月英语六级快速阅读实战练习 Abalanceddietisonethatprovidesanadequateintakeofenergyandnutrientsformaintenanceofthebodyandthereforegoodhealth.Adietcaneasilybeadequatefornormalbodilyfunctioning,yetmaynotbeabalanceddiet. Carbohydrates Carbohydratesarearapidsourceofenergy,the
2、yarethebody'sfuel.Thebulkofabalanceddietshouldbemadefromcarbohydrates.Ifeateninanexcessofthedietaryrequirementscarbohydratesareeasilystoredasfatsinthecells,althoughcarbohydrateisthefirstsourceofenergyinthebody. Anaverageadultrequiresabout12,000kJofenergyaday,mos
3、tofthisissuppliedbytherespirationofcarbohydratesinthecells. Carbohydratesareusedprincipallyasarespiratorysubstrates,i.e.tobeoxidisedtoreleaseenergyforactivetransport,macromoleculesynthesis,celldivisionandmusclecontraction.Carbohydratesaredigestedintheduodenumand
4、ileumandabsorbedasglucoseintocells.Sourcesofcarbohydratessuchasstarcharerice,potatoes,wheatandothercereals.Sugarsarealsocarbohydrates,sourcesofsugarsarerefinedsugar-sucrose,whichisafoodsweetenerandpreservativeandfruitsugars-fructose.Ifthedietlackscarbohydratestor
5、esoffataremobilisedandusedasanenergysource. Proteins Proteinisnotadirectsourceofenergyinthebody,itisusedprimarilyforgrowthandrepairofbodytissueswhileremaininganenergysourceasalastresort.Proteinsfulfillawidevarietyofrolesinthebody.Theyarebrokendowninthestomachan
6、dintestinestoaminoacidswhicharethenabsorbed.Thebodycanonlyform8aminoacidstobuildproteinsfrom,thedietmustprovideEssentialAminoAcids(EAAs)whicharesynthesisedintoproteinswhichcanbestructural,i.e.collageninbone,keratininhair,myosinandactininmuscle;metabolicenzymes,he
7、moglobin,protectiveantibodiesandcommunicativehormones. Sourcesofproteinincludemeat,fish,eggsandpulses.Thedietneedstoprovide8EAAsasthebodyisunabletosynthesisproteinswithoutthesemolecules.2otheraminoacidsaresynthesisedfromEAAssoifthedietlackstheoriginalEAAstheseot
8、hertwowillnotbepresenteither.Phenylalanineisconvertedtotyrosineandmethionineisconvertedtocysteine.Cellsdrawuponapoolofaminoacidsforproteinsynthesiswhicheitherc
