Obesity2.docx
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Obesity2.docx
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Obesity2
Causes
Atanindividuallevel,acombinationofexcessivefoodenergyintakeandalackofphysicalactivityisthoughttoexplainmostcasesofobesity.[62]Alimitednumberofcasesaredueprimarilytogenetics,medicalreasons,orpsychiatricillness.[63]Incontrast,increasingratesofobesityatasocietallevelarefelttobeduetoaneasilyaccessibleandpalatablediet,[64]increasedrelianceoncars,andmechanizedmanufacturing.[65][66]
A2006reviewidentifiedtenotherpossiblecontributorstotherecentincreaseofobesity:
(1)insufficientsleep,
(2)endocrinedisruptors(environmentalpollutantsthatinterferewithlipidmetabolism),(3)decreasedvariabilityinambienttemperature,(4)decreasedratesofsmoking,becausesmokingsuppressesappetite,(5)increaseduseofmedicationsthatcancauseweightgain(e.g.,atypicalantipsychotics),(6)proportionalincreasesinethnicandagegroupsthattendtobeheavier,(7)pregnancyatalaterage(whichmaycausesusceptibilitytoobesityinchildren),(8)epigeneticriskfactorspassedongenerationally,(9)naturalselectionforhigherBMI,and(10)assortativematingleadingtoincreasedconcentrationofobesityriskfactors(thiswouldnotnecessarilyincreasethenumberofobesepeople,butwouldincreasetheaveragepopulationweight).[67]Whilethereissubstantialevidencesupportingtheinfluenceofthesemechanismsontheincreasedprevalenceofobesity,theevidenceisstillinconclusive,andtheauthorsstatethattheseareprobablylessinfluentialthantheonesdiscussedinthepreviousparagraph.
Diet
Averagepercapitaenergyconsumptionoftheworldfrom1961to2002[68]
Thepercapitadietaryenergysupplyvariesmarkedlybetweendifferentregionsandcountries.Ithasalsochangedsignificantlyovertime.[68]Fromtheearly1970stothelate1990stheaveragecaloriesavailableperpersonperday(theamountoffoodbought)increasedinallpartsoftheworldexceptEasternEurope.TheUnitedStateshadthehighestavailabilitywith3,654 caloriesperpersonin1996.[68]Thisincreasedfurtherin2003to3,754.[68]Duringthelate1990sEuropeanshad3,394 caloriesperperson,inthedevelopingareasofAsiatherewere2,648 caloriesperperson,andinsub-SaharanAfricapeoplehad2,176 caloriesperperson.[68][69]Totalcalorieconsumptionhasbeenfoundtoberelatedtoobesity.[70]
Thewidespreadavailabilityofnutritionalguidelines[71]hasdonelittletoaddresstheproblemsofovereatingandpoordietarychoice.[72]From1971to2000,obesityratesintheUnitedStatesincreasedfrom14.5%to30.9%.[73]Duringthesameperiod,anincreaseoccurredintheaverageamountoffoodenergyconsumed.Forwomen,theaverageincreasewas335 caloriesperday(1,542 caloriesin1971and1,877 caloriesin2004),whileformentheaverageincreasewas168 caloriesperday(2,450 caloriesin1971and2,618 caloriesin2004).Mostofthisextrafoodenergycamefromanincreaseincarbohydrateconsumptionratherthanfatconsumption.[74]Theprimarysourcesoftheseextracarbohydratesaresweetenedbeverages,whichnowaccountforalmost25percentofdailyfoodenergyinyoungadultsinAmerica,[75]andpotatochips.[76]Consumptionofsweeteneddrinksisbelievedtobecontributingtotherisingratesofobesity.[77][78]
Associetiesbecomeincreasinglyreliantonenergy-dense,big-portions,andfast-foodmeals,theassociationbetweenfast-foodconsumptionandobesitybecomesmoreconcerning.[79]IntheUnitedStatesconsumptionoffast-foodmealstripledandfoodenergyintakefromthesemealsquadrupledbetween1977and1995.[80]
AgriculturalpolicyandtechniquesintheUnitedStatesandEuropehaveledtolowerfoodprices.IntheUnitedStates,subsidizationofcorn,soy,wheat,andricethroughtheU.S.farmbillhasmadethemainsourcesofprocessedfoodcheapcomparedtofruitsandvegetables.[81]
Obesepeopleconsistentlyunder-reporttheirfoodconsumptionascomparedtopeopleofnormalweight.[82]Thisissupportedbothbytestsofpeoplecarriedoutinacalorimeterroom[83]andbydirectobservation.
Sedentarylifestyle
Seealso:
Sedentarylifestyle andExercisetrends
Asedentarylifestyleplaysasignificantroleinobesity.[84]Worldwidetherehasbeenalargeshifttowardslessphysicallydemandingwork,[85][86][87]andcurrentlyatleast60%oftheworld'spopulationgetsinsufficientexercise.[86]Thisisprimarilyduetoincreasinguseofmechanizedtransportationandagreaterprevalenceoflabor-savingtechnologyinthehome.[85][86][87]Inchildren,thereappeartobedeclinesinlevelsofphysicalactivityduetolesswalkingandphysicaleducation.[88]Worldtrendsinactiveleisuretimephysicalactivityarelessclear.TheWorldHealthOrganizationindicatespeopleworldwidearetakinguplessactiverecreationalpursuits,whileastudyfromFinland[89]foundanincreaseandastudyfromtheUnitedStatesfoundleisure-timephysicalactivityhasnotchangedsignificantly.[90]
Inbothchildrenandadults,thereisanassociationbetweentelevisionviewingtimeandtheriskofobesity.[91][92][93]A2008meta-analysisfound63of73studies(86%)showedanincreasedrateofchildhoodobesitywithincreasedmediaexposure,withratesincreasingproportionallytotimespentwatchingtelevision.[94]
Genetics
Mainarticle:
Geneticsofobesity
A1680paintingbyJuanCarrenodeMirandaofagirlpresumedtohavePrader-Willisyndrome[95]
Likemanyothermedicalconditions,obesityistheresultofaninterplaybetweengeneticandenvironmentalfactors.Polymorphismsinvariousgenescontrollingappetiteandmetabolismpredisposetoobesitywhensufficientfoodenergypresent.Asof2006morethan41ofthesesiteshavebeenlinkedtothedevelopmentofobesitywhenafavorableenvironmentispresent.[96]PeoplewithtwocopiesoftheFTOgene(fatmassandobesityassociatedgene)hasbeenfoundonaveragetoweigh3–4 kgmoreandhavea1.67-foldgreaterriskofobesitycomparedtothosewithouttheriskallele.[97]Thepercentageofobesitythatcanbeattributedtogeneticsvaries,dependingonthepopulationexamined,from6%to85%.[98]
Obesityisamajorfeatureinseveralsyndromes,suchasPrader-Willisyndrome,Bardet-Biedlsyndrome,Cohensyndrome,andMOMOsyndrome.(Theterm"non-syndromicobesity"issometimesusedtoexcludetheseconditions.)[99]Inpeoplewithearly-onsetsevereobesity(definedbyanonsetbefore10 yearsofageandbodymassindexoverthreestandarddeviationsabovenormal),7%harborasinglepointDNAmutation.[100]
Studiesthathavefocuseduponinheritancepatternsratherthanuponspecificgeneshavefoundthat80%oftheoffspringoftwoobeseparentswereobese,incontrasttolessthan10%oftheoffspringoftwoparentswhowereofnormalweight.[101]
Thethriftygenehypothesispostulatesthatduetodietaryscarcityduringhumanevolutionpeoplearepronetoobesity.Theirabilitytotakeadvantageofrareperiodsofabundancebystoringenergyasfatwouldbeadvantageousduringtimesofvaryingfoodavailability,andindividualswithgreateradiposereserveswouldbemorelikelysurvivefamine.Thistendencytostorefat,however,wouldbemaladaptiveinsocietieswithstablefoodsupplies.[102]ThisisthepresumedreasonthatPimaIndians,whoevolvedinadesertecosystem,developedsomeofthehighestratesofobesitywhenexposedtoaWesternlifestyle.[103]
Medicalandpsychiatricillness
Certainphysicalandmentalillnessesandthepharmaceuticalsubstancesusedtotreatthemcanincreaseriskofobesity.Medicalillnessesthatincreaseobesityriskincludeseveralraregeneticsyndromes(listedabove)aswellassomecongenitaloracquiredconditions:
hypothyroidism,Cushing'ssyndrome,growthhormonedeficiency,[104]andtheeatingdisorders:
bingeeatingdisorderandnighteatingsyndrome.[2]However,obesityisnotregardedasapsychiatricdisorder,andthereforeisnotlistedintheDSM-IVRasapsychiatricillness.[105]Theriskofoverweightandobesityishigherinpatientswithpsychiatricdisordersthaninpersonswithoutpsychiatricdisorders.[106]
Certainmedicationsmaycauseweightgainorchangesinbodycomposition;theseincludeinsulin,sulfonylureas,thiazolidinediones,atypicalantipsychotics,antidepressants,steroids,certainanticonvulsants(phenytoinandvalproate),pizotifen,andsomeformsofhormonalcontraception.[2]
Socialdeterminants
Mainarticle:
Socialdeterminantsofobesity
Whilegeneticinfluencesareimportanttounderstandingobesity,theycannotexplainthecurrentdramaticincreaseseenwithinspecificcountriesorglobally.[107]Thoughitisacceptedthatenergyconsumptioninexcessofenergyexpenditureleadstoobesityonanindividualbasis,thecauseoftheshiftsinthesetwofactorsonthesocietalscaleismuchdebated.Thereareanumberoftheoriesastothecausebutmostbelieveitisacombinationofvariousfactors.
ThecorrelationbetweensocialclassandBMIvariesglobally.Areviewin1989foundthatindevelopedcountrieswomenofahighsocialclasswerelesslikelytobeobese.Nosignificantdifferenceswereseenamongmenofdifferentsocialclasses.Inthedevelopingworld,women,men,andchildrenfromhighsocialclasseshadgreaterratesofobesity.[108]Anupdateofthisreviewcarriedoutin2007foundthesamerelationships,buttheywereweaker.Thedecreaseinstrengthofcorrelationwasfelttobeduetotheeffectsofglobalization.[109]Amongdevelopedcountries,levelsofadultobesity,andpercentageofteenagechildrenwhoareoverweight,arecorrelatedwithincomeinequality.AsimilarrelationshipisseenamongUSstates:
moreadults,eveninhighersocialclasses,areobeseinmoreunequalstates.[110]
Manyexplanationshaveb
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