XII. Proteins and Amino Acids, Quality - Open Textbooks

Net protein utilization (NPU) measures percent of dietary protein retained in the body. Since NPU considers digestibility, it is a better estimate for protein ... Skiptocontent Theestimationofnutrientrequirementsforlivestockandthedeterminationoftheextenttowhichdifferentfeedstuffsourcessupplyessentialnutrientsareneededforefficient,economical,andsustainablefeeding.Inthiscontext,estimationofproteinrequirementsandtheextenttowhichdifferentproteinsourcessupplyessentialandnonessentialaminoacidsareresearchedindetail.   NewTerms Aminoacidscore Antagonism Bioavailability Biologicalvalue Digestibility Ilealdigestibility Limitingaminoacid Netproteinutilization Proteinefficiencyratio Proteinquality ChapterObjectives Tointroduceanddiscussthefactorsaffectingtheproteinqualityofanimalfeeds Tointroducedifferentmethodsforassessingproteindigestibility Todifferentiatebetweenfecalandilealdigestibilityanditsimportanceinrationformulationformonogastricanimals Thetermproteinqualityreferstotheavailabilityofaminoacidsthattheproteinsupplies,anddigestibilityconsidershowtheproteinisbestutilized.Therearevariousmethodsthatrankthequalityofdifferenttypesofprotein,andthischapterdescribessomeoftheinvivoanimalbioassaysandinvitrolaboratoryanalysiscommonlyusedinanimalnutrition. Proteinqualityisatermusedtodescribehowwellaproteinfromfeedmatchesanimalrequirements.Withhigh-qualityprotein,lessisneededinthediet. FactorsAffectingProteinQuality Aminoacidprofile Contentandbalanceofessentialandnonessentialaminoacids Contentoflimitingaminoacids Proteindigestibilityandbioavailability Severalfactorscanaffectproteinquality.Animalsdonothaveaproteinrequirement,rathertheyhaveanaminoacidrequirement.Thebasicfunctionofdietaryproteinistosupplyadequateamountsofrequiredaminoacids.Thusthequalityoffeedproteindependsontheaminoacidprofileofthediet,thecontentofessentialaminoacidsaswellthecontentoflimitingaminoacids,andthedigestibilityandphysiologicalutilizationofaminoacidsafterdigestion.Thesefactorsaremostimportantfornonruminants.Solubilityanddigestibilityarealsoimportantforruminantanimals.     Limitingaminoacidsaretheessentialaminoacidsthatinterruptproteinsynthesisduetoitslimitedamountandthegreatdemandforthem. Thetwomostcommonlimitingaminoacidsinanimalsfedcorn-andsoy-basedcommercialdietsintheUSarelysineandmethionine. ProteinDigestibilityandBioavailability Whatismeantbythedigestibilityofprotein? Thedigestibilityofproteincanbedefinedasthefractionoftheproteiningestedthatisabsorbedbytheanimal—thatis,notexcretedinfeces.Digestibilitycanthereforebecalculatedbymeasuringdietaryproteininputandfecaloutputasshowninthefollowingequation: Digestibilityofproteincanbedefinedasthefractionoftheproteiningestedbytheanimalandnotexcretedinfeces. aminoaciddigestibility(%)=aminoacidconsumed−aminoacidexcreted×100 /aminoacidconsumed Digestibilityassaysareafavoredtechniqueformeasuringtheavailabilityofaminoacids.Mostcommonandpublishedvaluesonthedigestibilityofproteinandaminoacidsinmonogastricanimalsarebasedonfecalorexcretaanalysisbecauseofitssimplicityandsamplesize,asalargenumberofanimalscouldbeusedwithoutsacrificingtheanimals.Fecal-baseddigestibilitymeasurements,however,sufferfromthemodifyingandvariableeffectsofthehindgut(microbes’dietaryproteinutilizationandthecontributionofmicrobialandendogenousproteinstoaminoacidexcretioninfecescontributingtofecalaminoacidoutput). Digestibilitycanbedividedintofecalandileal. Fecaldigestibilityassaysaredonethroughfecalcollectionandanalysis,whileilealdigestibilityassaysaredonethroughdigestacollectionattheilealjunction,eitherusingcannulatedanimalsorslaughterinsmallanimalssuchaspoultry. Thetermdigestibilityisoftenusedsynonymouslywithbioavailability.However,thisisnottrue.Bioavailabilityofaminoacidcanbedefinedasanaminoacidinaformsuitablefordigestion,absorption,andutilization.Bioavailabilitycanonlybeestimatedwithanimalgrowthbioassaysusingliveanimals. Bioavailabilityofaminoacidsissimplytherateatwhichaminoacidsareabsorbedandbecomeavailableforproteinsynthesis. Severalanimalandfeedfactorscanaffectbothdigestibilityandbioavailability.Forexample,thesourceofprotein,suchasanimalversusplantprotein,andfeedprocessingmethodscanaffectproteindigestibility.Animalproteinsaremorebalancedwithessentialaminoacidsandnonessentialaminoacids.Plantproteins,suchassoybeanmeal,canhaveantinutritionalfactors(e.g.,trypsininhibitor)thatcanaffectproteindigestibilityandavailabilityofaminoacids. Feedprocessingmethodssuchasheating,roasting,andextrusioncandisruptantinutritionalfactorsliketrypsininhibitorinsoybeanmeal.However,overheating(browning,orMaillardreaction)makesaminoacidsunavailableduetoacomplexreactionwithsugars.Inaddition,otherinterrelationshipssuchasantagonism,excessesofoneaminoacidinterferingwiththemetabolismofanotheraminoacid,makingthemunavailable(e.g.,lysineandarginine)canprecipitateaminoacidimbalance. MethodsforAssessingProteinQuality Proteinqualitycanbeassessedbybioassaysusingliveanimalsorchemicalassay. BiologicalValue(BV):Thisisameasureoftheproportionofabsorbedproteinfromafeedthatbecomesincorporatedintotheproteinsoftheanimal’sbody.Itdetermineshowreadilythedigestedproteincanbeusedinproteinsynthesisintheanimal.BVassumesproteinistheonlysourceofnitrogenandmeasurestheproportionofnitrogenabsorbedbythebodythatisthenexcreted(fecalandurine).Theremaindermusthavebeenincorporatedintotheproteinsoftheanimal’sbody.Aratioofnitrogenincorporatedintothebodyovernitrogenabsorbedgivesameasureofprotein“usability”orBV. Biologicalvalue(BV)measuresthepercentofabsorbedproteinretainedinthebody. NetProteinUtilization(NPU):Asthenameindicates,NPUdeterminestheratioofaminoacidconvertedtoproteinsinthebody(orretainedinthebody)totheratioofaminoacidssuppliedinthediet. NPUmeasurespercentofdietaryproteinretainedinthebody. Asavalue,NPUcanrangefrom0to1(or100),withavalueof1(or100)indicating100%utilizationofdietarynitrogenasproteinandavalueof0anindicationthatnoneofthenitrogensuppliedwasconvertedtoprotein.Eggproteinhasahighvalueof1(or100) NPU=bodyNcontentwithtestprotein−bodyNcontentwithprotein-freediet×100/Nintake ItturnsoutthatNPU=digestibility×BV.SinceNPUconsidersdigestibility,itisabetterestimateforproteinquality.   ProteinEfficiencyRatio(PER):ThePERwasthefirstmethodadoptedforroutineassessmentoftheproteinqualityoffood.ThePERisbasedontheweightgainofatestanimaldividedbyitsintakeofaparticularfoodproteinduringthetestperiod.PERassumesthatallproteinisusedforgrowth. PERmeasuresbodyweightgainperunitofproteinconsumed. BothBVandPER=0whennongrowinganimalsareused. Otherchemical(invitro)methodsofassessingproteinqualityincludetheaminoacidscore.Thisisdonethroughlaboratoryanalysisofaminoacidprofilesusinghigh-pressureliquidchromatography,andtheresultsarecomparedtoastandard(orreference)proteinsuchaseggprotein(albumen)andgivenascore.Althoughnoanimalsareinvolved,thesescorescangivesomeeasyandquickinformation.However,theydonotgiveanyinformationonpalatability,digestibility,oravailability.     KeyPoints Thenutritionalqualityofaproteinisdeterminedbydigestibility,essentialaminoacids,andlimitingaminoacidcompositionfornonruminants.Thetwomostlimitingaminoacidsarelysineandmethionine. Solubilityanddigestibilityareimportantfactorsforruminants.Ruminantshavenoaminoacidrequirement.Instead,theyhaveanitrogenrequirement. Proteinqualitycanbeassessedbybioassaysorchemicalassays.Biologicalvalue(BV)measurespercentofabsorbedproteinretainedinthebody.HighBVofaproteinmeanslessisneeded. Netproteinutilization(NPU)measurespercentofdietaryproteinretainedinthebody.SinceNPUconsidersdigestibility,itisabetterestimateforproteinquality.Anotherbioassayusedistheproteinefficiencyratio(PER),whichmeasuresbodyweightgainperunitofproteinconsumed. Aminoacidscoreorchemicalscorecomparestheamountofindividualessentialaminoacidstothatofanidealorreferenceprotein.Theaminoacidthathasthelowestscoreisassignedtothetestprotein.Thechemicalscoreiseasytoconductbutdoesnotconsiderdigestibilityorpalatabilityinliveanimals. Digestibilityandbioavailabilityofaminoacidscanbeaffectedbydisproportionateamountsofaminoacidsinthediet(excessorshortage),proteinsource,feedprocessing,andantagonism.Symptomsforbothdeficiencyandtoxicityofindividualaminoacidscanaffectanimalproductivity.Aminoacidantagonismsuchaslysine-arginineiscommoninpoultry. ReviewQuestions Whatisproteinquality?Listthefactorsthataffectproteinquality? Whatisalimitingaminoacid?Whatarethetwomostcommonlimitingaminoacids,andwhyaretheylimitinginanimaldiets? Differentiatebetweendigestibilityandbioavailability. Whatisproteinquality?Listtwoinvivotestsforproteinquality. Whichbioassaymethodwouldyouusetoassesproteinquality?Why? Whatisanaminoacidscore? Previous/nextnavigation Previous:XI.Proteins,Metabolism Next:XIII.Vitamins Backtotop License AGuidetothePrinciplesofAnimalNutritionbyGitaCherianislicensedunderaCreativeCommonsAttribution-NonCommercial4.0InternationalLicense,exceptwhereotherwisenoted. ShareThisBook ShareonTwitter