Intermittent Fasting 101 — The Ultimate Beginner's Guide

Intermittent fasting may slightly boost metabolism while helping you eat fewer calories. It's a very effective way to lose weight and belly fat. Health Benefits. HealthlineHealthConditionsDiscoverPlanConnectShopSubscribeNutritionHealthyEatingMealPrepVitamins&SupplementsLifestyleDietsWeightManagementConditionsProductsNutritionEvidenceBasedIntermittentFasting101—TheUltimateBeginner’sGuideWrittenbyKrisGunnars,BSc—MedicallyreviewedbyAmyRichter,RD,Nutrition—UpdatedonJune16,2022WhatItIsMethodsEffectsWeightLossBenefitsSimplerLifestyleWhoShouldn't?Safety&SideEffectsFAQGettingStartedShouldYou?Weincludeproductswethinkareusefulforourreaders.Ifyoubuythroughlinksonthispage,wemayearnasmallcommission.Here’sourprocess.Intermittentfasting(IF)iscurrentlyoneoftheworld’smostpopularhealthandfitnesstrends.Peopleareusingittoloseweight,improvetheirhealthandsimplifytheirlifestyles.Manystudiesshowthatitcanhavepowerfuleffectsonyourbodyandbrainandmayevenhelpyoulivelonger(1,2,3).Thisistheultimatebeginner’sguidetointermittentfasting.ShareonPinterestPhotographybyAyaBrackettWhatIsIntermittentFasting(IF)?Intermittentfasting(IF)isaneatingpatternthatcyclesbetweenperiodsoffastingandeating.Itdoesn’tspecifywhichfoodsyoushouldeatbutratherwhenyoushouldeatthem.Inthisrespect,it’snotadietintheconventionalsensebutmoreaccuratelydescribedasaneatingpattern.Commonintermittentfastingmethodsinvolvedaily16-hourfastsorfastingfor24hours,twiceperweek.Fastinghasbeenapracticethroughouthumanevolution.Ancienthunter-gatherersdidn’thavesupermarkets,refrigeratorsorfoodavailableyear-round.Sometimestheycouldn’tfindanythingtoeat.Asaresult,humansevolvedtobeabletofunctionwithoutfoodforextendedperiodsoftime.Infact,fastingfromtimetotimeismorenaturalthanalwayseating3–4(ormore)mealsperday.Fastingisalsooftendoneforreligiousorspiritualreasons,includinginIslam,Christianity,JudaismandBuddhism.SummaryIntermittentfasting(IF)isaneatingpatternthatcyclesbetweenperiodsoffastingandeating.It’scurrentlyverypopularinthehealthandfitnesscommunity.IntermittentFastingMethodsThereareseveraldifferentwaysofdoingintermittentfasting—allofwhichinvolvesplittingthedayorweekintoeatingandfastingperiods.Duringthefastingperiods,youeateitherverylittleornothingatall.Thesearethemostpopularmethods:The16/8method:AlsocalledtheLeangainsprotocol,itinvolvesskippingbreakfastandrestrictingyourdailyeatingperiodto8hours,suchas1–9p.m.Thenyoufastfor16hoursinbetween.Eat-Stop-Eat:Thisinvolvesfastingfor24hours,onceortwiceaweek,forexamplebynoteatingfromdinneronedayuntildinnerthenextday.The5:2diet:Withthismethod,youconsumeonly500–600caloriesontwononconsecutivedaysoftheweek,buteatnormallytheother5days.Byreducingyourcalorieintake,allofthesemethodsshouldcauseweightlossaslongasyoudon’tcompensatebyeatingmuchmoreduringtheeatingperiods.Manypeoplefindthe16/8methodtobethesimplest,mostsustainableandeasiesttostickto.It’salsothemostpopular.SUMMARYThereareseveraldifferentwaystodointermittentfasting.Allofthemsplitthedayorweekintoeatingandfastingperiods.HowItAffectsYourCellsandHormonesWhenyoufast,severalthingshappeninyourbodyonthecellularandmolecularlevel.Forexample,yourbodyadjustshormonelevelstomakestoredbodyfatmoreaccessible.Yourcellsalsoinitiateimportantrepairprocessesandchangetheexpressionofgenes.Herearesomechangesthatoccurinyourbodywhenyoufast:HumanGrowthHormone(HGH):Thelevelsofgrowthhormoneskyrocket,increasingasmuchas5-fold.Thishasbenefitsforfatlossandmusclegain,tonameafew(4,5,6,7).Insulin:Insulinsensitivityimprovesandlevelsofinsulindropdramatically.Lowerinsulinlevelsmakestoredbodyfatmoreaccessible(8).Cellularrepair:Whenfasted,yourcellsinitiatecellularrepairprocesses.Thisincludesautophagy,wherecellsdigestandremoveoldanddysfunctionalproteinsthatbuildupinsidecells(9,10)Geneexpression:Therearechangesinthefunctionofgenesrelatedtolongevityandprotectionagainstdisease(11,12).Thesechangesinhormonelevels,cellfunctionandgeneexpressionareresponsibleforthehealthbenefitsofintermittentfasting.SUMMARYWhenyoufast,humangrowthhormonelevelsgoupandinsulinlevelsgodown.Yourbody’scellsalsochangetheexpressionofgenesandinitiateimportantcellularrepairprocesses.AVeryPowerfulWeightLossToolWeightlossisthemostcommonreasonforpeopletotryintermittentfasting(13).Bymakingyoueatfewermeals,intermittentfastingcanleadtoanautomaticreductionincalorieintake.Additionally,intermittentfastingchangeshormonelevelstofacilitateweightloss.Inadditiontoloweringinsulinandincreasinggrowthhormonelevels,itincreasesthereleaseofthefatburninghormonenorepinephrine(noradrenaline).Becauseofthesechangesinhormones,short-termfastingmayincreaseyourmetabolicrateby3.6–14%(14,15).Byhelpingyoueatfewerandburnmorecalories,intermittentfastingcausesweightlossbychangingbothsidesofthecalorieequation.Studiesshowthatintermittentfastingcanbeaverypowerfulweightlosstool.A2014reviewstudyfoundthatthiseatingpatterncancause3–8%weightlossover3–24weeks,whichisasignificantamount,comparedtomostweightlossstudies(1).Accordingtothesamestudy,peoplealsolost4–7%oftheirwaistcircumference,indicatingasignificantlossofharmfulbellyfatthatbuildsuparoundyourorgansandcausesdisease(1).Another2011studyshowedthatintermittentfastingcauseslessmusclelossthanthemorestandardmethodofcontinuouscalorierestriction(16).However,keepinmindthatthemainreasonforitssuccessisthatintermittentfastinghelpsyoueatfewercaloriesoverall.Ifyoubingeandeatmassiveamountsduringyoureatingperiods,youmaynotloseanyweightatall.SUMMARYIntermittentfastingmayslightlyboostmetabolismwhilehelpingyoueatfewercalories.It’saveryeffectivewaytoloseweightandbellyfat.HealthBenefitsManystudieshavebeendoneonintermittentfasting,inbothanimalsandhumans.Thesestudieshaveshownthatitcanhavepowerfulbenefitsforweightcontrolandthehealthofyourbodyandbrain.Itmayevenhelpyoulivelonger.Herearethemainhealthbenefitsofintermittentfasting:Weightloss:Asmentionedabove,intermittentfastingcanhelpyouloseweightandbellyfat,withouthavingtoconsciouslyrestrictcalories(1,13).Insulinresistance:Intermittentfastingcanreduceinsulinresistance,loweringbloodsugarby3–6%andfastinginsulinlevelsby20–31%,whichshouldprotectagainsttype2diabetes(1).Inflammation:Somestudiesshowreductionsinmarkersofinflammation,akeydriverofmanychronicdiseases(17).Hearthealth:Intermittentfastingmayreduce“bad”LDLcholesterol,bloodtriglycerides,inflammatorymarkers,bloodsugarandinsulinresistance—allriskfactorsforheartdisease(20).Cancer:Animalstudiessuggestthatintermittentfastingmaypreventcancer(22,23,24,25).Brainhealth:IntermittentfastingincreasesthebrainhormoneBDNFandmayaidthegrowthofnewnervecells.ItmayalsoprotectagainstAlzheimer’sdisease(26,27,28,29).Anti-aging:Intermittentfastingcanextendlifespaninrats.Studiesshowedthatfastedratslived36–83%longer(30,31).Keepinmindthatresearchisstillinitsearlystages.Manyofthestudiesweresmall,short-termorconductedinanimals.Manyquestionshaveyettobeansweredinhigherqualityhumanstudies(32).SUMMARYIntermittentfastingcanhavemanybenefitsforyourbodyandbrain.Itcancauseweightlossandmayreduceyourriskoftype2diabetes,heartdiseaseandcancer.Itmayalsohelpyoulivelonger.MakesYourHealthyLifestyleSimplerEatinghealthyissimple,butitcanbeincrediblyhardtomaintain.Oneofthemainobstaclesisalltheworkrequiredtoplanforandcookhealthymeals.Intermittentfastingcanmakethingseasier,asyoudon’tneedtoplan,cookorcleanupafterasmanymealsasbefore.Forthisreason,intermittentfastingisverypopularamongthelife-hackingcrowd,asitimprovesyourhealthwhilesimplifyingyourlifeatthesametime.SUMMARYOneofthemajorbenefitsofintermittentfastingisthatitmakeshealthyeatingsimpler.Therearefewermealsyouneedtoprepare,cookandcleanupafter.WhoShouldBeCarefulOrAvoidIt?Intermittentfastingiscertainlynotforeveryone.Ifyou’reunderweightorhaveahistoryofeatingdisorders,youshouldnotfastwithoutconsultingwithahealthprofessionalfirst.Inthesecases,itcanbedownrightharmful.ShouldWomenFast?Thereissomeevidencethatintermittentfastingmaynotbeasbeneficialforwomenasitisformen.Forexample,a2005studyshowedthatitimprovedinsulinsensitivityinmen,butworsenedbloodsugarcontrolinwomen(33).Thoughhumanstudiesonthistopicareunavailable,olderstudiesinratshavefoundthatintermittentfastingcanmakefemaleratsemaciated,masculinized,infertileandcausethemtomisscycles(34,35).ThereareanumberofanecdotalreportsofwomenwhosemenstrualperiodstoppedwhentheystarteddoingIFandwentbacktonormalwhentheyresumedtheirpreviouseatingpattern.Forthesereasons,womenshouldbecarefulwithintermittentfasting.Theyshouldfollowseparateguidelines,likeeasingintothepracticeandstoppingimmediatelyiftheyhaveanyproblemslikeamenorrhea(absenceofmenstruation).Ifyouhaveissueswithfertilityand/oraretryingtoconceive,considerholdingoffonintermittentfastingfornow.Thiseatingpatternislikelyalsoabadideaifyou’repregnantorbreastfeeding.SUMMARYPeoplewhoareunderweightorhaveahistoryofeatingdisordersshouldnotfast.Thereisalsosomeevidencethatintermittentfastingmaybeharmfultosomewomen.SafetyandSideEffectsHungeristhemainsideeffectofintermittentfasting.Youmayalsofeelweakandyourbrainmaynotperformaswellasyou’reusedto.Thismayonlybetemporary,asitcantakesometimeforyourbodytoadapttothenewmealschedule.Ifyouhaveamedicalcondition,youshouldconsultwithyourdoctorbeforetryingintermittentfasting.Thisisparticularlyimportantifyou:Havediabetes.Haveproblemswithbloodsugarregulation.Havelowbloodpressure.Takemedications.Areunderweight.Haveahistoryofeatingdisorders.Areawomanwhoistryingtoconceive.Areawomanwithahistoryofamenorrhea.Arepregnantorbreastfeeding.Allthatbeingsaid,intermittentfastinghasanoutstandingsafetyprofile.Thereisnothingdangerousaboutnoteatingforawhileifyou’rehealthyandwell-nourishedoverall.SUMMARYThemostcommonsideeffectofintermittentfastingishunger.Peoplewithcertainmedicalconditionsshouldnotfastwithoutconsultingwithadoctorfirst.FrequentlyAskedQuestionsHereareanswerstothemostcommonquestionsaboutintermittentfasting.1.CanIDrinkLiquidsDuringtheFast?Yes.Water,coffee,teaandothernon-caloricbeveragesarefine.Donotaddsugartoyourcoffee.Smallamountsofmilkorcreammaybeokay.Coffeecanbeparticularlybeneficialduringafast,asitcanblunthunger.2.Isn’tItUnhealthytoSkipBreakfast?No.Theproblemisthatmoststereotypicalbreakfastskippershaveunhealthylifestyles.Ifyoumakesuretoeathealthyfoodfortherestofthedaythenthepracticeisperfectlyhealthy.3.CanITakeSupplementsWhileFasting?Yes.However,keepinmindthatsomesupplementslikefat-solublevitaminsmayworkbetterwhentakenwithmeals.4.CanIWorkoutWhileFasted?Yes,fastedworkoutsarefine.Somepeoplerecommendtakingbranched-chainaminoacids(BCAAs)beforeafastedworkout.YoucanfindmanyBCAAproductsonAmazon.5.WillFastingCauseMuscleLoss?Allweightlossmethodscancausemuscleloss,whichiswhyit’simportanttoliftweightsandkeepyourproteinintakehigh.A2011studyshowedthatintermittentfastingcauseslessmusclelossthanregularcalorierestriction(16).6.WillFastingSlowDownMyMetabolism?No.Olderstudiesshowthatshort-termfastsactuallyboostmetabolism(14,15).However,longerfastsof3ormoredayscansuppressmetabolism(36).7.ShouldKidsFast?Allowingyourchildtofastisprobablyabadidea.GettingStartedChancesarethatyou’vealreadydonemanyintermittentfastsinyourlife.Ifyou’veevereatendinner,thensleptlateandnoteatenuntillunchthenextday,thenyou’veprobablyalreadyfastedfor16+hours.Somepeopleinstinctivelyeatthisway.Theysimplydon’tfeelhungryinthemorning.Manypeopleconsiderthe16/8methodthesimplestandmostsustainablewayofintermittentfasting—youmightwanttotrythispracticefirst.Ifyoufinditeasyandfeelgoodduringthefast,thenmaybetrymovingontomoreadvancedfastslike24-hourfasts1–2timesperweek(Eat-Stop-Eat)oronlyeating500–600calories1–2daysperweek(5:2diet).Anotherapproachistosimplyfastwheneverit’sconvenient—simplyskipmealsfromtimetotimewhenyou’renothungryordon’thavetimetocook.Thereisnoneedtofollowastructuredintermittentfastingplantoderiveatleastsomeofthebenefits.Experimentwiththedifferentapproachesandfindsomethingthatyouenjoyandfitsyourschedule.SUMMARYIt’srecommendedtostartwiththe16/8method,thenperhapslatermoveontolongerfasts.It’simportanttoexperimentandfindamethodthatworksforyou.ShouldYouTryIt?Intermittentfastingisnotsomethingthatanyoneneedstodo.It’ssimplyoneofmanylifestylestrategiesthatcanimproveyourhealth.Eatingrealfood,exercisingandtakingcareofyoursleeparestillthemostimportantfactorstofocuson.Ifyoudon’tliketheideaoffasting,thenyoucansafelyignorethisarticleandcontinuetodowhatworksforyou.Attheendoftheday,thereisnoone-size-fits-allsolutionwhenitcomestonutrition.Thebestdietforyouistheoneyoucansticktointhelongrun.Intermittentfastingisgreatforsomepeople,notothers.Theonlywaytofindoutwhichgroupyoubelongtoistotryitout.Ifyoufeelgoodwhenfastingandfindittobeasustainablewayofeating,itcanbeaverypowerfultooltoloseweightandimproveyourhealth.ReadthearticleinSpanishLastmedicallyreviewedonMarch28,2022FEEDBACK:WrittenbyKrisGunnars,BSc—MedicallyreviewedbyAmyRichter,RD,Nutrition—UpdatedonJune16,2022ReadthisnextYouCan’tExerciseYourWayOutofanUnhealthyDietEvenifyouregularlyexercise,anewstudyfindsthathavinganunhealthydietcanstillincreaseyourriskofseveraldiseasesandearlydeath.READMOREThe14BestGluten-FreeBreadBrandsAccordingtoDietitiansWrittenbyKelliMcGrane,MS,RDandEllenLandes,MS,RDN,CPTIfyou’reonagluten-freediet,youlikelyknowhowdifficultitcanbetofindabreadthat’sbothnutritiousandtasty.Herearethe14bestgluten-READMOREThe11BestKetoBooksof2022WrittenbySaVannaShoemaker,MS,RDN,LDIfyou'reinterestedindoingthelowcarb,highfatketodiet,youmaywanttodiveintoagoodbookonthesubjecttolearnallyoucanaboutit…READMOREVeganIntermittentFasting:IsItRightForYou?WrittenbyAlinaPetre,MS,RD(NL)Intermittentfastingandveganismareincreasinglypopulareatingpatterns,butcanyoucombinethem?Thisarticlereviewsthepros,cons,safety,and…READMORESprinlyReview:Pros,Cons,Cost,andMoreWrittenbyRachaelLink,MS,RDSprinlyisamealdeliveryserviceofferingpremadeveganmealsdeliveredtoyourdoor.ThisarticlereviewstheprosandconsofSprinly.READMOREIdealProteinDietReview:DoesItWorkforWeightLoss?WrittenbyRyanRaman,MS,RDandJillianKubala,MS,RDTheIdealProteinDietisamodifiedketodietthatmustbeundertakenwithone-on-onecoaching.ThisarticletakesacloselookatwhethertheIdeal…READMOREMediterraneanDietvs.PaleoDiet:What'stheDifference?WrittenbyTristaChan,MHScRDTheMediterraneandietandthepaleodiethavealotincommonbutdifferinafewways.Thisarticlereviewstheirsimilaritiesanddifferencesand…READMOREWhatIsthePineappleDiet,andShouldYouTryIt?WrittenbyJillianKubala,MS,RDThepineapplediet,alsoknownastheSexyPineappleDiet,isafaddietpeoplehaveusedsincethe1970s.Thisarticleexplainswhyyoushouldn'tuse…READMORE8oftheBestKeto-FriendlyProteinBarsYouCanBuyin2022WrittenbyKelliMcGrane,MS,RDandLaurenPanoff,MPH,RDProteinbarsmakeagreatsnack,butfindingketo-friendlyonescanbeachallenge.Hereare8ofthebestketoproteinbarsof2022.READMOREStudyFindsFishLinkedtoSkinCancerRisk,ButYouDon’tNeedToGiveUponSeafoodGreaterconsumptionoftunaandothernon-friedfishwasassociatedwithanincreasedriskofmelanoma,themostserioustypeofskincancer…READMORE