营养学基础.ppt

第一篇营养学PART1NUTRITION 1 营养 是指人体摄取 消化吸收 利用食物中营养物质和排除废物的生物学过程 Itisabiologicalprocesswhichnutrientsandfunctionalcompositionsinthefoodsareingested digested absorbed transported utilizedandthewasteareexcreted 2 营养素 1 定义 食物中可给人体提供能量 机体构成成分和组织修复以及有生理调节功能的化学成分 Chemicalsubstancesobtainedfromfoodandusedinthebodytoprovideenergy structuralmaterials andrepairofthebody stissues Nutrientsmayalsoreducetheriskofsomediseases 2 Classification proteinlipidsmacronutrientscarbohydrate dietaryfiber nutrientswatervitaminmicronutrientsmacroelementsminerals核酸nucleicacid microelements 3 合理营养 RationalDiet 通过合理的膳食和科学地烹调加工 能向机体提供足够数量的热能和各种营养素 并保证各种营养素之间的数量平衡 以满足人体的正常生理需要 并保持人体健康 Nutrientsintakesmeettherequirementsofthebody 5 膳食营养供给量 RecommendedDietaryAllowance RDA 是对各种人群提出的保证人体营养需要的膳食中应含有的热能和营养素的适宜量 Theaveragedailyamountofanutrientconsideredadequatetomeettheknownnutrientneedsofpracticallyallhealthypeople agoalfordietaryintakebyindividuals 4 营养生理需要量 NutritionalRequirement 是指能保持人体健康 达到应有发育水平和能充分有效地完成各项体力和脑力活动所需要的热能和各种营养素的必需量 Thelowestcontinuingintakeofenergyandnutrientsthatmaintainthespecifiedcriterionofadequacy thatwillmaintainthespecificbiochemicalandphysiologicalfunctionsofthebody Chapter1BasicNutrition第一章营养学基础 含量 16 19 更新 动态平衡 不断更新 每天约3 肠和骨髓最快 Section1Protein 蛋白质 一 蛋白质 一 人体中蛋白质 二 功能 1 机体构成成分2 构成各种生理活性物质和重要的生理功能3 提供热能 三 Thefirststructure 蛋白质 protein 氨基酸 Aminoacid AA 非必需AA nonessential 9种半必需AA 条件必需AA 2种 semi orconditionally 必需AA essentional 9种 多肽 poly 10AA以上寡肽 oligo 4 10AA以下3肽 tri 3AA2肽 di 2AA 肽 peptide 二 氨基酸和必需AA essentialaminoacid EAA 1 种类 20种 1 必需AA EAA 2 非必需氨基酸 NEAA 3 半必需氨基酸 semi EAAorconditionalEAA 半胱酸 Cysteine 蛋氨酸 Methionine 酪氨酸 Tyrosine 苯丙氨酸 Phenylalanine 2 必需氨基酸 人体不能合成或合成速度不能满足机体需要 必需从食物中直接获得的AA Aminoacidsthatthebodycannotsynthesizeinamountsufficienttomeetphysiologicalneedsandhastoobtainthemfromfoods 表1 1人体内的氨基酸 组氨酸为婴儿必需氨基酸 成人需要量可能较少 摘自ModernNutritioninHealthandDisease 第9版 第14页 1999 三 氨基酸模式 AAPattern 和限制氨基酸 limitingAA 1 氨基酸模式 AAPattern 蛋白质中各种必需氨基酸的构成比例 表1 2几种食物和人体蛋白质氨基酸模式 数据摘自营养与食品卫生学 第3版 第9页 2 限制氨基酸 limitingaminoacid 与人体蛋白质模式比较 食物蛋白质中含量相对较低的必需AA Theessentialaminoacidfoundintheshortestsupplyrelativetotheamountneededforproteinsynthesisinthebody FourAAaremostlikelytobelimiting Lysine Methionine Threonine Tryptophan 3 参考蛋白 完全蛋白和蛋白质的互补作用 参考蛋白 用于测定食物蛋白质质量的标准蛋白 ReferenceProtein Astandardagainstwhichtomeasurethequalityofotherproteins 完全蛋白 必需氨基酸模式与人体蛋白质相对一致 能满足机体蛋白质合成需要的食物蛋白质 CompleteProtein Adietaryproteincontainingalltheessentialaminoacidsinrelativelythesameamountsthathumanbeingrequired includinganimalproteinandsoyprotein Incompleteprotein 蛋白质互补 将两种或两种以上的食物蛋白质混合事用 使不同食物中必需氨基酸以多补少 从而提高膳食蛋白质的营养价值 ComplementaryProtein Twoormoreproteinswhoseaminoacidsassortmentscomplementeachotherinsuchawaythattheessentialaminoacidsmissingfromonearesuppliedbytheother 几种食物混合后蛋白质的生物价 三 蛋白质的消化 吸收和代谢 一 消化和吸收 digestionandabsorption GastricJuice 1 Acid denaturation2 Pepsin nothighlyspecificbutpreferentialattackonbondsadjacenttoaromaticAAandleucine 3 Carboxypeptidase terminalaromaticorbranch chained arginineorlysine neutral dietaryProtein Panc elastase 弹性蛋白酶 arometicAAreatictrypsin 胰蛋白酶 NH2 juicechymotrypsine 糜蛋白酶 lysine arginine dipeptide tripeptideandfreeAA absorbedthroughthemucosalcellsofsmallintestinepeptidase throughthreeactivefreeAAtransportsystemsneutral basic andacidic Liverandotherorgans 二 代谢与氮平衡 metabolismandnitrogenbalance 1 代谢 摄入蛋白质90克 消化道 30 肌肉 50 器官 体液 其它 机体蛋白质 氨基酸池 粪便10g 1 6gN 尿75g 12gN 其它5g 0 8gN 20 肠道内源性蛋白质70g 消化 吸收蛋白质150g 图1 1蛋白质代谢及氮平衡 2 氮平衡 Nitrogenbalance 必要的氮损失 obligatorynitrogenlosses 2 氮平衡 B I U F S Positivenitrogenbalance Nin Nout Nin Nout Nitrogenequillibrium Negativenitrogenbalance Nin Nout 四 食物蛋白质营养学评价 1 真消化率 true 一 蛋白质含量 content 2 表观消化率 apparent 二 蛋白质消化率 digestibility 三 蛋白质利用率 utilization 1 Biologicalvalue BV 生物价 Theamountofproteinnitrogenthatisretainedforgrowthandmaintenance expressedasapercentageoftheproteinnitrogenthathasbeendigestedandabsorbed Todeterminetheactualvalueofanabsorbedproteinasitisusedbythebody 2 NetProteinUtilization NPU蛋白质净利用率 Theamountofproteinnitrogenthatisretainedfromagivenamountofproteinnitrogeneaten Todeterminetheaminoacidcompositionanddigestibilityofatestprotein 3 ProteinEfficiencyRatio PER 蛋白质功效比值 Ameasureofproteinqualityassessedbydetermininghowwellagivenproteinsupportsweightgainingrowingrats Usedtoestablishtheproteinqualityforinfantformulasandbabyfoods 4 AminoAcidScore AAS氨基酸评分 ThelowestscoreisthescoreoflimitingAAwhichistheAASoftheprotein Amethodofevaluatingproteinqualitybycomparingatestprotein saminoacidpatternwiththatofareferenceprotein sometimescalledchemicalscoring 表1 4几种食物和不同人群需要的氨基酸评分模式 摘自WTOTechnicalReportSeries724 第12页 1985年 表1 5常见几种食物蛋白质质量 摘自 营养与食品卫生学 第3版 第11页 5 ProteinDigestibilitycorrectedAAS PDCAAS 经消化率校正的氨基酸评分 AmeasureofproteinqualityassessedbycomparingtheaminoacidscoreofafoodproteinWiththeaminoacidrequirementsofpreschool ageChildrenandthencorrectingforthetruedigestibilityoftheprotein RecommendedbytheFAO WHOandusedtoestablishproteinqualityoffoodsfordailyvaluepercentageonfoodlabels 表1 6几种食物蛋白质的PDCAAS 摘自UnderstandingNutrition 第7版 第215页 1996年 五 蛋白质营养不良 protein energymalnutrition PEM Kwishiorkor AGhanaianwordmeaning theevilspiritthatinfectsthefirstchildwhenthesecondchildisborn Thesecondchildsuddenlyswitchedfromnutrientdense protein richbreastmilktoastarchy protein poorcereal soonbeginstosickenanddie Marasmus Aseveredeprivationoffoodoveralongtime chronicPEM andthereforeiscausedbyaninadequateenergyandproteinintake andbyinadequateessentialfattyacids vitamins andmineralsaswell 蛋白质营养不良 Kwashiorkor Marasmus Marasmus Kwashiorkor Infancy lessthan2yr olderinfantsandyoungchildren 1 3yr Developsslowly chronicPEM rapidonset acutePEM Severeweightloss someweightloss Severemusclewasting withfat Somemusclewasting withretentionofsomebodyfat Growth 60 weight for age growth 60 80 weight for age Nodetectableedema edema Nofattyliver enlargedfattyliver Anxiety焦急 apathy淡漠 apathy misery痛苦 irritability易怒 sadness Goodappetitepossible lossofappetite Hairissparse thinanddry Easilypulledout hairisdryandbrittle easilypullout changescolor becomesstraight Skinisdry thin andeasilywrinkles skindevelopslesion 3 高蛋白质膳食过多的危害 problemsofexcessprotein AdultBoneLoss Osteoporosis骨质疏松 Calciumexcretionrisesasproteinintakeincreases Recommen dationofcalcium to proteinismorethan20 mg to1 g Cancer populationstudiessuggestacorrelationbetweenhighintakesofanimalproteinandsometypecancer cancerofcolon breast kidneys pancreas andprostate GainWeight Protein richfoodsareoftenfat richfoods Others HeartDisease Foodsrichinanimalproteintendtoberichinsaturatedfatsandcholesterol homocystei 高半胱氨酸 maybeanindependentriskfactorforheartdisease 六 蛋白质供给量和食物来源 DRIsandFoodsource RNI 1 数量2 质量 优质蛋白质 Assessmentofproteinnutritioninhumanbody 1 Serumalbumin 35 50g L 2 Serumtransferritin 2 2 4g L 3 Retinol bindingprotein 26 76mg L 4 Thyroxine binding albumin 280 350mg L3 FoodSource animalandplantfoods MilkandSoybeen 中国居民膳食蛋白质的推荐摄入量 RNI与RDA RNI与RDA 续表 Section2Lipids 脂类 固醇类 sterols 脂类 甘油三脂 triglycerides 磷脂 phospholipids 99 inbody 95 infood 99 inbody 95 infood 一 脂类的分类Classification 一 脂类的功能 function Structure 1glycerol 3fattyacidsFunction 一 triglycerides 甘油三脂 TG 1 triglyceridesinthebody A Storingandprovidingthebodywithenergy 1g9 46kcal 两个特点 B Thelayeroffatbeneaththeskinhelpskeepthebodywarm insulatingthebody C ProtectingandproppinguptheorgansinthebodyD Helpingthebodyuseitstwootherenergynutrients carbohydrateandprotein sparingprotein E TheveryimportantcompositionofthebodyF Incretion leptin TNF IL 6 IL 8 adiponectin resistin estrogen Insulin likegrowthfactor IGF angiotensinogen 血管紧张素 IGFbindingprotein3 plasminogenactivatorinhibitor PAI 纤维蛋白溶酶原激活因子抑制物 2 Triglyceridesinthefoods A Providingbodywithenergyandessentialfattyacid B Improvingthecolor flavor tasteandshapeoffoods C Becausedietaryfatcausesthestomachtoemptymoreslowlythaneithercarbohydrateorprotein itimpartssatietyandsatisfaction D Tosupplybodywithfat solublevitaminsandaidintestinalabsorptionofthem 3 riskofhigh fatdiet Heartdisease Foodsaturatedfat especially12 14 16carbons raisesbloodcholesteroldramatically whichcausesheartattackorstroke Obesity肥胖 High fatdiettendtostorebodyfatefficiently 3 Cancer Prostatecancerisastrongassociationwithfat Breastcancerisalittleornoassociationwithdietfat 4 Riskfromtrans fattyacids TheyraiseLDLandlowerHDL 二 fattyacids 脂肪酸 FA chainlength long chain 10 14carbonsormore medium chain 6or8 short chain 2 4or2 6 Saturation 饱和度 saturatedFA SFA monounsaturatedFA MUFA polyunsaturatedFA PUFA positionofdoublebond n or n 3 theinitialdoublebondsituatedbetweenthethirdand4thcarbonatomsfromthemethylend n 6 theinitialdoublebondisbetweenthe6and7isomericforms异构体 cis顺式 trans反式 1 Classification 2 Essentialfattyacid 必需脂肪酸 EFA Definition Classification 1 linoleicacid 亚油酸 C18 2 n 6 2 linolenicacid 亚麻酸 C18 3 n 3 Fattyacidsneededbythebody butnotmadebyittomeetphysiologicalneeds SynthesizeofMUFAinbody linoleicacid C18 2 n 6 linolenicacid C18 3 n 3 6去饱和酶 linolenicacid C18 3 n 6 十八碳四烯酸 C18 4 n 3 碳链延长酶bi homotype linolenic二十碳四烯酸 C20 4 n 3 acid C18 3 n 6 5去饱和酶arachidicacid AA timnodonicacid EPA C20 4 n 6 C20 5 n 3 碳链延长酶二十二碳四烯酸 C22 4 n 6 clupanodonicacid C22 5 n 3 4去饱和酶二十二碳五烯酸 C22 5 n 6 docosahexenoicacid DHA C22 5 n 3 FunctionofEFA Tomaintainthestructuralpartsofcellmembranes itisapartofphospholipids Usedtosynthesizemanyhormonelikesubstances eicosanoids 二十烷 prostaglandins PG 前列腺素 Thromboxane TXA 凝血恶烷 leukotrienes LT 白三烯 hydroxyfattyacids andlipoxins 脂素 3 Lowerbloodcholesterolandriskofheartattackorstroke especiallyEPA eicosapentaenoicacid andDHA docosahaxaenoicacid Theymodulatecardiovascular pulmonary immune reproductive andsecretoryfunctionsinmanycells DeficiencyofEFA Affectdevelopment production liver kidney nerve vision andskin 三 磷脂 phospholipid 1 Structure Itconsistofaglycerolbackbonewithoneortwoattachedfattyacids plusadifferentstructurethatincludesaphosphategroupattachedtotheotherhydroxylgroup s Themostimportantmemberislecithins 卵磷脂 2 Function 1 Importantconstituentsofcellmembranes Theythusallowfat solublesubstances includingvitaminsandhormones topasseasilyinoroutofcells 3 Asemulsifiers 乳化剂 inthebodyorfoodmanufacturers 4 Tosupplybodywith9kcaloriespergram 5 Healthfunction differentpoints 四 固醇 Sterols 1 Structure Asamemultiple ringwithdifferentsidegroupsattached Cholesterolisthemostimportant 2 Function Essentialtothestructureandfunctionofcells especiallythecellmembrane about90 oftotalbodycholesterol Cholesterolistheoriginalmaterialforsynthesisofmanyvitallyimportantbodycompoundssuchasbile sexhormones adrenalhormone andvitaminD 3 Harmfuleffects Itformsdeposits 沉积物 inthearterywallswhichleadtoatherosclerosis 动脉粥样硬化 adiseasethatcausesheartattacksandstrokes 二 脂类代谢 metabolism mouth salivarygland 唾液腺 lipase 脂肪酶 Itplaysasmallroleindigestioninadults butefficientlydigeststheshort andmedium chainfattyacidsofmilkininfants Stomach Stomach schurning搅拌actionmixesfatwithwaterandacid Agastriclipasehydrolyzesaverysmallamountoffat Smallintestine emulsificationoffatbybile PancreaticlipaseIntestinallipase bile trappedbydietaryfibersandcarriedoutofthebodywiththefeces ReabsorbedintothebloodliverGallbladder 胆囊 Monoglyceride fattyacidsandglycerol Long chainfattyacidsMonoglycerides Glycerol short andmedium chainFattyacidsabsorbeddirectlyintoblood Merge 结合 intomicelles 微胶粒 Merge 结合 intomicelles 微胶粒 Easilydiffuse 扩散 intotheintestinalcellsandreassembled 重新合成 intonewtriglycerides Withcholesterolandphospholipids Chylomicronswithintheintestinalcells Absorbedintolymph thenintobloodCellsalloverthebodyremovelipidsfromthechylomicrons 乳糜微粒 astheypassby andmakethemsmallerandsmaller protein Livercellscollectthechylomicronremnants 残体 andpackthemwithlipids cholesterolandproteinsasVLDLwhichwilltravelthroughthebodyintheblood HDLmadeinthelivercarrycholesterolsandphospholipidsfromthecellsbacktotheliver TheylosetriglyceridesandgathercholesterolfromOtherlipoproteinscirculatingthroughthebloodstreamAndbecomeLDL Metabolismofcholesterol 三 FoodsourceandRDA脂类的食物来源及供给量 1 Foodsource 1 Animalfat plentyofSFA 2 Vegetableoil plentyofMUFA LinoleicAcid n 6 leafyvegetables seeds nuts grains vegetableoil corn safflower红花 soybean cottonseed sesame芝麻 sunflower ArachidonicAcid n 6 meats canbesynthesizedfromlinoleicacid linolenicacid n 3 fatsandoil soybean walnut胡桃 wheatgerm EPAandDHA n 3 Humanmilk shellfishandfish mackerel鲐鱼 tuna金枪鱼 salmon大麻哈鱼 sturgeon鲟鱼 herring鲱鱼 trout鳟鱼 sardines沙丁鱼 canbemadefromlinolenicacid 2 DRIs basicprinciple percentageoftotaldietaryenergy 20 30 SFA totaldietaryenergy 10 n 6 totaldietaryenergy 3 n 6 n 3 4 1Cholesterol 300mg d 3 Substituteoffat 蔗糖聚酯sucrosepolyester Olestra燕麦素Ostrim AIsofdietaryfat percentageoftotaldietaryenergy 注1 SFA为饱和脂肪酸 MUFA为单不饱和脂肪酸 PUFA为多不饱和脂肪酸注2 n 6 n 3表示n 6系列不饱和脂肪酸与n 3系列不饱和脂肪酸的比例 Section3 carbohydrate碳水化合物 一 分类与食物来源Classificationandfoodsource 单糖 monosaccharide 双糖 disaccharide 寡糖 oligosaccharide 多糖 polysaccharide 一 单糖 monosaccharide 1 Glucose D andL Alittleinthenaturalfoods 2 Fructose 果糖 主要存在于水果和蜂蜜中 人工制造的玉米糖浆中含40 90 Itisfoundinfruits honey abouthalffructoseandhalfglucose andhigh fructosecornsyrupwhichisusedintheproductionofsoftdrinks frozendesserts andconfections 糖果和蜜饯等 InmostAmericandiets fructoseaccountsforabout8 10 oftotalenergyintake 3 Galactose 半乳糖 以乳糖形式存在于天然食物中 Itisnotusuallyfoundfreeinnatureinlargequantities butrathercombineswithglucosetoformlactose 4 Others ribose 核糖 deoxyribose 脱氧核糖 arabinose 阿拉伯糖 xylose 木糖 sugaralcohols 糖醇 sorbitol 山梨醇 mannitol 甘露醇 dulcitol 卫矛醇 inocitol 肌醇 二 双糖 disaccharide 1 Sucrose 蔗糖 葡萄糖 果糖 foundinsugarcane 甘蔗 andsugarbeets 甜菜 2 Maltose 麦芽糖 葡萄糖 葡萄糖 淀粉水解产物3 Lactose 乳糖 葡萄糖 半乳糖 乳及乳制品中4 Trehalose 海藻糖 葡萄糖 葡萄糖 真菌 fungus 及细菌中 三 寡糖 oligosaccharide 3 10单糖组成 有些具有重要的生理功能 四 多糖 polysaccharide 1 糖原 glycogen 动物淀粉2 淀粉 starch 直链淀粉 amylose 老化 支链淀粉 amylopectin 胡化 3 纤维 fibre Nonstarchpolysaccharidesinplantfoodsthatarenotdigestedbyhumandigestiveenzymes butsomearedigestedbyGItractbacteria Fiberinfoodsisalsocalleddietaryfiber definition Insolublefiber Cellulose 纤维素 Itistheprimaryconstituentofplantcellwallsandthereforeoccursinallvegetables fruitsandlegumes 豆类 Hemicellulose 半纤维素 Itisthemainconstituentofcerealfiber Themanybackbonesandsidechainsmakethehemicelluloseadiversegroup somearesoluble manyofthemareinsoluble Lignin 木质素 Itisanonpolysaccharidefiber Itoccursinthewoodypartsofvegetablesuchascarrotsorthesmallseedsoffruitssuchasstrawberries lignin hemicellulose solublefiber 1 Pectins 果胶 Itis 1 4 D galacturonicacidpolymersmethylate 被甲酯化至一定程度的半乳糖醛酸多聚体 Commonlyfoundinvegetablesandfruits especiallycitrus柑橘andapples Pectinsmaybeisolatedandusedbythefoodindustrytothickenjelly becausetheyreadilyformgelsinwater 2 Gumsandmucilages 树胶和粘胶 Gumsarecomposedofvariousmonosaccharidesandtheirderivatives Gumssuchasgumarabic 阿拉伯胶 areusedasadditivesbythefoodindustry Mucilagesaresimilartogumsinstructure Theyincludeguar 瓜拉胶 whichareaddedtofoodasstabilizers Foodsourceofdietaryfiber 二 碳水化合物的功能Function 1 Functionofcarbohydrateinbody 1 Componentofcellsandtissues glycoprotein 糖蛋白 andglycolipid 糖脂 mucin 粘蛋白 andDNA RNA 2 StoringglucoseasglycogenandusingitforenergyTheliverstoresone thirdofthebody stotalglycogen Whenbloodglucosefalls thelivercellsdismantle 拆除 释放 theglycogenintosinglemoleculesofglucoseandreleasethemintothebloodstreamtobeusedbycentralnervoussystemandotherorgans Musclecellscanalsostoreglucoseasglycogen theothertwo thirds buttheyuseitjustforthemselvesduringexercise 3 Sparringproteinfromuseasenergysource 节约蛋白质作用 Ifbodycannotconsumeenoughcarbohydratetoyieldglucose itisforcedtomakeglucosefromprotein thisprocessistermedglyconeogenesis糖异生 becausefatsgenerallycannotbeconvertedintoglucose Soenoughcarbohydratescansparebodyproteinwhichcanbeusedtomakebodytissuesandtoperformothervitalprocesses 即足够的碳水化物可以使体内蛋白质不被分解作为热能而消耗的作用 sparingproteinaction 4 Preventingketosis 抗生酮作用 Alowcarbohydrateintake withtheresultingdeclineinreleaseoftheinsulin leadstoincompletebreakdownoffattyacidsintheliver smetabolicpathwayandtoformationofketones Soenoughcarbohydrateisnecessaryforthecompletemetabolismoffatsinthebody 2 Functionofcarbohydrateinfoods 1 SupplyingenergyCarbohydrateisthecheapestandbroadestfoodenergysourceintheworld 4kcal g 2 Imparting 把 给予 color flavortofoodsandchangingtheirshape Thesugaristhemostimportantsweetener Modifiedstarchisrichinamylopectin 支链淀粉 andmaymakefoodstablewhichisusedinbabyfoods saladdressings andinstant 速溶 puddings 几种糖及糖醇的相对甜度 摘自 PerspectiveinNutrition 第三版 第77页 1996 3 Supplyingdietaryfiber 1 Weightcontrol Fiberslowstherateoffoodleavingthestomachanddrawwaterintotheintestine whichcandelaythestomach semptyingandglucoseabsorption 2 Lowingserumcholesterol Solublefibercanbindwithbileandfoodcholesterolwhichwoulddecreasefatandcholesterolabsorption ThenliversynthesizesmorebilefromcholesterolandexcretedintheGItract whichreducesbloodcholesterolfurther Theshort chainfattyacidsresultingfrombacterialdegradationsolublefiberalsoreducecholesterolsynthesisintheliveroncethesefattyacidsareabsorbedandentertheliver Functionofdietaryfiber Thesixthnutrient 3 Makingeliminationoffecesmucheasier Toolittlefibereatingmakesstoolsmallandhard andconstipation 便秘 mayresultwhichcanforceonetoexertexcessivepressureinthelargeintestineduringdefecation 排便 Thishighpressurecanformmanysmallpouches 囊 onthelargeintestinewallcalleddiverticula 憩室 andhemorrhoid 痔疮 Enoughfibercanmakestoollargeandsoftbecauseitattractswater Thelargesizestimulatestheintestinalmuscles whichaidselimination 4 playingakeyroleinpreventingcoloncancerofthelargeintestine Itmaybeduetothefermentationofresistantstarch 抗性 fiber inthecolon whichlowersthepH AdecreasedpHinthecolonisassociatedwithdecreasedcoloncancerrisk Itisunclearwhethertheprotectionderivesfromthefiberorthevitamins Fibermayhelppreventcoloncancerbydiluting binding andrapidlyremovingpotentiallycancer causingagentsfromthecolon 三 碳水化合物的消化吸收DigestionandAbsorption 1 Theprocessesofdigestion 2 InthestomachToasmallextent thestomach sacidcanbreakstarchdown butitsjuicescontainnoenzymestodigestcarbohydrates Fiberstendtolinger 逗留 inthestomach delayinggastricemptying Thisprovidesafeelingoffullnessandsatiety 饱腹感 1 InthemouthThesalivaryenzymeamylase