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1 ACriticalLiteratureReviewonEnergyRegulationofSolidFoods,LiquidFoods,and aRelationshipbetweentheTwo HannahRapport Fall,2011 A Critical Literature Review submitted in partial fulfillment of the requirements for the Senior Research Thesis. 2 Abstract Theaimofthepresentreviewistoexaminetheresearchconcerningenergy regulationofsolidfoods,energyregulationofliquidfoods,andacomparison betweenthetwofoodforms.Energyregulationofsolidfoodsseemstobegoverned byamultitudeoffactors.Thegoverningfactorsthatwillbeconsideredinthe presentreviewincludehormones,macronutrientcontent,andenergydensity. Energyregulationinliquidfoodsdoesnotappeartobeaseffectiveasenergy regulationofsolidfoods;however,viscosityofthebeverage,macronutrientcontent (especiallyfibercontent),andspeedatwhichthebeverageisconsumedallappear toregulateenergyconsumptionofliquidfoods. Thereisnoclearconsensusinthescientificcommunityaboutthefactors influencingthedifferencesinenergyregulationbetweenliquidandsolidfoods; however,itisclearthatwhencomparablesolidsandliquidsareconsumed,thesolid foodleadstogreaterfeelingsofsatietyandlesserfeelingsofhunger.Foodform seemstogreatlyaffecthungerandsatiety. Theimplicationsofthisresearcharevast.AspeopleintheUnitedStates continuetoconsumemorehigh‐calorieandhighsugarbeverages,theinadequate energyregulationofliquidfoodsleadstopositiveenergybalanceandweightgain. However,inotherclinicalpopulations,suchasthosewithanorexianervosaand olderadultswithweightlossduetolackofappetite,thepositiveenergybalance fromconsumingliquidsmaybebeneficialasitcanleadtoweightgainwithoutthe patientbeingawareofhisorherincreasedcalorieconsumption. 3 ACriticalLiteratureReviewonEnergyRegulationofSolidFoods,LiquidFoods,and aRelationshipbetweentheTwo Humansobtainenergyfromawidevarietyoffoodsources.Thesefood sourcesfallintotwobasiccategories:liquidsandsolids.However,themechanisms thatgoverntheenergyregulation,oramountofenergyapersonconsumesfroma meal,andthesignalsthatleadtoaninitiationandterminationofeatingsolidfoods orliquidfoodsarenotthesame(Mattes&Campbell,2009). Energyregulationofsolidfoodsisgovernedbyamultitudeoffactors. Hormonesignalsfromthegut,especiallyghrelinandleptin,arecorrelatedwith feelingsofhungerandsatiety.Specifically,highlevelsofghrelinarecorrelatedwith feelingsofhungerandhighlevelsofleptinarecorrelatedwithfeelingsofsatiety (Huda,2009).Lowlevelsofleptinarealsoassociatedwithmodulationofactivityin thestriatalregionofthebrainandincreasedrewardingvalueoffood,causinga persontoseekoutfood(Farooqetal.,2007). Alsoinvolvedintheregulationofenergyinsolidfoodformsismacronutrient content.Differentfoodscontaindifferentmixturesoffats,proteins,carbohydrates, sugars,andothermacronutrients.However,thesemacronutrientsdifferentially affectfeelingsofhungerandsatiety.Inordertodeterminetheeffectsofdifferent macronutrientsonfeelingsofhungerandsatiety,researchershavegivensubjects specificmacronutrientsandexaminedtheeffectsofeach.Fromthisresearch,it becomesclearhowdifferentmacronutrientsaffecthunger. Energyregulationofliquidsislessunderstood.Energyregulationinliquidsis alsoincomplete,meaningthathumansarenotcapableofaccountingforenergy 4 consumedfromliquidsandadjustingsubsequentenergyconsumptionto compensate.Theresultofthisisliquidmealsandbeveragesaddingtothetotal amountofenergyandtotalnumberofcaloriesconsumedratherthansubstituting forlatercalories.Thispositiveenergybalancecanultimatelyleadtoweightgain (Tiekenetal.,2007).However,themechanismsbehindwhyenergyregulationof liquidsisincompleteasopposedtoenergyregulationofsolidsarenotfully understood.Someresearchhasindicatedthatthehormonesinvolvedininitiating andterminatingeatingsession,ghrelinandleptin,arenotadequatelystimulatedby liquids(Leidyetal.,2010).Viscosity,energydensity,macronutrientcontent,and fibercontenthavealsobeenimplicatedaspossiblemechanismsbywhichenergy regulationoccurs,althoughthisregulationisincomplete;howeverresultsremain inconclusive. Whencomparableliquidsandsolidsareconsumed,differencesarestriking. Whensolidsandliquidsarematchedacrossvariousdimensions,suchas macronutrientcontent,energydensity,andvolumeoffoodconsumed,solidfoods stillremainmoresatiatingthanliquidfoods.Researchinthisareaisentirely inclusive.Moreresearchcomparingthesatiatingeffectsofsolidsandliquidsis neededtoanswerthisquestion. The following paper serves as a review of the research that has been conducted regarding energy regulation of solid foods and the regulation of liquid foods, and how energy regulation between the two groups is not equivalent, and why. 5 EnergyRegulationinSolidFoods Whendiscussinghowhumansregulateenergyconsumptionfromsolidfoods, awidevarietyofvariableshavetobeconsidered.Macronutrientcontent,energy density,speedatwhichfoodiseatenaresomeofthefactorsthatgreatlyinfluence howpeopleregulatetheconsumptionofsolidfoods.Consumptionofsolidfoodsis alsoregulatedbyvarioushormones,suchasghrelin,leptin,Glucagon‐likepeptide‐1 (GLP‐1)andcholecystokinin(CCK),glucose,andinsulin;thesehormoneshavean incompleteaffectinthegoverningofenergyconsumptionfromliquids,whichwill bediscussedinalatersection. Hormonesregulatingenergyconsumptiondictatewhenapersoninitiatesan eatingsessionandterminatesaneatingsession,aswellashowrewardingfoodsare perceivedtobeatvarioustimes.Itisnotsurprisingthatwhenapersonishunger, foodisperceivedasbeingmorerewardingandapersonismoremotivatedtoseek outfood.Themechanismbehindwhythisoccursappearstobegovernedby hormoneregulation,specificallyleptinregulation(Farooq,2007).Ghrelinhasalso beenimplicatedinfeelingsofhungeranddecreasedghrelincorrelateswithfeelings ofsatiety(Cummings,2001). Macronutrientsdifferentiallyaffectthesehormones,aswellasdifferentially affectingfeelingsofhungerandsatiety.Carbohydratesappeartosuppressghrelin morethanotherhormones;however,fathasbeenshowntosuppressghrelin,as well.Interestingly,fatcausesasharpdropinghrelinlevels,butthatisfollowedbya largerspikeinghrelinlevelsquickly,causingapersontoeatmoreinhisorhernext eatingsession(Erdmann,2003). 6 Thefactorsinfluencingenergyconsumptioninsolidfoodsarediscussed morefullyinthefollowingsection. Hormonesregulatingappetite Leptin Leptinisoftenreferredtoasthesatietyhormone.Thishormoneisreleased byadipocytesinresponsetothepresenceoffoodinthegastrointestinaltract. Leptinisreleaseinthebloodstream,andistransportedthebrain,wherereceptors invariousareashavebeenimplicatedintheterminationofeatingsessions. However,locationsactivatedbyleptinarewidelydebated.Researchhas demonstratedthatmereexposuretofoodleadstoincreasedserumleptininthe hypothalamusandincreasedactivationofhypothalamicneurons,suggestingarole ofthehypothalamusinenergyregulation(Karhunenetal.,1999).Extendingthese findings,researchhasshownthatleptinactsonthestriatalregion,whichmodulates therewardpropertiesoffood.Adecreaseinleptinenhancestherewardproperties offood.Butwhenhigherconcentrationsofleptinispresent,whenfoodreachesthe gastrointestinaltractandleptinisreleased,causesleptinincreasesinstriatalregion, decreasingtherewardingvaluesoffoodandterminatinganeatingsession(Farooq etal,2007). Thisservesasanexplanationforthemoreappetitivenatureoffoodwhena personishungry.Whenapersonishungry,meaningthereisnofoodinthe gastrointestinaltract,therewouldbenoleptinbeingsecreted.Thisdecreasein leptinactsonthestriatalregiontoenhancetherewardingpropertiesoffood,which wouldcauseapersontoseekoutfood,asfoodisnowdeemedrewarding. 7 Duringfastingsessions,serumleptinlevelsdropdramatically.However, onceapersoninitiatesaneatingsession,serumleptinlevelsincreasedramatically. Whatisinteresting,though,isthathigherlevelsoffasting,meaninglongerlengthof fastandhighersubjectivehungerratings,donotcorrelatewithgreaterincreasesof leptinonceaneatingsessionhasbeeninitiated(Marsetal.,2005,Chanetal.,2003, Chanetal.,2008).Thiswouldimplyathresholdforleptinrelease.Perhaps,oncea certainamountoffoodhasenteredthegastrointestinaltract,acertainamountof leptinwillbereleased.Whenmorefoodentersthegastrointestinaltract,nomore leptincanbereleasedasleptinhasalreadyreachedthreshold. Thiswouldalsoimplythatmorethanonehormoneaffectssatiation.Ifa personiseatingmorefoodafteranextendedfast(upto72hours)ratherthana normalfast,suchastimespentasleep,butthesameamountofleptinisbeing secreted,whichshouldterminateaneatingsession,thenotherhormonescertain havetoaffecttheamountoffoodbeingconsumedorelseapersonwouldeatthe sameamountafteraneighthourfastanda72hourfast. However,researchbyCarlsonandcolleagues(2009)supportedthefindings ofKarhunenetal.,showingthatleptinsecretioniscorrelatedwiththetermination ofaneatingepisode.Researchersmonitoredsubjects’guthormoneslevelsbefore aneatingsession,throughoutaneatingsession,andattheterminationofaneating sessionandfoundthatserumleptinlevelswerelowpriortotheinitiationofan eatingsessionbutincreaseddramaticallyduringaneatingsessionandpeaked followingeatingperiods.Inthefastingperiodthatfollowedaneatingsession,leptin 8 droppeddramatically,leadingresearcherstosupporttheassumptionthatleptinis animportanthormoneininitiatingsatietyandendingeatingperiods. Intheaforementionedstudies,measuresofcalorieconsumptionalwaysused solidfoods.Solidfoodscausedistentionofthegastrointestinaltract,whichhas beenimplicatedincausingadipocytestosecreteleptinintothebloodstream (Karhunnenetal.,1999).Becauseliquidscauselessgastrointestinaldistentionthan solidfoods,liquidfoodsdonotcauseasufficientreleaseofleptin,whichallowsthe drinkingsessiontoinvolveconsumingmorecaloriesthananeatingsession.More concerningleptinregulationofliquidfoodswillbediscussedintheregulationof liquidcaloriessection. Ghrelin Ghrelinisanimportanthormoneinvolvedinregulatinghungerandhas oppositeaffectsofleptin.Researchhasdemonstratedthathumansreceive peripheralinjectionsofghrelin,adlibitumfoodintakeatasubsequentmeal increasesinbothleanandoverweightadults(Druceetal.,2005;Hudaetal.,2005; Wrenetal.,2001).Researchershavealsomonitoredguthormonesthroughouta twenty‐fourhourperiodandfoundthatghrelinishighestjustbeforeamealand lowestjustafterameal,suggestingthatghrelinplaysavitalroleininitiatingmeals andregulatinghungerinhumans(Cummingsetal.,2001).Manystudieshave suggestedthatoverweightchildrenandadultsmayhaveahypersensitiveresponse toghrelin,whichwouldleadtoovereating(Mittelmanetal.,2010). However,inthepreviouslymentionedresearch,subjectswereanticipatinga mealtime,whichmayhavealsoledtoincreasedhungerandresponsivenesstofood. 9 Inordertoaccountforthislimitation,Cummingsetal.(2004),conductedastudyin whichsubjectswererequiredtorequestfoodpriortoreceivingfood.Researchers fedsubjectsamealatlunch,andsubsequentlymonitoredguthormonelevelsuntil thesubjectrequesteddinner.Researchfoundasimilarcharacteristicriseinghrelin levelsjustpriortotheinitiationofeatingthesecondmeal,suggestingavitalroleof ghrelinininitiatingmealsevenwithoutanticipationoffood.Researchersalsofound thatincreasingghrelinlevelswererelatedtoincreasedhungerratingsforall subjects. Asdiscussedinthemacronutrientsectionthatfollows,macronutrient contentgreatlyaffectsthesecretionofleptin. Cholecystokinin(CCK)andGlucogon‐likepeptide‐1 Ithasalsobeensuggestedthatcholecystokinin(CCK)mayhaveimplications interminatingmealsandinitiatingsatiation.StudiesshowthatwhenCCKis injectedinhumans,amountoffoodconsumedgreatlydecreases.However,previous researchhasusedlevelsofCCKhigherthanthatwouldbenormallyfoundin physiologically.Inordertoaccountforthelimitationsinpreviousstudies, researchersconductedastudyinwhichsubjectswerereceivedintraduodenal injectionsofasalineorfatinlevelscomparabletowhatwouldoccurinaneating session,aswellasinjectionsofeithersalineoraCCKantagonist.Whensubjects receivedinjectionsofintraduodenalfatandaCCKantagonist,CCKdidnotreduce foodintakeaswhensubjectsreceivedinjectionsofonlyintraduodenalfat.This suggeststhatCCKplaysavitalroleinterminatingeatingsessionsandinitiating satiation(Gokeetal.,1999) 10 GLP‐1hasbeenimplicatedininitiatingsatietyinhumansalso.GLP‐1is releasedfromthegutwhenfoodinpresentinthegut.GLP‐1hasalsobeen demonstratedtoreduceeatinginratsifinjectedintothecerebrospinalfluidofthe centralnervoussystem,butnotifinjectedintothebloodstreamoftheanimal (Gutzwilleretal.,1999).WhenaGLP‐1antagonisthasbeeninjectedintothe cerebrospinalfluid,ratsthatwerenothungrybecausethathadjustpreviously terminatedaneatingsessionbegantoeat.InordertostudytheeffectsofGLP‐1in humans,researchersgavesubjectsvariousdosesofGLP‐1intravenouslywhile monitoringhungerlevelsandfoodintake.Resultsindicatethatascirculatinglevels ofGLP‐1increase,hungerlevelsandeatingratesdrop.Interestingly,researchers alsonotedthatfluidingestiondecreasedinresponsetoGLP‐1increase. ResearchersconcludedthatGLP‐1playsanimportantroleininitiatingsatietyin humans(Guitzwilleretal.,1999). Thefactorsthatinfluencetheregulationofenergyconsumptionfromsolid foodsinhumansarevast,andfartoomuchlargeforthescopeofthisarticle. However,importantfactorssuchasinsulinandglucoselevels,top‐downprocessing, orosensoryproperties,mechanicalprocessoffood,andamultitudeofotherfactors playaroleintheregulationofhungerandsatietyofsolidfoods. Inregardstothefactorsdiscussedinthissection,moreresearchisyettobe done.Anovelareaofresearchthatrequiresmuchmoreresearchistheeffectsof top‐downprocessingonenergyregulation.Ithasbeendemonstratedanecdotally andinlaboratorysettingsthatpeoplemayeatwhentheydonotactuallyfeel compelledtoeat.Certainstudieshaveshowndifferencesincirculatinghormone 11 levels,specificallyleptin,inresponsetothemerepresenceoffood(Marsetal., 2005). Macronutrientcontentandenergydensity Thehumandietiscomposedofavarietyofmacronutrients.Undernormal conditions,humansconsumeamixtureofthesemacronutrientsintheirmeals. However,thesedifferentmacronutrientsaffecthungerandsatiationtovarying degrees.Inordertoteaseapartthedifferentialeffectsofdifferentmacronutrients onhungerandsatiety,researcherswilloftenprovidesubjectswithamealeither entirelycomposedofonemacronutrientoramealthatisexceptionallyhighinone macronutrient,suchasahighproteinbreakfast.Fromthiswork,researchershave discoveredthatcarbohydrate‐richmealssuppressghrelinmoresothanmealshigh othermacronutrients.Similarly,highfatmealsreduceghrelinlevelsintheshort term,butalsoleadtohighersubsequentghrelinlevelsandgreaterenergy‐intakein subsequentmeals(Erdmann,2003).Surprisingly,insomeresearch,proteinhasnot beendemonstratedtoreduceghrelinlevelstosameextentasfatorcarbohydrates (Erdmannetal.,2003,Misraetal.,2009,Lomenick,etal.,2009).Intestingallowing subjectstoeatvariousmacronutrientsuntilsatiation,acarbohydrate‐richmealwas theonlymealtoreducecirculatingghrelinlevelsafteringestion.Ingestionoffat, proteins,andfruitsandvegetablesactuallyledtoanincreaseincirculatingghrelin. However,fat,proteins,andcarbohydratesledtolesscaloriesconsumedina subsequentmealthaningestionoffruitandvegetables(Erdmannetal.,2004). Behavioralstudieshavesupportedfindingsthatcarbohydratessuppress hungermorethanothermacronutrients.Whenresearchersgavesubjectsapre‐ 12 loadofbread,acarbohydrate‐richfood,overallenergyconsumption,includingthe pre‐load,waslessthanenergyconsumptionwithanyotherornopre‐load(Lorian Kohen,2011). However,theresearchinthisareaisnotentirelyconclusive.Someresearch hasdemonstratedgreatersuppressionofghrelinandgreaterreductioninhunger associatedwithhighproteinmeals.However,carbohydratesstillsuppressed ghrelinmorethanfatandalsoledtogreatersatiety(Foster‐Shubertetal.,2008). InastudybyGoetzeet.al(2006),researchersusedamagneticresonance imaging(MRI)tostudytheeffectsofmacronutrientsongastricvolumeresponses andgastricemptying.Gastricvolumeisassociatedwithafeelingoffullness,and gastricemptyingisassociatedwithincreasedhungerwerefat,glucose,andprotein, whichmakeupthemajorityofthehumandiet.Equalsamplesofthesemixtures wereadministeredthroughanasogastrictubeinordertoremovetheeffectsof orosensoryperceptionofthenutrients. Resultsindicatedthatgastricemptyingwashigherfatandproteinthenfor glucose.Immediategastricvolumeresponsesweresignificantlyhigherforglucose thanforproteinorfat.However,theperceptionofsatietywasnotrelatedtowhich macronutrientwaspresent,butratherthantheamountofmacronutrientpresent, whichiscontrarytowhatwasinitiallypredicted(Goetzeetal.,2006). Energydensityisdirectlyrelatedtomacronutrientcontent.Energydensity isdefinedasthecaloriesinagivenunitoffood,andmacronutrientcontentdirectly affectsthis.Energydensityhasbeenimplicatedintheregulationofenergy consumptionindependentlyofmacronutrientcontent.High‐energydensefoods 13 tendtobemorepalatablethanlow‐energydensefoods,andmayleadtoovereating (Belletal.,1998).Mealviscosityinsolidfoodshasalsobeenshowntohavean effectonfeelingofsatietybecausemoreviscousmealsinduceagreatergastric responsetothepresenceoffood,whichmeansthatthemoreviscousfoodis perceivedasbeingmoresatiating(Marcianietal.,2000).Viscosityhasalsobeen showntoreduceadlibitumfoodintake,presumablybecausemoreviscousmeals areeatenmoreslowly(Zijlstraetal.,2008) Whentakentogether,theresearchpresentedclearlyindicatesawidevariety ofinfluencesthataffectenergyconsumptionfromsolidfoods.Hormones, macronutrientcontent,andenergydensityoffoodsallinfluencethesatiating propertiesoffoods.Aswillbeseeninthefollowingsection,thesefactorsarenotthe samefortheregulationofenergyconsumptionfromliquids. RegulationofEnergyfromLiquids Themechanismscontrollinghungerandsatiationwithliquidfoodsaremuch lessunderstoodthanthemechanismscontrollingsatiationandhungerwithsolid foods.Someresearchershavepostulatedthatviscosityoftheliquiddeterminesthe satiationqualitiesofliquidfood,andmoreviscousliquidsslowthegastricemptying ratesmorethanlessviscousliquidsdo(Marcianietal,2000).However,other researchersspeculatethatthemacronutrientmakeupofliquidbeveragesinfluence thesatiationabilitiesofliquidsfood(Almironetal.,2005,Bolton,1981,Lylyetal, 2010)withproteinsandcarbohydratesbeingthemostsatiatingmacronutrientsand sugarsbeingtheleastsatiating.However,thereisotherresearchthatsuggeststhat thefibercontentofbeveragesinfluencesthesatiationqualityofliquidfoods. 14 Researchersconcludedthatliquidfoodshighinfiberreducehungerandleadto longersatiationthanliquidfoodswithlittletonofiber(Juvonenetal., 2009,Perringueetal.,2010,Zijlstraetal,2009). However,whatisapparentintheresearchconcerningliquidfoodsisthe additivenatureofliquidenergyconsumption.Ratherthanliquidfoodsbeinga substituteforsolidfoods,itappearsthathumansdonothavetheabilityto compensateforthecaloriesconsumedinbeverages,thusbeveragesacttoadd caloriestothedietratherthansubstituteforcalories.(Tiekenetal.,2009) Viscosity Researchershavespeculatedthatmoreviscousliquidshavegreatersatiatingeffects thanlessviscousliquids.Whenmatchedformacronutrientcontentbutvaryingin viscosity,theeffectsofconsuminglessviscousormoreviscousbeveragesappear verysimilarintheirabilitiestoappeasehungerinshorttermtesting.However,as timepassed,itbecameapparentthatmoreviscousliquidsweresuperiorat reducingsubjectivehungerratingsandsuppressingghrelinsecretionovertimethan lessviscousliquids(Juvonenetal.,2009,Perringueetal.,2010). Moreviscousdrinks,e.g.milkshake,havebeenshowntosuppressghrelin moresothanlessviscousdrinkswhenbothdrinksarematchedformacronutrient content(Juvonenetal.,2009,Zijlstraetal.,2009).Asdiscussedinprevioussections, decreasedghrelinlevelsareassociatedwithdecreasedhungerratings.Because moreviscousdrinksareassociatedwithdecreaseinghrelinlevels,theyarealso associatedwithdecreasedsubjectivefeelingsofhunger. 15 Viscosityofabeveragehasalsobeenshowntoaffectserumleptin concentrations.Researchhasshownthatconsumingsoftdrinks,whicharehighin caloriesbutnotviscous,resultsindecreasedsecretionofleptinasopposedtomore viscousliquidswithsimilarmacronutrientandcaloricprofiles(Brayetal.,2004, Stanhope&Havel,2008,Kimberetal.,2008).Leptinisinvolvedinthecessationof eatingsessionsandthefeelingofsatiety.Ifasoftdrinkwithanequalnumberof caloriesandmacronutrientcontentasasolidfoodisconsumedbutdoesnotresult inthesecretionofleptin,apersonwillstillfeelhungryandcompelledtoeat.This couldresultinapositiveenergybalanceandweightgain. Orosensoryproperties,suchasmoreviscousorlessviscous,cangreatly influencedigestion,aswell.Thisappearstobetheeffectsoftop‐downprocessing onenergyregulation.Asageneralheuristic,thegreatertheviscosityofabeverage, themoreenergythatbeveragewillcontain.Moreviscousliquidsgenerallycontain morecaloriesandaremoreenergydense,resultinginmoreenergybeingderived fromthatliquid.Thisheuristicmayacttoinfluencehungerandsatietyindependtly ofactualenergycontent(Mattes&Rothacker,2001). However,thespeedwithwhichbeveragesareconsumedmayimpacthow foodsaffectsatiety,aswell.Becauseliquidsareswallowedanddonotrequirethe mechanicalprocessinginvolvedwithconsumingsolidfoods,consumingmore energyrequireslesstime.Morecaloriesareconsumedbeforethefoodisprocessed bythegastro‐intestinaltractandbeforeghrelinissuppressedandleptinissecreted. Ithasbeendemonstratedthatconsumingabeverageataslowerrateleadsto increasesinghrelinsuppressionandleptinsecretion(Kokkinosetal.,2010).Amore 16 viscousliquidcannotbeconsumedasrapidlyasalessviscousliquid;therefore moretimeconsumingtheproductmayleadtoincreasedghrelinsuppressionand leptinsecretion. Alsoimportantintheingestionoffoodandtheregulationofenergyintake fromfoodisGLP‐1andCCK,bothofwhichareresponsiblefortheuptakeof nutrientsintheintestinesafterfoodingestionandslowingofgastricmotility. Interestingly,researchersfoundnosignificanteffectsofviscosityoffluidson differenthormoneslevels,includingCCK,andGLP‐1.However,subjectsstillrated moreviscousliquidsasbeingmoresatiatingthanlessviscousliquids.Itappears thatviscosityofbeveragesisinvolvedwithsubjectiveratingsofhungermorethan thephysiologicalmechanismscontrollinghungerandsatiating(Zijlstaetal.,2009). However,itisexceptionallydifficulttodifferentiallyaffectviscosityofa liquidwithoutaffectingotherfactors,suchasmacronutrientcontent.Often,liquids thatareviscousarehigherinmacronutrientssuchascarbohydratesorfatsas opposedtolessviscousliquids.However,someresearchershavemanagedtotease apartthesevariablesandanalyzetheeffectsofjustviscosityonfoodintake.In Viscosityandadlibitumfoodintake(Ziljastraetal.,2008),subjectsconsumed significantlymoreoftheliquidfoodthanthesemi‐liquidandsignificantlymoreof thesemi‐liquidfoodthanthesemi‐solidfoodbeforereportingfeelingsatiated,even thoughbeverageswerematchedformacronutrientcontent.Researchersconcluded thatthedifferencesineatingwereduetoeatingrates,timethefoodwasintheoral cavity,andtheamountofthefoodconsumedperminute.Liquidfoodremainedin 17 theoralcavityforfarlesstimethaneitheroftheotherfoods,whichresultedinless timeexposedtosensoryreceptorsintheoralcavity. Macronutrientcontent Researchhassuggestedthatdifferentmacronutrientshave differentialeffectsonhungerandsatiety.Ithasbeenproposedthatspecificallythe additionofprotein,fat,andfiber(fiberisdiscussedinfurtherdetailinthenext section)allreducehungerandincreasesatietywhenaddedtoaliquid;itis presumedthatthesemacronutrientsacttoincreasesatietyanddecreasehungerby slowingtherateofgastricemptying(DellaValleetal.,2005).Milkhasespecially beenimplicatedinthesestudies.Becauseofitsfatandproteincontent,milkhas beenshowntogreatlyreduceenergyintakeatsubsequentmeals.However,ina studybyDellaValleandcolleagues(2005),researcherssawnodifferencesin subsequentenergyconsumptionwhensubjectsconsumed1%milk,fruitjuice, water,dietsoda,orsoda.Theonlydifferenceseenwasinhungerratingsfor consumingnobeverageatall,whichledtogreaterhungerbeforethemeal. Researchersdidseesignificantdifferencesintheamountofcaloriesconsumedafter differentbeveragepre‐loads,butthiswasonlyduetothedifferencesincaloric intakefromthedifferentbeverages,notdifferentintakesatsubsequentmeals. Researchersfoundthatconsumingcaloricbeveragesmayleadtoapositiveenergy balancebecauseoftheinabilitytoregulatecaloricintakefrombeverages.The macronutrientpropertiesofbeverageshadnoeffectonsubsequentmealintake, meaningtheamountofprotein,fat,andfiberineachdrinkdidnotaffectsubsequent energyintake(DellaValleetal.,2005).ResearchbyAlmiron‐RoidandDrewnoski 18 (2003)reportedverysimilarfindingsinthe1%milk,orangejuice,andahigh caloriesodadidnothavedifferentialeffectsofsubsequentfoodintakes,butallthese beveragesreducedsubsequentfoodintakemorethanwater.Whileconflictingwith thefindingsofthestudybyDellaValleandcolleagues,thisimpliesthatwhile macronutrientcontentdoesaffectthesatiatingpropertiesofliquids,regulationof liquidenergyconsumptionisnotcompleteandleadstoapositiveenergybalance. However,inthisstudy,theamountoffoodconsumedinthemealwasnotdifferent betweenanyofthegroups,includingwater.Thisdemonstratedthatthesensory qualityofdrinksdidnothaveasignificanteffectonhungerandsatiety(Almiron‐ Roid&Drewnowski,2003). FiberContent Thepresenceoffiberhasbeenshowntoincreasesatietyanddecrease hunger,presumablybecausethesolublefiberdecreasesthespeedofgastric emptying.Pectinisatypeofsolublefiberfoundinfruitsandvegetables,andhas beenimplicatedinthereductionofhungerwhenconsumed.Manystudieshave focusedontheeffectsofaddingfibertoliquidsonsatiationandhunger. Perringeandcolleagues(2010)comparedtheeffectsoffruitjuicewithout pectintofruitjuicewithpectinonintakeatasubsequentmeal.Researcherssawno effectoffiberonsubjectiveratingsofhungerordesiretoeat;however,subjectswho drankafruitjuicewithpectinconsumedlessatalatermealthansubjectswho drankfruitjuicewithoutpectin.InanexperimentbyLylyandcolleagues(2010), researchersfoundthataddingfibertobeveragesincreasedfeelingsofsatietyand decreasedfeelingsofhungermorethanabeverageofequalcaloriesbutnofiber. 19 Interestingly,whenresearchersdoubledtheamountoffiberaddedtoabeverage, satietyratingsdidnotincreaseandhungerratingsdidnotdecreasefromtheratings associatedwithlessfiber(Lylyetal.,2010).Perhapsthesefindingssuggesta thresholdforthesatiatingabilityoffiberinliquids.Moreresearchinthisareawill needtobedoneinordertodetermineifathresholdforthesatiatingabilitiesoffiber trulyexists,butifdoublingtheamountoffiberinabeveragedoesnotincreasethe satiatingabilities,therecertainlyappearstobeevidenceofthisfact. Similarfindingshavebeenseenwhensolublefiberisaddedtobothlow‐ energy‐denseyogurtandhigh‐energy‐denseyogurt.Infact,whenthesolublefiber wasaddedtothelow‐densityyogurt,itincreasedtheyogurt’ssatiatingabilitiesto thatofthehigherenergy‐denseyogurtwithoutfiber.Theadditionoffiberdid increasethesatiatingabilitiesofthelow‐energy‐denseyogurtbutnotthesatiating abilitiesofthehigh‐energy‐denseyogurt,itisnecessaryformoreresearchtobe focusedonwhatcombinationoffat,protein,andfiberleadstothegreatestsatiety (Perringeetal.,2009). Themannerinwhichfiberactstoreducehungerhasbeenwidelydebated; however,researchpointstotheeffectsoffiberontheproductionofCCKafter ingestionofaviscousliquid.InastudybyGelevaandcolleagues(2003), researchersaddedcellulose,asolublefibercommonlyfoundinvegetables,to beveragesandmeasuredtheeffectsofaddedfiberonCCKlevelsinthesmall intestines. Asdiscussedearlier,CCKisahormonereleasedbythesmallintestinein responsetofoodbeingdigested.CCKisreleasedrapidlyandhaseffectsongut 20 motilityandcontrolsgastricemptying.Becausefiberincreasesviscosityof beverage,ithasbeenproposedthatthemethodbywhichfiberincreasessatietyand decreaseshungerisbycreatingamoreviscousfluidthatdoesnotpassthroughthe smallintestineasrapidly.Becauseamoreviscousfiberrequiresmoretimetopass throughthesmallintestine,morenutrientscanbeabsorbedbythemicrovillae,and moreresponsefromCCKcanbeelicited(Lyly,2003). Addingfibertobeverageshasalsobeendemonstratedtosuppressghrelin. Byaddingfibertobeverages,researchershavemeasureddecreasedlevelsof circulatingghrelininsubjects.Decreasesinghrelinweregreaterinbeverageswith fiberaddedthancomparedtobeverageswithnoaddedfiber(Gruendeletal.,2006). Resultsfromstudiesinvolvingleptinarelessconclusive.Researchhas demonstratedthatincreasesinfiberinliquidfoodsdonotleadtoincreasesinleptin levels(Tiekenetal.,2007).Thiswouldsuggestthatevenwhenfiberisaddedtoa beverage,whichinitiallysuppressesghrelinsecretion,suppressinghunger, increasedleptinsecretiondoesnotfollow.Therefore,apersonmaynotfeelhungry, buttheymaynotfeelsatiatedeither.Thelackofresponseofleptintobeverages, evenwithresponseofghrelintobeverages,mayaccountfortheincompleteenergy regulationseeninbeverageconsumption.However,becauseleptinlevelsremain low,foodmaystillhavehighrewardingproperties,whichcauseapersonto continuetoseekoutfoodthoughthepersonhasconsumedenergyanddoesnotfeel hungry. Whentakentogether,researchclearlyindicatesthatenergyregulationof beverageconsumptionisincomplete;howeverthereasonsbehindtheineffective 21 regulationofenergyconsumptionarenotwhollyunderstood.Researchhas indicatedthatmechanismssuchasorosensorypropertiesofliquidsmayeffect energyregulation,aswellasviscosity,fiber,andmacronutrientcontent.However, whatisclearisthatliquidandsolidcaloriesarenotequal. Itmaybeassumedthatbeverages,particularlycaloricallydensebeverages, mayhaveanegativeimpactonhumanhealthyasthesebeveragesleadtoapositive energybalance.Buttheinabilityofhumanstoregulateenergyconsumptionfrom beverages,thusthefactthatbeveragesaddcaloriestothedietratherthan substitutecaloriesmayinfactbebeneficial.Thepositiveenergybalancecould positivelyaffectmanyclinicalpopulations. Forexample,olderadultsbenefitgreatlyfromtheadditionofcalorie‐heavy beveragestotheirdiets.Olderadultstendtonotfeelhungryandthusmaynot consumeasmuchfoodasmaybenecessaryforthemtosustainhealthyweightsand skeletalmusclemasses.However,whengivenliquidnutritionsupplements, subjectswerenotabletocompensateforintakeofliquidcalories,asexpectedfrom previousresearch.Thisinabilitytocompensateforliquidcaloriesconsumedledto apositiveenergybalanceandweightgain,whichcanpositivelyaffectfortheelderly population(Wouters‐Wesselingetal.,2003). Comparisonbetweenliquidandsolidcalories Whenresearcherscomparetheeffectsofconsumingcaloriesfromaliquid withconsumingcaloriesfromasolid,thedifferencesarestriking.Asdiscussed earlier,themechanismsforregulatingenergyintakeforaliquidascomparedtoa soliddiffergreatly,however,whenliquidsandsolidsarematchedacrossvarious 22 dimensions,suchasmacronutrientcontent,fibercontent,andvariousother macronutrients,itisclearthatliquidcaloriesarenotequivalenttosolidcaloriesand mayultimatelyleadtoapositiveenergybalance. Fruitisoftenusedtostudytheeffectsofliquidversussolidfoodonsatiety andhungerbecausetheinherentpropertiesfruitallowforavarietyof manipulations.Fruitisnormallyverylowenergydensity,which,asexplained earlieristheamountofenergyintheformofcaloriespervolume.Thisisbecauseof largeamountwaterpresentinfruits.Fruitsalsotendtocontainalargeamountof fiberinthesolidform,butnotintheliquidform.Pureedfruits,suchasapplejuice, tendtohavenegligibleamountsoffiberinit,becausethepeels,whichhavethemost fiberareremoved,andbecausethemechanicalprocessingofthefruitdestroysthe fiberinthefruit.However,usingapplesallowstheresearcherstomanipulatemany thevariablesimplicatedinsatiety.Forexample,applescanbepresentedinthe whole,asasemisolidinapplesauce,andasaliquidinapplejuice.Macronutrient content,otherthanfiber,isequivalent,andfibercanbeaddedtoanystate(Flood‐ Obbagy&Rolls,2009). InastudybyFlood‐ObbagyandRolls(2009),researcherscontrolledfora widevarietyofvariablesinfoodform,includingweight,energy,fibercontent,and energydensity.Subjectsconsumeapplespeeledandcut,applesaucewithsome fiber,applejuicewithaddedfiber,andapplejuicewithnofiber,ornopre‐loadasa control.Subjectsthenconsumedanablibitumtestmeal.Subjectswhoatetheapple pre‐loadconsumedlessatlunchthanthosewhoateapplesauce;thosewhoate applesauceconsumedlessthanthosewhodrankeitherjuice;thosewhodrank 23 applejuicewithaddedpectinconsumedthesameasthosewhodrankapplejuice withoutaddedpectin;thosewhodrankapplejuicewithorwithoutaddedpectin consumedlessthanthesubjectswhohadnopreload.Researcherssawasimilar patternasdescribedwithconsumptiondifferences,wherebysubjectswhohad consumedapplesinwholewerelesshungrythanthosewhoconsumedtheother pre‐loads,andallpre‐loadsresultedinsignificantlowerhungerratingsthannopre‐ load(Flood‐Obbagy&Rolls,2009). Hungerratingsweresignificantlylowerforanyofthepreloadsasopposedto nopre‐load.However,incontrasttotheresultsfromtheamountoffoodconsumed, applejuicewithaddedpectinshowedsignificantlylowerhungerratingsthanthose whoconsumedapplejuicewithoutaddedpectin.Resultsfromsatietyratings mirroredthoseofthehungerratings:satietyratingswerehighestforapplesin wholeandlowestforapplejuicewithnoaddedpectin,butallpre‐loadshadhigher satietyratingsthanthesatietyratingsforsubjectswhohadconsumednopre‐load atall(Flood‐Obbagy&Rolls,2009). Consumingdifferentformsofthesamefruitwithsimilarenergydensities andenergycontentandweightcanhaveverydifferenteffectsonhungerandsatiety andfoodconsumption.Researchersspeculatedthatdifferencesinhungerand satietymaybeduetothestructuralpropertiesoffoodthatleadtofoodtobe mechanicallyprocesseddifferent.Foodsrequiringchewingareeatenmoreslowly andrequiremoremechanicalprocessingthanfoodthatisswallowedwithout mechanicalprocessing(Flood‐Obbagy&Rolls,2009). 24 Resultsfromotherstudiesinvolvingconsumptionofapples,applesauceand applejuiceindicatedthatintheinitialthirtyminutes,therewasnosignificant differenceinhungerandsatietyratingsbetweenthethreegroups.However,thirty minutesafterconsumption,differencesbegintoemerge.Hungerwassignificantly lessforsubjectswhohadconsumedappleasopposedtoapplesauceorapplejuice. Andafterthetwo‐hourwaitingperiod,subjectswhoconsumedapplesauceorapple juicehadsignificantlylowersatietyratingsthanthosewhohadconsumedwhole apples.Therewasalsoashorterinter‐mealintervalaftertheapplejuiceand applesauceasopposedtothewholeapples.However,therewasnoeffectoffood formontheamountoffoodeatenatasubsequentmeal.Sowhilesubjectswho consumedapplejuiceandapplesauceatetheirnextmealsoonerthanthosewho hadconsumedwholeapples,themealsizebetweenallthegroupswasnot statisticallydifferent(Mattes&Campbell,2007). However,oneimportantlimitationinthisstudywouldneedtobeaddressed infuturestudies.Thelatencytimesbetweenconsumingthepre‐loadofapplejuice, applesauce,orawholeappleandeatingmayhavebeentoolongtoreallycapture thedifferencesinsatiatingpowerofthevariousfoodforms.Infuturestudies,it wouldbeimportanttoreducetheamountoftimebetweenapre‐loadandafollow‐ upmeal(Mattes&Campbell,2007) However,theseresultshavebeensupportedinstudiesutilizingorangesas comparedtoorangejuice,aswellasgrapesversusgrapejuice.Researchers measuredplasmalevelsofglucoseandinsulinafterconsumptionoftheliquidor solidformofthefruit.Withwholefoodstherewasasmallerinsulinresponsethan 25 withjuices,andasmallerpost‐absorptivedropinglucoseafteringestingwhole foodsasopposedtothefruitjuices.Sincetheamountofsugarinthefruitandfruit juiceswascomparable,researchersconcludedthatfibercontentinthefood consumedhadasignificanteffectonthelevelsofplasmaglucoseandplasmainsulin, moreofanimpactthanglucosecontent(Boltonetal.,1981).Researchwith grapefruit,grapefruitjuiceandwaterfurthersupportsthesefindings.Subsequent mealintakeaftereatinggrapefruitwaslowerthanforgrapefruitjuice,whichledto weightloss(Silveretal.,2011) Fromtheaforementionedstudies,itcanbeconcludedthatwhenbeverages areconsumedalone,theyhaveweakerappetitiveeffectsthanwhensolidsare consumedalone.Energy‐densebeveragesmaythereforeleadtoandmaintaina positiveenergybalanceinhumans,andmayultimatelyleadtoweightgain.People arenotcompensatingfortheamountofcaloriesinabeveragethatisbeing consumed,thereforetheyeatmorecaloriesthantheywouldhaveinitiallyeatenif theyhadnothadthebeverage. Inastudymoreapplicabletotheclinicalsetting,researchersexaminedthe effectsofliquid‐mealreplacementproductsandsolid‐mealreplacementproducts onhungerandsatietyrating,andappetite‐regulatinghormonesinolderadults (Tiekenetal.,2007).Liquidmealreplacementproductsareproductsthatprovide anappropriatebalanceofmacronutrientcontentandcaloriestoreplaceanentire meal,butinliquidform.Popularliquid‐mealreplacementproductsinclude SlimFast©mealreplacementshakes,proteinshakes,Ensure©mealreplacement shakes.Solid‐mealreplacementproductsareproductsthatprovideanappropriate 26 balanceofmacronutrientcontentandcaloriesandreplaceanentiremeal,butarea singleproduct.Popularsolid‐replacementmealproductsincludeClif©energybars andSpecialK©energybars. Thisissueofhungerandsatietyinolderadultsisofgreatimportanceinthe clinicalcommunity.Aspeopleage,theytendtoconsumelessenergy,whichmay resultinolderadultsbeingunderweight.Beingunderweightcanleadto malnutrition,frailty,andultimatelydeath.However,ashasbeenexplainedearlier, energyregulationisincompleteinliquidfoodsascomparedtoenergyregulationin solidfoods.Liquidfoodselicitweakersatiationcuesthansolidfoods,whichmay meanthatolderadultscanconsumeliquidfoodsinordertoaddtotheirtotalenergy consumption(Tiekenetal.,2009). Inthisstudy,researchersmeasuredtheghrelin,leptin,glucose,andinsulin levels,aswellassubjectivehungerratings,atvariouspointsintimeafterolder adultsconsumedeitheraliquidorsolidmealreplacementproduct.Resultsfrom thisstudyindicatethatsatietyratingswerehigherandremainedhigherthelength ofthestudyforsolid‐mealreplacementsthanliquid‐mealreplacements;as expected,hungerrateswerehigherandremainedhigherforliquid‐meal replacementproductsthansolid‐mealreplacementproducts.Resultsalsoindicated lowerdesirestoeatforsubjectswhoconsumedtosolid‐mealreplacementsas opposedtoliquid‐mealreplacements.Plasmaglucoselevelswerehigherinitially forsubjectswhoconsumedliquid‐mealreplacementproductsthanforsubjectswho consumedsolid‐mealreplacementproducts,butwithin60minutes,glucoselevels forsolid‐mealreplacementproductsreachedcomparablelevelstothoseofliquid‐ 27 mealreplacementproducts,andtheglucoselevelsofsubjectswhoconsumedsolid‐ mealreplacementproductswerenodifferentthanthoseofsubjectswhoconsumed liquid‐mealreplacementproducts.Ghrelinlevelstendedtobelowerforsubjects whoconsumedsolid‐mealreplacementproductsasopposedtothosewho consumedliquid‐mealreplacementproducts,butthoselevelsneverreached significance.Therewerealsonosignificantdifferencesinleptinbetweensubjects whoconsumedliquid‐mealreplacementproductsandthosewhoconsumedsolid‐ mealreplacementproducts(Tiekenetal.,2009). Therewerecertainlimitationsinthisstudythatwouldneedtobeaddressed inorderforthisstudytobeasinfluentialaspossible.Researchersreportedthatthe macronutrientcontentofliquid‐mealreplacementproductsandsolid‐meal replacementproductswerenotmatched.Theliquid‐mealproductscontainedmore carbohydrates,protein,andfiber.However,thisisveryinterestingbecause,as describedearlier,thesemacronutrientsshouldcauseaproducttobemoresatiating, whichmaymeanthattheresultsofthisstudycouldbeunderstatedcomparedto whatmayactuallybetranspiring.Fromthisstudy,researchersconcludedthatthe physicalpropertiesoffoodmaybeagreaterdeterminantinhungerandsatietythan macronutrients(Tiekenetal.,2007). Inasimilarstudy,subjectswhoconsumedaliquidmealasopposedtoasolid mealhadgreaterhungerratingsandreportedhavingagreaterdesiretoeat. Ghrelinlevelsgreatlydecreasedforsubjectswhohadconsumedsolidsasopposed toliquids(Leidyetal.,2010). 28 InanotherstudybyFloodandRolls(2007),researchersreplicatedascenario seeninmanyrestaurantsandmeals.Researchershadsubjectsconsumesoupatthe beginningofameal.Thesoupwasabowlofbrothalongsideabowlofvegetables andbutter;achunkysoupconsistingofthebrothwiththesameamountof vegetablesandbutteraddedin;apureedsoupconsistingofthesameamountof vegetablesandbutterblendedtogetherfor15seconds,sothesoupwasnotentirely smooth;andacompletelypureedsoupwiththesameamountofvegetablesand butter,butinacompletelyliquidform.Inacontrolcondition,researchershad subjectssitquietlyandreadfor15minutespriortoamealbeingserved;subjectsin thisconditionreceivednosouppre‐load.Researchersthenprovidedsubjectswitha testmealofcheesetortellini15minuteslater. Resultsindicatedthatsubjectsconsumedsignificantlylessinthesubsequent testmealwhenasouppre‐loadwaspresentedasopposedtonopre‐loadcondition, andtotalenergyintakewassignificantlylesswhensubjectsconsumedasouppre‐ loadthannopre‐load.Increasedviscosityofthesoupwascorrelatedwitha decreaseinamountoffoodconsumedinasubsequentmeal.Themoreviscousa soup,likeachunkysoup,thelessfoodconsumedatasubsequentmeal. Fromthisresearch,aclearpatternemerges:liquidandsolidcaloriesarenot equal.Peoplearenotabletocompensateforenergyconsumedfromliquidsas opposedtosolids.Peoplecannotcompensatefortheamountofenergyconsumed fromliquids,whichmayultimatelyleadtopositiveenergybalance.However,this maynotalwaysbeanegativeconsequenceofconsumingliquidcalories.Consider thecaseofolderadults,whohavelosttheirappetites.Addingliquid‐meal 29 replacementproductstotheirmealsmayultimatelyleadtoweightgainbecauseof theirinabilitytocompensateforthecaloriesconsumed. Discussion Thisresearchisbecomingincreasinglymoreimportantforusasratesof obesitycontinuetoincreaseintheUnitedStates.Consumptionofhighcaloricand highsugarbeverageshasincreased,andportionsizeshaveincreased,aswell.This hasledtoincreasedratesofobesitybeginninginchildhood(Piernas&Popkin, 2011).Asclearlyillustratedfromtheresearchconcerningenergyregulationfrom beverages,thesebeveragesleadtopositiveenergybalancebyaddingtoenergy intakeratherthansubstitutingforothersourcesofenergy,leadingtoweightgain. Theimplicationsfromtheresearchconcerningintakeofsolidfoodswillbe importantincombatingincreasesinobesityandpositiveenergybalances.Gut hormonesleptinandghrelinhavebeenrecognizedasoflateasbeingimportant hormonesintheregulationofeatingandsatiation.Obeseindividualstendtoshow increasedsensitivitytoghrelinanddecreasedamountsofcirculatingleptin,aswell asdecreasedresponsivenesstoleptininthehypothalamus. Futureresearchneedstofocusontheclinicalapplicationsofleptinand ghrelinforcombatingobesity.Leptinreplacementtherapymaypotentiallybe usefulincombatingobesity,andhasbeenshowntobeeffectiveinnormalizingbody weightandhelpingindividualsreachaveragebodyweight(Pazetal.,2010). However,thepositiveenergybalanceandincompleteregulationofenergy consumptionthatresultsfromconsumingliquidcaloriesmayhavepositive implicationsformanyclinicalpopulations.Inpatientswithanorexianervosaand 30 bulimianervosa,havingpatientsconsumedietsconsistingofmostlyliquidmeals mayresultinpositiveenergybalanceandweightgain.Becausethispatientsarenot abletoregulatetheamountofcaloriesbeingconsumedfromaliquidmeal replacement,theseindividualsmaybeabletoconsumemorecaloriesbecausethey arenotabletoperceiveandaccountfortheamountofcaloriesconsumed. Therefore,thesepatientsmayup‐regulatetheamountofcaloriesconsumedbecause ofincompleteenergyregulationofliquids. Anotherclinicalpopulationthatmaybenefitfromconsumingmoreliquid mealreplacementsisolderadults.Asdiscussedearlier,olderadultsriskbecoming frailandmalnourishedbecauseoftheymaynotfeelhungryorfeeladesiretoeat. However,havingtheseolderadultsconsumeliquidmealreplacementsmaybea veryeffectivemethodofup‐regulatingtheamountofenergyconsumedinaday. Olderadultswhoconsumeliquidmealreplacementproductsmaynotexperience satietyfromtheseproducts,sowhiletheirsolidfoodintakedecreases,liquidmeal replacementmayincrease,allowingthesepatientstohaveapositiveenergybalance andincreasedweightgain. Researchintheareaofliquidandsolidcaloriesisfarfromconclusive,nordo researchersfullyunderstandthemechanismsthatgovernenergyregulationin liquidandsolidfoods.Itisclearthatmanydifferenthormones,suchasghrelinand leptin;propertiesoffoodandbeverage,suchasviscosity,energydensity,and macronutrientcontent;andtop‐downprocessing,suchasexpectationsabout consumingfood,suchasmoreviscousfoodshavemorecalories;andmechanical mechanism,suchasmechanicalprocessingoffoodallplayanimportantrolein 31 regulatingenergyintakeinbothliquidandsolidfoods.However,foravarietyof reasons,thisregulationisincompleteinliquidfoodsascomparedtosolidfoods. Moreresearchisrequiredinordertoelucidatethesefindsfurther,andthereisno clearagreementinthescientificcommunityaboutwhattheexactmechanismsare thatdifferbetweenliquidandsolidregulation. 32 WorksCited Almiron‐RoidE,DrewnoskiA.(2008)Hunger,thirstandenergyintakesfollowing consumptionofcaloricbeverages.Physiol&Behav.79(4‐5):767–773. 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