Download Energy Regulation of Solid Foods, Liquid Foods, and a Relationship

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ACriticalLiteratureReviewonEnergyRegulationofSolidFoods,LiquidFoods,and
aRelationshipbetweentheTwo
HannahRapport
Fall,2011
A Critical Literature Review submitted in partial fulfillment of the
requirements for the Senior Research Thesis.
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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.
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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
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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.
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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).
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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.
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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
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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.
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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)
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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
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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‐
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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
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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.
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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.
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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
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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
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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
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(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.
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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
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