ArticlePDF AvailableLiterature Review

The Safe and Effective Use of Plant-Based Diets with Guidelines for Health Professionals

Authors:

Abstract

Plant-based diets, defined here as including both vegan and lacto-ovo-vegetarian diets, are growing in popularity throughout the Western world for various reasons, including concerns for human health and the health of the planet. Plant-based diets are more environmentally sustainable than meat-based diets and have a reduced environmental impact, including producing lower levels of greenhouse gas emissions. Dietary guidelines are normally formulated to enhance the health of society, reduce the risk of chronic diseases, and prevent nutritional deficiencies. We reviewed the scientific data on plant-based diets to summarize their preventative and therapeutic role in cardiovascular disease, cancer, diabetes, obesity, and osteoporosis. Consuming plant-based diets is safe and effective for all stages of the life cycle, from pregnancy and lactation, to childhood, to old age. Plant-based diets, which are high in fiber and polyphenolics, are also associated with a diverse gut microbiota, producing metabolites that have anti-inflammatory functions that may help manage disease processes. Concerns about the adequate intake of a number of nutrients, including vitamin B12, calcium, vitamin D, iron, zinc, and omega-3 fats, are discussed. The use of fortified foods and/or supplements as well as appropriate food choices are outlined for each nutrient. Finally, guidelines are suggested for health professionals working with clients consuming plant-based diets.
Nutrients2021,13,4144.https://doi.org/10.3390/nu13114144www.mdpi.com/journal/nutrients
Review
TheSafeandEffectiveUseofPlantBasedDietswith
GuidelinesforHealthProfessionals
WinstonJ.Craig
1,
*,AnnReedMangels
2
,UjuéFresán
3
,KateMarsh
4
,FaythL.Miles
1,5
,AngelaV.Saunders
6
,
EllaH.Haddad
1
,CelineE.Heskey
1
,PatriciaJohnston
1
,EnetteLarsonMeyer
7
andMichaelOrlich
5
1
CenterforNutrition,HealthyLifestyles,andDiseasePrevention,SchoolofPublicHealth,LomaLinda
University,LomaLinda,CA92354,USA;fmiles@llu.edu(F.L.M.);ehaddad@llu.edu(E.H.H.);
cheskey@llu.edu(C.E.H.);pjohnston@llu.edu(P.J.)
2
VegetarianResourceGroup,Baltimore,MD21203,USA;reedmangela@gmail.com
3
eHealthGroup,InstitutodeSaludGlobalBarcelona(ISGlobal),08036Barcelona,Spain;
ujue.fresan@isglobal.org
4
PrivatePractice,Chatswood,NSW2067,Australia;drkatemarsh@gmail.com
5
SchoolofMedicine,LomaLindaUniversity,LomaLinda,CA92354,USA;morlich@llu.edu
6
NutritionInsights,SanitariumHealthFoodCompany,BerkeleyVale,NSW2261,Australia;
angela.saunders@sanitarium.com.au
7
HumanNutrition,Foods,andExerciseVirginiaTech,Blacksburg,VA24061,USA;enette@vt.edu
*Correspondence:wcraig@llu.edu
Abstract:Plantbaseddiets,definedhereasincludingbothveganandlactoovovegetariandiets,
aregrowinginpopularitythroughouttheWesternworldforvariousreasons,includingconcerns
forhumanhealthandthehealthoftheplanet.Plantbaseddietsaremoreenvironmentally
sustainablethanmeatbaseddietsandhaveareducedenvironmentalimpact,includingproducing
lowerlevelsofgreenhousegasemissions.Dietaryguidelinesarenormallyformulatedtoenhance
thehealthofsociety,reducetheriskofchronicdiseases,andpreventnutritionaldeficiencies.We
reviewedthescientificdataonplantbaseddietstosummarizetheirpreventativeandtherapeutic
roleincardiovasculardisease,cancer,diabetes,obesity,andosteoporosis.Consumingplantbased
dietsissafeandeffectiveforallstagesofthelifecycle,frompregnancyandlactation,tochildhood,
tooldage.Plantbaseddiets,whicharehighinfiberandpolyphenolics,arealsoassociatedwitha
diversegutmicrobiota,producingmetabolitesthathaveantiinflammatoryfunctionsthatmayhelp
managediseaseprocesses.Concernsabouttheadequateintakeofanumberofnutrients,including
vitaminB12,calcium,vitaminD,iron,zinc,andomega3fats,arediscussed.Theuseoffortified
foodsand/orsupplementsaswellasappropriatefoodchoicesareoutlinedforeachnutrient.Finally,
guidelinesaresuggestedforhealthprofessionalsworkingwithclientsconsumingplantbaseddiets.
Keywords:plantbaseddiets;vegetarian;vegan;sustainability;microbiome;vitaminB12;CV
disease;diabetes;bonehealth;lifecycle
1.Introduction
Interestinplantbaseddietshassoaredinthepastdecadeforamyriadofreasons[1].
Peopleareconcernedaboutissuessuchastheirhealth,climatechange,thesustainability
ofthefoodproductionsystem,andthewelfareofanimals.Aplantbaseddietisdefined
invariousways.Forsomeitmeanseatingfoodsmostly,butnotentirely,ofplantorigin,
whileforothersitmeanseatingonlyplantbasedfoods.Inthismanuscript,wechoseto
restrictthetermplantbasedtoincludebothvegandiets(totalplantbasednutrition)and
lactoovovegetariandiets(thisallowsfortheconsumptionofdairyproductsandeggs).
Wedonotincludeadiscussionofflexitarian,semivegetarian,orpescovegetarianeating
patterns,astheydonotfitintoourdefinitionofplantbaseddiets.
Citation:Craig,W.J.;Mangels,A.R.;
Fresán,U.;Marsh,K.;Miles,F.L.;
Saunders,A.;Haddad,E.;Heskey,C.;
J
ohnston,P.;Larso
n
Meyer,E.;etal.
TheSafeandEffectiveUseofPlant
BasedDietswithGuidelinesforHealth
Professionals.Nutrients2021,13,4144.
https://doi.org/10.3390/nu13114144
AcademicEditor:StephenIves
Received:27October2021
Accepted:17November2021
Published:19November2021
Publisher’sNote:MDPIstays
neutralwithregardtojurisdictional
claimsinpublishedmapsand
institutionalaffiliations.
Copyright:©2021bytheauthors.
LicenseeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsand
conditionsoftheCreativeCommons
Attribution(CCBY)license
(https://creativecommons.org/license
s/by/4.0/).
Nutrients2021,13,41442of30
Thispaperwilldiscusstheenvironmentalissuesandthebenefitsfortheplanetof
significantlyreducingoreliminatingmeatanddairyfoodsfromourdiet.Inaddition,it
outlinesthetherapeuticadvantagesofaplantbaseddietformanagingthechronic
diseasesofWesternsociety,suchasobesity,cardiovasculardisease(CVD),cancer,and
diabetes.Aplantbaseddietisalsoshowntohaveasubstantialimpactuponthe
compositionandfunctionofthegutmicrobiome,whichinturninfluencesouroverall
health.Furthermore,thesafetyoffollowingaplantbaseddietduringallstagesofthelife
cycleisaddressed.Finally,questionswereraisedabouttheadequacyofaplantbaseddiet
withrespecttoeightkeynutrients.Thesearediscussedindetail,withsolutionssuggested
astohowonecanmeetthedietaryrequirementsthroughfoodchoicesand/or
supplementation.Somesimpleguidelinesaregivenforhealthprofessionalstoeffectively
servethegrowingpopulationofthoseconsumingaplantbaseddiet.
2.CurrentTrends
Internationally,theprevalenceoffollowingavegetariandietvariesbycountry,but
itisgenerallyestimatedtobelessthan10%ofthepopulation.TheexceptionisIndia,
where20%ormoreofadultsarevegetarian[2,3].IntheUnitedStates,anationwidepoll
in2020foundthatapproximately6%ofadultsfollowedavegetariandiet,withhalfof
thembeingvegans[4].AsimilarU.S.pollfoundthatapproximately2%of8‐to17year
oldchildrenfollowedavegandiet,and3%followedanonveganvegetariandiet[5].
Globally,themarketforalternativestodairyproductsisexpectedtoreach$US25
billionby2026[6].U.S.retailsalesofplantbasedfoods(plantbaseddairyalternatives
andplantbasedmeats)increased27%between2019and2020,withatotalplantbased
marketvalueestimatedat$7billion[7],suggestingagrowingconsumerinterestinnon
animalproducts.
The2020–2025DietaryGuidelinesforAmericansendorsesa“HealthyVegetarian
DietaryPattern”asoneofthreedietarypatternsthatcan“betailoredtomeetculturaland
personalpreferences”[8].Thereareversionsofthisplanforagesoneyearandolder.The
GuidelinesalsoencourageallAmericanstoeatmoreplantfoods,includingdriedbeans,
wholegrains,fruits,vegetables,andnuts.Manyothercountriespromoteplantbased
dietsintheirdietaryguidelines[9].
Withagrowinginterestinvegetarianeating,establishmentssuchascollegesand
universities,schoolfoodservices,airlines,restaurants,prisons,employeefoodservices,
nursinghomes,andhospitalsareincreasinglyprovidingvegetarianoptions[10–12].
3.EnvironmentalSustainabilityofVegetarianDiets
Theproductionofdifferentfoodscanhaveverydiverseenvironmentalimpacts.
Thereisalargevariationintheecologicalfootprintofanimalbasedproducts,with
ruminantmeatbeingespeciallydetrimentalfortheenvironmentascomparedwithother
productssuchaspork,whitemeat,oreggs[13–15].Anincreasingbodyofdataprovides
evidencethatenvironmentaldegradation,throughtheemissionsofgreenhousegas
(GHG)andotherpollutants,andtheuseofearth’sresources,suchaswaterandland,in
theproductionofplantbasedfoodsaresignificantlylowerthanthatfromanimalbased
foods[13–15].Certainly,theeffectsofthelowestimpactanimalproductsaretypically
greaterthanthoseofplantbasedalternatives,eveninthecaseofhighlyprocessedplant
basedmeatanalogs[14,16].Theproductionofplantbasedproductsismoreefficient
regardlessofwhetherthecomparisonismadebyweightofproduct,perserving,per
calories,orevenproteincontent[14,15,17,18].Producingthesameamountofproteinfrom
tofu(soybeans)incomparisontobeefproteinrequires74timeslesslandandeighttimes
lesswater,whiletheGHGemissionsare25timeslowerandtheeutrophication(aprocess
drivenbytheenrichmentofwaterbynutrients,especiallycompoundsofnitrogenand/or
phosphorus,leadingtoanincreasedgrowth,primaryproductionandbiomassofalgae;
changesinthebalanceoforganisms;andwaterqualitydegradation[19])potentialis
reducedby39times[14,18].Evenifcomparedtoeggprotein,tofuproteinrequiresalmost
Nutrients2021,13,41443of30
threetimeslesslandandsixtimeslesswater,whiletheGHGemissionsareonlyhalfof
thatfromeggprotein,andtheeutrophicationpotentialisfivetimeslower[14,18].
Likewise,areductionofanimalbasedfoodsinthedietgoeshandinhandwitha
decreaseinthedietaryenvironmentalimpact[20,21].Vegetariandiets,bothlactoovo
vegetarianandvegan,havebeendescribedasmoreenvironmentallysustainablethan
thosedietsincludingmeat.Areviewstudyconcludedthattheadoptionoflactoovo
vegetariandietscouldreducethedietaryGHGemissionsby35%,landuseby42%,and
freshwateruseby28%[20].Adoptingavegandietwouldleadtoaroundonehalfofboth
GHGemissionsandlanduseofthatofcurrentdietarypatterns.Oneshouldnotethat
thereissubstantialvariabilityinthedietaryenvironmentalimpactofthoseconsuming
vegetariandiets.Inthefinalanalysis,anyenvironmentalbenefitswoulddependonthe
quantityandthespecificfoodsconsumed.Overconsumptionofcalories,ahighintakeof
fruitstransportedbyplane,ortheconsumptionoflargequantitiesoffattydairyproducts,
suchascheeseorbutter,inlactoovovegetariandietscouldjeopardizeanypotential
benefitfromtheavoidanceofmeat.
Studiessuggestthattheadoptionofnutritionallybalancedvegetariandiets,bothin
developedanddevelopingcountries,couldbeaneffectivestrategyforreducingGHG
emissionsworldwide[22,23].Thisdietarytransitionwouldbemoderatelyeffectivein
reducingfertilizerapplicationandwoulddecrease,althoughtoalesserextent,cropland
andfreshwateruse[23].Altogether,embracingabalancedvegetariandiet,especiallyin
developedcountries,couldbeaneffectivestrategyforreducingthefoodsystem’s
environmentaldegradationandreducingouruseoftheearth’sresources.
4.PlantBasedDietsandChronicDiseases
4.1.CVD,IncludingHyperlipidemia,IschemicHeartDisease,Hypertension,andStroke
CVDcontinuesasthemostcommoncauseofdeathanddisabilityintheU.S.and
globally[24,25].TheleadingriskfactorsforCVDincludedyslipidemia,excessweight,
hypertension,glucometabolicdisorders,anddiabetesandareattributedtopoordiets
[26,27].Comparedtoomnivorousdiets,vegetarianandplantbaseddietsrichinwhole
grains,legumes,vegetables,fruits,nuts,andseedshavebeenassociatedwithsubstantial
reductionsinseveralmodifiableriskfactors,includingbodymassindex(BMI)andwaist
circumference[28,29],atherogeniclipoproteinconcentrations[29,30],bloodglucose[28],
inflammation[31],andbloodpressure[32].
Theresultsofrandomizedcontrolledtrials(RCT)ofveganandvegetarian
interventionsalongwithsystematicreviewsandmetaanalysesofsuchstudiesshow
improvementsinseveralintermediatecardiometabolicriskmarkers,includingbody
weightandbloodlipids[33–37],andcardiometabolicriskprofiles[38].Datafrom
relativelylongterm(years)clinicalinterventionstudieswithintensivelowfatvegetarian
[39]andvegandiets[40]showreversalsincoronaryarterydiseaseinindividualswith
CVD.Duetolowersaturatedfatandcholesterollevelsandmoreoptimalplantsteroland
fibercontent,greaterfavorableeffectsofvegandietsonheartdiseaseriskfactorsare
expected.Avegandiet,comparedtotheAmericanHeartAssociation(Dallas,TX,USA)
dietforcoronaryheartdisease(CHD),resultedinsimilarreductionsinBMI,waist
circumference,markersofglycemiccontrol,bloodlipids,anda32%lowerhighsensitivity
Creactiveprotein(aproinflammatorymarker)[31].
IntheAdventistHealthStudy2(AHS2),vegetarianshada13%and19%lowerrisk
ofCVDandischemicheartdisease(IHD)mortality,respectively,comparedwithnon
vegetarians.Thisdifferenceoccurredinspiteofthefactthatthenonvegetariansinthe
cohortconsumedlessmeatthanthegeneralpopulation.Bloodpressurelevelsinvegans
andvegetarianswerealsolowerthanthoseoftheomnivores.Metabolicsyndromeand
type2diabetes(T2D)areprimeriskconditionsforCVDandstroke.Areducedprevalence
oftheseconditionswasobservedinveganandvegetarianparticipantsofAHS2[28,41].
Nutrients2021,13,41444of30
TheEPICOxfordstudyofvegetarians,vegans,andhealthconsciousindividuals
reportedthattheriskofincidentIHDhospitalizationsanddeathscausedbycirculatory
diseasewas32%lowerinvegetariansthaninnonvegetarians[42].The18yearfollowup
showedlowerratesofIHDinvegetariansbuthigherratesofhemorrhagicandtotalstroke
[43].Redmeatintake,bothprocessedandunprocessed,wasassociatedwithCHDriskin
malehealthprofessionals[44].Inthelargeprospectivecohortofmenandwomenofthe
USNationalInstitutesofHealth—AARPDietandHealthStudy,higherplantprotein
intakewasassociatedwithreducedCVDmortality[45].IntheAtherosclerosisRiskin
Communitiesstudy,higherintakesofplantbaseddietsscoredashealthywereassociated
withalowerriskofincidentCVDandCVDmortality[46].
PooleddatafromsevenprospectivecohortstudiesshowedareducedCHDincidence
andmortalityof28%and22%,respectively,associatedwithvegetariandiets.No
associationwithCVDorstrokemortalitywasseen[47].Similarly,acomprehensive
reviewandmetaanalysisof10prospectivecohortstudiesshoweda25%reducedriskof
incidenceormortalityfromIHDinvegetarianandvegandietsbutnotoftotalCVDand
strokemortality[48].CVDandstrokemortalityoutcomesmaybeinfluencedbylifestyle
factorsotherthandietandbyaccesstocardiovascularhealthcare.
4.2.Type2Diabetes
Observationalstudiesinavarietyofpopulationshaveconsistentlyshownthat
comparedtononvegetarians,thosefollowingavegetarianorvegandiethavea
significantlylowerriskofT2D[41,49–53].A2017systematicreviewandmetaanalysisof
14studiesfoundapooledoddsratiofordiabetesinvegetariansvs.nonvegetariansof
0.73[54].A2020systematicreviewsimilarlyfoundthatavegandietwasassociatedwith
lowerprevalenceorincidenceofT2D,althoughinsomestudiesitwasnotpossibleto
determineifthebenefitswereduetothevegandietaloneorcombinedwithotherhealthy
lifestylehabits[55].
A2018systematicreviewofnineRCTsfoundthat,comparedtocontroldiets
(includingthoseofseveraldiabetesassociations),plantbaseddietswereassociatedwith
significantimprovementinemotionalwellbeing,physicalwellbeing,depression,quality
oflife,generalhealth,HbA1clevels(ameasureoflongtermbloodglucoselevels),weight,
andtotalandLDLcholesterollevels[56].Anearliersystematicreviewandmetaanalysis
ofsixstudiesfoundthattheconsumptionofvegetariandietswasassociatedwitha
significantreductioninHbA1ccomparedtocontroldiets[57].Similarly,areductionin
HbA1chasbeenobservedwithplantbaseddiets,includingvegetarian,vegan,
Mediterranean,andDietaryApproachestoStopHypertension(DASH)diets,compared
tocontrolorconventionaldiets[58].
Thereareseveralpossibleexplanationsforthebenefitsofplantbaseddietsfor
diabetespreventionandmanagement.Comparedtomostwesterndiets,vegetarianand
vegandietsaregenerallyhigherindietaryfiberandarelikelytoincludemorewhole
grains,legumesandnuts,allofwhichhavebeenassociatedwithareducedriskofT2D
[59].Thereisalsoevidenceforaninverseassociationbetweenhigherintakesofgreenleafy
vegetablesandfruitandtheriskofT2D[60–63].
Theabsenceorlimitedintakeofanimalproteinandredmeatalsolikelyplaysarole.
Atleast25studieshavebeenpublishedassessingtherelationshipbetweenmeatintake
andT2Drisk,withthemajorityshowingapositiveassociationbetweenredmeatand/or
processedmeatintakes.A2013metaanalysisfoundanassociationbetweenhigher
intakesoftotalmeat,unprocessedredmeat,andprocessedmeatandT2Drisk[64].There
isalsoconsistentevidenceforanassociationbetweentotaldietaryhemeironintakeand
hemeironintakefromredmeatandriskofT2D,andhighserumferritinlevelsare
associatedwithinsulinresistanceandT2Drisk[65].A2019metaanalysisofprospective
cohortstudieslookingatdietaryproteinintakeandsubsequentriskofT2Dfoundhigh
totalproteinandanimalproteinintakestobeassociatedwithanincreasedriskofT2D,
whileamoderateplantproteinintakewasassociatedwithadecreasedrisk[66].Anearlier
Nutrients2021,13,41445of30
systematicreviewandmetaanalysisof13RCTsinpeoplewithdiabetesfoundthat
replacinganimalproteinwithplantprotein(around35%oftotalprotein/day)resultedin
significantreductionsinHbA1c,fastingglucose,andfastinginsulinlevelscomparedto
controlgroups[67].
ExcessweightisasignificantcontributortoinsulinresistanceandT2Drisk,and
weightlossisakeycomponentofthemanagementofT2D[68].Followingavegetarianor
vegandiet,oneislesslikelytobeoverweight[69].
4.3.Cancer
Eachoftheplantfoodgroupshasshownthattheypossesschemoprotective
properties.Systematicreviewsandmetaanalyseshaveshownthatanincreasednut
consumptionwasassociatedwithbothadecreasedriskofallcancerscombined[70]and
decreasedcancermortality[71].Inthesamemanner,anincreasedintakeoffruitsand
vegetablesandofwholegrainswasshowntodecreasetheriskoftotalcancerincidence
[72]andtotalcancermortality[73],respectively.Furthermore,ahigherintakeoflegumes
(beansandlentils)wasassociatedwithadecreaseintheriskofgastrointestinalcancers
andallcancersitescombined[74].Manyplantfoodsarerichinhealthpromoting
phytochemicals,someofwhichhavebeenshowntobeusefulinthetreatmentofhuman
cancer[75,76].
Ontheotherhand,theconsumptionof100–120g/dayofredmeatsignificantly
increasedtheriskofmanycancers(comparedtoeatingnomeat):11%forbreastcancer,
17%forcolorectalcancer,and19%foradvancedprostatecancer[77].Fortheconsumption
of50g/dayofprocessedmeat,theriskwasincreased4%fortotalprostatecancer,9%for
breastcancer,18%forcolorectalcancer,19%forpancreaticcancer,and8%forcancer
mortality[77].IntheFrenchNutriNetSantécohortstudy,redmeatintakewasassociated
withincreasedriskofoverallcancers(HR1.31)andbreastcancer(HR1.83),butnot
prostatecancer[78].IntheNationalInstitutesofHealth(Rockville,MD,USA)—AARP
DietandHealthStudycohortofhalfamillionpeople,aged50to71yearsatbaseline,
higherredandprocessedmeatintakeswereassociatedwithmodestincreasesintotaland
cancermortality[79].
Withtheeliminationofmeatandagreateruseofprotectiveplantfoods,vegetarians
mayhaveareducedriskofcancer.EpidemiologiccohortstudiesintheU.S.andUKhave
providedhighqualityevidenceregardingtheassociationofvegetariandietarypatterns
withcancerrisk.IntheUSbasedAHS–2,veganshadloweroverallcancerriskcompared
tononvegetarians(HR0.84);overallcancerriskforlactoovovegetarianswasnot
significantlydifferentfromnonvegetarians[80].Vegansshowedalowerriskofprostate
cancer(HR0.65)[81]andalower(butnotstatisticallysignificantlylower)riskofbreast
cancer[82].Neitherlactoovovegetariansorveganshadasignificantlylowerriskof
colorectalcancer[83].
IntheUKbasedEPICOxfordstudy,comparedwithmeateaters,vegans(HR0.82)
andlactoovovegetarians(HR0.90)hadlowerriskofallcancerscombined[84].For
prostatecancer,whilevegans(HR0.61)andvegetarians(HR0.86)hadlowerrisk,they
werenotsignificantlydifferentfrommeateaters[84].Forcolorectalcancerandfemale
breastcancer,riskforthevegetariangroupsagaindidnotsignificantlydifferfrommeat
eaters[84].IntheUKWomen’sCohortStudy,comparedwithredmeateaters,theriskof
breastcancerforvegetarianswasnotsignificantlylower(HR0.85)[85].
Takenasawhole,suchresultsseemtosupporttheideathatvegetarians(including
vegansandlactoovovegetarians)haveamodestbutpotentiallyimportantreducedoverall
cancerriskcomparedtotheirnonvegetariancounterparts.Findingsforcommonindividual
cancers(colorectal,prostate,breast)arelessconsistentandwarrantfurtherstudy.
Nutrients2021,13,41446of30
4.4.OverweightandObesity
Over70%ofadultsintheU.S.areoverweightorobese[86],andtrendsshowthat
overweightandobesityareincreasingworldwide[87].Observationalstudiesshowthat
vegansandvegetarianstypicallyhavealowerBMIthanomnivores[88,89],and
vegetariandietsorplantbasedtypedietarypatternsareprotectiveagainstadultweight
gainand/ortheriskofoverweightorobesity[90,91].Veganstypicallyhavethelowest
BMIorlowestprevalenceofoverweightorobesityinstudiesthatcomparemultiple
dietarypatterns,includingvegetariansandomnivores[88,92].Goggaetal.noted
differencesinpercentbodyfatbetweenvegans,lactoovovegetarians,andomnivores,
eventhoughallgroupBMIvalueswerewithinthenormalrange[93].Interventionsusing
vegan[94–99],vegetarian[97,100],orwholefoodplantbaseddietary[101]treatments
havebeenfoundtolowerBMI,weight,orfatmasscomparedtosubjectsonameatbased
diet.A4.8%weightlosswasreportedforoverweightandobesesubjectsrandomizedtoa
veganorvegetariandietfor2months,comparedtoa2.2%lossseeninthoseconsuming
anomnivorousdiet[97].Weightlossof3to5%isclinicallymeaningfulandmay
contributetochronicdiseaseriskfactorreduction[102].
Thequalityoftheplantbaseddietisalsoanimportantconsideration.Subjectswho
adheretoahealthyplantbaseddietarereportedtohavealowerBMI,waist
circumference,andvisceralfatthanthosewhoadhereto‘unhealthy’plantbaseddiets
[103,104].Researchershavenotedthatdietqualitymaybemoreimportantthandietary
patternswhencomparingvegans,vegetarians,andomnivores,astheadiposityvaluesdid
notdiffersignificantlybetweenthesegroups[105].Theweightlossexperiencedona
hypocaloriclactoovovegetariandietwassimilartothatobservedwithahypocaloric
Mediterraneandiet[106].
Mechanismsthatexplaintheweightmanagementbenefitsofplantbaseddiets
includerelativelyhigherfiber,fruit,andvegetableconsumptioncomparedtoomnivorous
diets[88,107].Thisfoodpatternmayleadtobeneficialalterationstoappetitehormones
[93,108]andthegutmicrobiota[109],bothofwhichmayhaveanimpactonbodyweight.
4.5.BoneHealth
Healthybonesrequireavarietyofessentialnutrientsandhealthylifestylepractices
tomaximizepeakbonemassduringgrowthandminimizebonelosslaterinlife[110].
WhilecalciumandvitaminDarewellrecognizedasimportantcontributorstobone
health,othernutrients,includingmagnesium,potassium,vitaminK,vitaminC,andzinc,
aswellasbioactivecompoundsfoundinfruitsandvegetables,havebeensuggestedas
contributingtobonehealthand/orreducedriskoffracture[111–114].Somehavereported
greaterbenefitfromvegetables,especiallycruciferousandalliumvegetables,thanfrom
fruit[115,116].
Therelationshipofproteinintaketobonestatusiscomplex.Earlierstudiesshowed
highintakescausealossofcalcium,whilearecentreviewfound“noadverseeffectsof
higherproteinintakes”andsomepositivetrendsatmostbonesites[117].Arecentreview
andmetaanalysisfoundnodifferencebetweensoyandanimalproteinonbonemineral
density(BMD)andcertainmarkersofboneturnover[118].Otherssuggestthelowacid
loadofvegetariandiets,partlyduetothepotassiumandmagnesiumcontentfroman
increasedfruitandvegetableintake,isbeneficialtobonehealth[119].Someelderly
vegetariansandafewvegansmaynotconsumesufficientproteinformaintainingoptimal
bonehealth[114,120,121].
Theimpactofavegetariandietonbonehealthhasmanydimensions.Reportscan
varyconsiderablyinstudydesign,populations,andconclusions.Somefindsignificantly
lowerBMDinvegetarians,especiallyvegans,whichcouldincreasefracturerisk[122],
whileothersseenodifferenceinbonehealth,providedthatcalciumandvitaminDis
adequate[123],andconcludethatvegetarianfoodcanprovideasolidfoundationfor
healthybonesandpreventingfractures[124].
Nutrients2021,13,41447of30
AlargeprospectiveUKstudyfoundthatfisheatersandvegetarianshadahigher
riskofhipfracturescomparedtomeateaters,whileveganshadagreaterriskoftotal,hip,
leg,andvertebralfractures[125].Someofthedifferencesmayhavebeenpartlydueto
lowerBMIandpossiblylowerintakeofcalciumandproteininthevegans.
Asystematicreviewofsome20studiesinvolving37,134subjectsfoundvegetarians
andveganshadlowerBMDatthefemoralneckandlumbarspinecomparedtoomnivores
[126].Theeffectwasgreaterinveganswhoalsohadhigherfracturerates[127,128].
Anotherreviewconcludedthatthebalancebetweenprotectivefactorsinvegetarianand
vegandietsandpotentialnutrientshortfallsmayleavevegetarians,andespeciallyvegans,
atincreasedriskofbonelossandfractures[129].Potentialnutrientshortfallscanbe
remediedbyappropriatefoodselections(includingfortifiedfoods)containingcritical
nutrientsorbytakingsupplements.Moreresearchdataonthebonehealthofvegansare
neededbeforedefinitiverecommendationscanbemade.
5.EatingDisorders
Previoususeofavegetarianorvegandietapparentlydoesnotincreasetheriskof
developinganyeatingdisorder,suchasanorexianervosa,bulimianervosa,andbinge
eatingdisorder[130,131].Thosewithpreexistingdisorderedeatingtendenciesmayselect
vegetarianorpartiallyvegetariandietsasawaytolimitfoodintakeinasocially
acceptablefashion[130,132].Semivegetariansappeartobeathigherriskfordeveloping
eatingdisordersthanvegetariansandvegans[130,133].Thosevegetarianswhose
motivationisweightcontrolreportmoresymptomsofdisorderedeatingthandothose
withothermotivations[134].Commonlyusedassessmenttoolsmayincorrectlyassess
dietaryrestraintoreatingdisorderpsychopathologyinvegetarians[130].
6.PlantBasedDietsandtheGutMicrobiome
Thehumangutmicrobiotaisahighlycomplexcommunityofsome1014
microorganisms.Diethasasignificantimpactuponthemicrobiotacompositionand
function[135,136].Themicrobiomehasaprofoundimpactonone’spersonalhealthand
wellbeing[137].Manipulatingthegutmicrobiotahasbeenviewedasawaytomodulate
theriskofchronicdiseasessuchasobesity,T2D,cancer,andCVD[135,137].
Gutmicrobiotahaveamajorroleinthefermentationofnondigestiblecarbohydrates,
namelyresistantstarch,soluble/insolubledietaryfiber,includingplantwall
polysaccharidesandoligosaccharides.Fermentationofthesenondigestiblecarbohydrates
isassociatedwithahigherabundanceofmicrobesthatproducebutyrateandothershort
chainfattyacids,whichhaveanantiinflammatoryfunction,strengthentheintestinal
barrierfunction,andimproveoverallguthealth[138–141].Forexample,theconsumption
offiberrichfoodssuchasbarley,wheatbran,brownrice,andotherwholegrains,aswell
asfructooligosaccharidesandotherprebiotics,arereportedtoincreasebutyrate
producingmicrobes[137,142–146].Vegetarianswouldbeexpectedtohaveanincreased
abundanceofthesemicrobes,astheirfiberrichdietsaretypicallyhighinwholegrains,
fruits,vegetables,nuts,andlegumes[107].
Theseplantfoodsalsocontainpolyphenols—lignans,isoflavones,anthocyanins,and
flavonols—inadditiontootherphytochemicalssuchascarotenoidsandphytosterols
[147–149].Thesearemetabolizedintobioactivecompoundsbyvariousmicrobes[150],
somewithhealthbenefitsandantiinflammatoryorantioxidantactivity.Phytochemicals
increasebeneficialbacteria,includingLactobacillusandBifidobacterium,whicharethe
primaryspeciespresentinprobioticsupplementsthataretakentoimproveguthealth
[151],inadditiontosomebutyrateproducers[150].Amongfiberrichplantfoods,nutsin
particular(walnuts,almonds,pistachios)havebeenfoundtohaveprebioticeffectsand
areassociatedwithincreasesinbutyrateproducingmicrobesandotherbeneficial
microbes[152].Hence,thegutmicrobialcompositionisgreatlyinfluencedbydietaryfiber
aswellasbypolyphenolsandotherphytochemicalsandtheirmetabolites,allofwhich
aremorehighlyconsumedbyvegetarians.
Nutrients2021,13,41448of30
Studieshavesupportedthevalueoftwosocalledenterotypes,orclustersofmicrobes
drivenbydistinctgenera,indistinguishingdietarypatterns.Accordingly,Bacteroidesare
associatedwithanimalfatandhighproteindiets[153–157],andPrevotellaareassociated
withfiberrichfoodsandcarbohydrates,typicalofaplantbaseddiet[158–160].Higher
abundanceofPrevotellaandotherpolysaccharidedegradingorpotentialbutyrate
producingmicrobeshasbeenseenparticularlyinagrarianculturessuchasthosein
Tanzania,thePeruvianAmazon,andBurkinaFaso,comparedtoU.S.orWestern
populations,reflectingthehigherconsumptionoffiberrichplantfoodsbythesesocieties
[160–162].Hence,enterotypesmayhavesomeutilityindistinguishingplant‐andanimal
baseddiets.Plantbasedandhighfiberdietsarealsoassociatedwithincreasesinthe
Bacteroidetesphylum[160,162],ortheBacteroidetes/Firmicutesratio,aswellasmicrobial
richness/diversity[142,155,160,162–164],incontrasttodietshighinfat[165–169].Thisis
relevantinthatvariousmicrobesfromtheBacteroidetesphylumencodecarbohydrate
activeenzymes(CAZymes)necessaryfordegradingindigestiblecarbohydrates[139],and
theBacteroidetes:Firmicutesratiomayhaveimplicationsforobesityandmetabolicdiseases,
althoughtherelationshipisnotclearasfindingshavebeeninconsistent[170,171].
Differencesingutmicrobialcompositionarenotalwaysobservedincrosssectional
studiescomparingvegansorothervegetarianswithnonvegetarians[172].IntheAHS2
cohort,onlysubtledifferenceswerenotedinthemicrobiome[173].However,vast
differenceswerediscoveredintheplasmametabolome,withvegansshowinghigher
abundanceofantiinflammatoryplant/polyphenolormicrobialrelatedmetabolites[174].
Nonvegetariansontheotherhandmayhavehigherabundanceofaminoacidsandlipids
conceivablyassociatedwithcardiometabolicphenotypes[174–176].Intestinalmicrobiota
convertcholineandLcarnitine,derivedfrommeat,fish,dairy,andeggs,into
trimethylamine,whichisoxidizedbythelivertotrimethylamineNoxide(TMAO),apro
inflammatorycompoundthathasbeenassociatedwithincreasedcardiometabolicrisk
[177–180].Thus,itmaybethatmicrobialfunctionismorerelevantthancomposition,with
metabolicprofilesshowingmuchgreaterdifferences,reflectingphenotypicchanges.
Therearephysiologicalconsequencesofdietinducedshiftsinthemicrobiome.Low
consumptionofplantbasedfoodsmayleadtoincreasedpenetrationoftheintestinal
barrier,asalowfiberdiettriggersashiftfromfiberdegradingtomucusdegradingbacteria
[181].Thisinturncouldpromoteahyperactiveimmuneresponse,conceivablywiththe
productionofproinflammatorymetabolitesthatfueldiseaseprocesses[182].However,
muchremainstobeunderstoodabouthowvegetarianandplantbaseddietarypatterns
impactthemicrobiomeandassociatedmetabolicresponsestoinfluencediseaseprocesses.
7.PlantBasedDietsandtheLifeCycle
Vegetarian,includingvegan,dietscansatisfythenutritionalrequirementsofall
stagesofthelifecycle.Theycanpromotenormalgrowthanddevelopmentininfancy,
childhood,andadolescenceandmeettheneedsforenergyandnutrientsoftheselifecycle
stagesaswellasthoseofpregnancy,lactation,andolderadulthood.
7.1.PregnancyandLactation
Vegetariandietscaneffectivelymeetenergyandnutrientneedsinpregnancyand
lactation[183,184].Severalreviews,whilenotingthelimitedamountofinformationabout
vegetarian,includingvegan,dietsinpregnancy,haveconcludedthat,withadequate
nutrientintake,thesedietsaresafeinpregnancy[183,185].Whenfoodaccessis
satisfactory,infantbirthweightsandthedurationofgestationaresimilarinvegetarian
andnonvegetarianpregnancy[186,187].Somestudiesreportthatvegetariansaremore
likelytohaveinfantswhoaresmallfortheirgestationalage[188–190].Thisfindingmay
beduetolowermeanprepregnancyBMI,lowerweightgain,orinadequateweightgain
inpregnancy.Wellnourishedvegetariansproducenutritionallyadequatebreastmilkthat
supportsinfantgrowthanddevelopment[191].
Nutrients2021,13,41449of30
Healthbenefitsofvegetariandietsinpregnancyincludealowerriskofexcessive
weightgainandhigherfiberandfolateintakes[188,192,193].Dietarypatternsthatare
highinplantfoodsareassociatedwithareducedriskofgestationaldiabetesmellitus,
hypertensivedisordersofpregnancy,andpretermbirth[194].
Nutrientrequirementsinvegetarianpregnancyandlactationgenerallydonotdiffer
fromthosefornonvegetarians[195].Vegetariansmayespeciallybenefitfromguidanceon
sourcesofiron,zinc,vitaminB12,iodine,anddocosahexaenoicacid(DHA).Although
ironabsorptionincreasesinpregnancy[196],ironneedsarehigh,soironrichfoodsand
lowdoseironsupplementsarerecommendedforallwomen[197,198].Theincreasedneed
forzinccanbemetthroughacombinationofincreasedintakeandabsorption[199].
Phytate’sinhibitoryeffectonzincabsorptionismarkedlyreducedinlatepregnancyand
earlylactation[200].Inadditiontotheuseofiodizedsalt,a150μg/diodinesupplement
isrecommendedforallpregnantandlactatingwomen[201].
Duringpregnancy,bloodDHAconcentrationsareoftenlowerinvegetariansthanin
nonvegetarians[202];cordbloodDHAislowerininfantsofvegetarians[202].Breastmilk
DHAconcentrationsofvegetariansandvegansarelowerthanworldwideaverages[203].
DHAoromega3supplementationisassociatedwithgreatergestationaldurationanda
reducedriskofpretermbirth[204,205].SupplementalDHAderivedfrommicroalgae
shouldbeusedinvegetarianpregnancyandlactation[195].
AdequatevitaminB12intakeisespeciallyimportantduringperiodsofgrowthsuch
aspregnancyandbreastfeeding.Infantsborntolongtermveganmothersandwhoare
breastfedareatriskofB12deficiency.Thisisespeciallytruewhenthemother’sdietisnot
wellsupplemented.SymptomsofB12deficiencyinbreastfedinfantsandsmallchildren
fedavegandietincludedevelopmentaldelayorpsychomotorregression,lethargy,
anemia,neurologicalissues,andfailuretothrive[206].Pregnantandlactatingvegetarians
shouldconsumereliablesourcesofvitaminB12,suchassupplementsand/orfortified
foods,onadailybasis[195].
7.2.Infants,Children,andAdolescents
Vegetarian,includingvegan,dietsthatarenutritionallyadequateareappropriatefor
useininfancy,childhood,andadolescenceandsupportnormalgrowth[184,207,208].
Healthbenefitsofvegetariandietsinchildhoodandadolescenceincludethepotentialfor
exposuretoawidevarietyofplantfoods,lowerriskforchildhoodobesity[209],and
higherconsumptionoffruitsandvegetables[210,211].Veganchildrenappeartohave
lowerintakesoftotalandsaturatedfatandcholesterolcomparedtononveganchildren
[211].Alowfatvegandiethaseffectivelytreatedchildrenwithobesityandelevatedblood
pressure[212].
Exclusivebreastfeedingisrecommendedforinfantsforthefirst6monthsafterbirth,
withbreastfeedingcontinuinguntilatleast12monthsofage[213].Ifbreastfeedingor
exclusivebreastfeedingisnotpossible,commercialinfantformulashouldbeusedasthe
primarybeverageforthefirstyear.Plantmilks,unmodifiedcow’smilk,othermilks,and
homemadeformulasshouldnotbeusedtoreplacebreastmilkorformuladuringthefirst
year.Standardpracticesshouldbeusedwhenintroducingcomplementaryfoodsto
vegetarianinfants.Vegetableproteins,suchaspureedbeansortofu,areusedinplaceof
pureedmeats.Afterthefirstyear,iftoddlersaregrowingnormallyandeatingavariety
offoods,fortifiedsoyorpeaproteinmilkordairymilkcanbestarted[195].
Severalnutrientsrequirespecialattentionintheplanningofnutritionallyadequate
dietsforyoungvegetarians,includingiron,zinc,iodine,andvitaminB12;calciumand
vitaminDmayalsorequireattention,dependingondietarychoicesandotherfactors.
Proteinrecommendationsforveganchildrenmaybesomewhathigherthanstandard
recommendationsbecauseoffactorsincludingproteindigestibilityandaminoacid
composition[195].Proteinneedsofvegetarianorveganchildrenandadolescentsare
generallymetwhentheirdietscontainadequateenergyandavarietyofplantprotein
sources.Deficienciesofironandzincarerarelyseeninvegetarianchildreneatingvaried
Nutrients2021,13,414410of30
diets[207].Zincsupplementationmaybeneededwhencomplementaryfoodsare
introduced,iffoodsaremainlythosewithlowzincbioavailability[214].Ironandzinc
statusininfants,children,andadolescentsshouldbemonitored,andfortifiedfoods
and/orsupplementsusedasneeded.Iodizedsaltisareliablesourceofiodineforchildren
andteens.IfmaternalvitaminB12intakeorstatusareinadequate,breastfedinfants
shouldbegivensupplementalvitaminB12[206].Vegetarianchildrenandadolescents
shouldusevitaminB12fortifiedfoodsorsupplementstosupplyadequatevitaminB12.
Calciumsourcesforchildrenandadolescentsincludefortifiedplantbasedmilks,green
leafyvegetables,anddairyproducts.
7.3.OlderAdults
Olderadultsgenerallyhavedecreasedenergyrequirements,althoughnutrient
requirementsareoftensimilarto,orhigherthan,thoseofyoungeradults.Theselectionof
nutrientdensedietsisespeciallyimportantforolderadults.Limitedresearchindicatesthat
nutrientintakesofoldervegetariansarecomparabletothoseofoldernonvegetarians[195].
Recommendationsforcalcium,vitaminB6,andvitaminDarehigherforolderadults
[215,216].Thereissomeevidencethatproteinneedsincreaseaswell[217].Higherprotein
foodssuchassoyproducts(includingtofu,soybeverage,soyyogurtalternative,etc.),
legumes,nutsandseeds,andmeatanalogsshouldbeusedtwotothreetimesadayby
oldervegetarians.VitaminB6recommendationsforallolderadultsarehigherdueto
decreasedabsorptionandalterationsinmetabolism[216].Vegetariansgenerallyhave
adequateintakesofvitaminB6.Sourcesincludepotatoes,bananas,fortifiedbreakfast
cereals,andspinach.Severalfactorsincreaseolderadults’riskforvitaminDinsufficiency,
includingreduceddermalandrenalsynthesis[218,219],inadequatedietaryintake,and
limitedsunexposure.Fortifiedfoodsand/orsupplementsmaybeneededforolderadults
tomeetrecommendationsforcalciumandvitaminD.
ThemaincauseofvitaminB12deficiencyinolderadultsisimpairedabsorptionof
vitaminB12fromfoods[220].AbsorptionofpurifiedvitaminB12fromfortifiedfoodsand
supplementsisnottypicallyimpaired,sorecommendationscallforolderadultstouse
fortifiedfoodsandsupplementsastheirprimarysourcesofvitaminB12[216].
8.AthleticPerformance
Vegetariandietscanmeettheneedsofathletesatalllevels,fromrecreationaltoelite
athletes[221,222],andhavebeenfollowedbyathletesthroughouthistory[223].Whilea
nutritionallyadequateplantbaseddietisthoughttohelpoptimizetrainingand
performance,dueinparttoitshighcarbohydrate[223–225]andhighphytochemical
content[225],limitedevidencefromwellcontrolledstudiessuggeststhatvegetariandiets
neitherenhancenorimpairperformance[225].Additionalresearchisneededtodetermine
whethersuchdietsenhancerecoveryandattenuatetheoxidativedamageand
inflammationthatoccurwithheavytraining.
Nutritionrecommendationsforathletesshouldconsidereachathlete’straining
volume(intensityandfrequency),sport,season,performancegoals,andfoodpreferences.
Vegetariandietsthatmeetenergyneedsandcontainavarietyofplantbasedprotein
sources,includingsoyfoods,driedlegumes,nuts,seeds,quinoa,andothergrains,can
provideadequateproteintosupportmosttrainingneeds.Thereissomeevidencethat
plantderivedproteinsresultinlowerpostprandialmuscleproteinsynthesisresponses
comparedwithequivalentamountsofanimalderivedproteins[226];thisresponsemay
beimprovedbyconsumingblendsofdifferentplantderivedproteins[226].Milkandeggs
[227–229]cansupplementplantbasedsourcesforvegetarianathletes.
Dependingonfoodpreferences,athletesneedtoensuretheyconsumeadequate
amountsofthenutrientsthatareeitherfoundlessabundantlyinvegetarianfoodsorare
lesswellabsorbedfromplantscomparedtoanimalsources.Thesenutrientsinclude
calcium,iron,zinc,iodineandvitaminB12.Forexample,femaleathletesandendurance
athletesshouldensuresufficientconsumptionofironrichplantfoodsalongwithdietary
Nutrients2021,13,414411of30
factorsthatenhanceratherthaninhibitironabsorption[230–232].Femaleathleteswith
restrictedintakeandamenorrhea(i.e.,lowenergyavailability)[233]mayrequire
additionalcalcium(1500mg/dayalongwith1500–2000IUvitaminD)tooptimizebone
health[234].MaintainingadequatevitaminDstatusisimportantforathletesduetoits
roleinimmunefunction,inflammatorymodulation,physicalperformance,andoverall
health[235–238].Vegetarianathletesmayhavelowerbloodandmusclecreatinineand
carnitineconcentrations[239–242]comparedtoomnivoresduetolowerdietaryintake.
Athletesparticipatinginresistancetrainingandboutsofhighintensityexercisemay
benefitfromcreatinesupplementation[243],butthereisnorecognizedbenefittocarnitine
supplementation.Vegetarianathletes,likemostothers,maybenefitfromeducationabout
foodchoicestooptimizehealthandpeakperformance[244].
9.NutrientsofConcerninaPlantBasedDiet
9.1.Calcium
Inadditiontoitsroleinbonemineralization,calciumisrequiredforbloodclotting,
musclefunction,nervetransmission,hormonerelease,intracellularsignaling,and
regulatingkeyenzymes[245].Typically,vegansconsumesubstantiallylesscalciumthan
othervegetariansandomnivores[192,246].Whencalciumintakesarelow,thebodycan
compensatesomewhatbyincreasingthefractionalcalciumabsorption[247]and
decreasingurinarycalciumexcretion[215].However,anyone,includingalltypesof
vegetarians,withinadequatecalciumintakeneedstoconsistentlyusecalciumfortified
foods,suchasfortifiedbreakfastcereals,fortifiedfruitjuices,andfortifiedplantbased
beverages,ortakeacalciumsupplement,tomeettheircalciumneeds.Vegandietsinthe
UKhavebeenassociatedwithaclinicallysignificantincreasedriskoffracturewhenthe
calciumintakewasinadequate[248].
Phyticandoxalicacidsinplantfoodsarebothinhibitorsofcalciumabsorption.The
calciumabsorptionfromoxalaterichvegetables(spinach,Swisschard)maybeaslow5%;
frombeans,almonds,tahini,andfigs20–25%;fromdairyproducts32%;fromsoy
products(tofu,fortifiedsoybeverages),itissimilartodairymilk;andfromlowoxalate
vegetables(kale,Chinesecabbage,broccoli,bokchoy,etc.)50–60%[249–251].Boilingcan
reduceoxalatecontentingreenleafyvegetables[252].Avegetariandiet,withitshigh
intakeoffruitandvegetables,isrichinantiinflammatoryphytonutrients,specifically
carotenoidsandflavonoids,andpotassiumandmagnesium.Carotenoidsandflavonoids
areassociatedwithanimprovedBMDandlowerbonefractures[253–256].
Comparedtoavegetariandiet,consumingananimalproteindietisassociatedwith
anincreasedlossofurinarycalcium[257].
9.2.Iron
Inadditiontoitsabilitytotransferoxygenbymeansofhemoglobinandmyoglobin,
ironfunctionsasacofactorformanyimportantenzymes(suchasmyeloperoxidase,
importantforimmunefunction)andhasaroleinthyroidhormonesynthesisandamino
acidmetabolism[245].Sincehemeironisgenerallybetterabsorbed(15–30%)thannon
hemeiron(typically5–10%),omnivoresareassumedtohavebetterironstatus.However,
vegetarianswhoeatavariedandwellbalanceddietdonotappeartobeatanygreater
riskofirondeficiencyanemiathanomnivores[258,259].Hemoglobinlevelsofthetwo
dietarygroupsnormallyshownosignificantdifferences[259,260].Additionalstudiesof
irondeficiencyinvegetariansareneededbeforedefinitiveconclusionscanbereached.A
varieddietthatisrichinwholegrains,legumes,nuts,seeds,driedfruits,ironfortified
cerealproducts,andgreenleafyvegetablesprovidesanadequateironintake.Vegetarian
dietsgenerallycontainasmuchormoreironthanomnivorediets[92,195].
Nonhemeironabsorptionissignificantlyaffectedbyseveraldietarycomponents
[261,262].VitaminC,otherorganicacids(citric,malic,lactic,tartaricacids),anderythorbic
acid(anantioxidantusedinprocessedfood)allenhanceabsorption[196,230,259,263,264].
Nutrients2021,13,414412of30
Plantferritin,foundinsoyandotherlegumes,isaneasilyabsorbedsourceofiron(22–
34%).Whilephytates(foundinlegumes,nuts,andwholegrains)caninhibitnonheme
ironabsorption,theirinhibitoryeffectisdiminishedbybaking,soaking,leavening,and
germination[184,265].Furthermore,theoveralllongtermeffectofenhancersand
inhibitorsofironmaybelessimportantthanoncethoughtwhenthefoodsareeatenas
partofawholediet[266,267].
Absorptionofnonhemeironisalsoinverselyrelatedtothebody’sironstatus[196].
Whenstoresarelowandtheneedforironincreases,compensatorymechanismsfacilitate
greaterabsorptionofiron.Absorptioncanbeaslowas2–3%inpeoplewithgoodiron
storesbutashighas14–23%inpeoplewithlowironstores[268].
Humanshavealimitedabilitytoexcreteexcessstorediron[258],soconsuminglarge
amountsofhemeironmaybeunhealthyduetoitsprooxidantnature.Consumptionof
hemeironhasbeenassociatedwithanincreasedriskofchronicdiseasessuchasdiabetes,
metabolicsyndrome,andcolorectalcancer[269–272].Vegetarianstypicallyhavelower
ironstores(asreflectedinlowerserumferritinlevels),whichmaybeanadvantageas
lowerserumferritinlevelsmaybeassociatedwithimprovedinsulinsensitivityand
reducedriskofT2D[258,273].
Ironabsorptionfromanomnivorousdietisclaimedtobeabout18%,whereasfora
plantbaseddietitissaidtobeabout10%[196].Hence,thecurrentDietaryReference
Intake(DRI)forironforvegetarianshasbeenset1.8timeshigherthanthatfornon
vegetarians.Thisincreasedrequirementisbasedonlimitedresearch,whichhasbeen
unabletoaccuratelymeasureadaptiveabsorptionratesofnonhemeironinvegetarians
[267,274].Furtherresearchisneededtoreassesstheironrequirementrecommendedfor
vegetarians[232].
9.3.Zinc
Zincactsasacoenzymeformultipleenzymesinvolvedwithgrowth,immunity,
cognitivefunction,bonefunction,andregulationofgeneexpression[275,276].Zinc
deficiencycausesstuntedgrowth,poorappetite,dermatitis,alopecia,endocrine
dysfunction,andimpairedimmunity[276].Zincdeficiencyisnotanymorecommonly
seeninvegetariansthaninnonvegetarians[277].Zincintakeandserumlevelsfor
adolescentandadultvegetariansindevelopedcountriesarethesameorslightlylower
thanforomnivores,butwithinthenormalrange[214,231,275,278,279].Indeveloping
countries,vegansandvegetariansaremorelikelytoshowmarginalzincstatus[278].
Thebioavailabilityofzincfromplantfoodsmaybereduced.However,zinc
absorptionandretentioncanberegulatedbyhomeostaticmechanisms,adaptingtolower
intakesbyreducinglossesandincreasingabsorption[275].Duringperiodsofhigh
demand(pregnancy,infancy),absorptionbecomesmoreefficient[280].
Phytatesincerealsandlegumeslowerabsorptionofzinc,butleavening,soaking,
fermenting,orsproutingreducesthephytatelevelsandmakeszincmorebioavailable
[281].Sulfurcontainingaminoacidsandorganicacidsinavarietyofplantfoodswillalso
enhancezincabsorption[279,282].
Vegetarianfoodsourcesforzincincludenuts,seeds,wholegrains,legumes,tofu,
tempeh,anddairyproducts[283].Theuseofsupplementsandfortifiedfoods(suchas
fortifiedbreakfastcereals)maybenecessaryforveryrestrictedvegandiets[214,246].
9.4.Iodine
Iodineisessentialforthyroidhormones,whichregulatemetabolicactivity.Iodineis
especiallyimportantinpregnancyforfetaldevelopmentandduringearlychildhood.
Iodinedeficiencyinchildhoodcanpreventchildrenfromattainingtheirfullphysical
potentialandintellectualcapacity[284].
Majordietarysourcesofiodineincludeiodizedsalt,seafood,anddairyproducts
[284].Theiodinecontentofseaweedsanddairyproductscanvarywidely[195,285].Sea
salt,Himalayansalt,andthesaltusedinprocessedfoodstypicallydonotcontainiodine
Nutrients2021,13,414413of30
[195].Althoughfoodssuchassoybeans,cruciferousvegetables,andsweetpotatoes
containnaturalsubstancesthatinterferewithiodineuptakebythethyroid,thesefoods
havenotbeenassociatedwiththyroiddysfunctioninhealthypeople,providediodine
intakeisadequate[196,286].
Veganswhodonotuseiodizedsaltand/orseavegetablesmayhavelowiodine
intakesandmaybeatriskforiodinedeficiency[287,288].
9.5.VitaminB12
VitaminB12isrequiredforredbloodcellformation,DNAsynthesis,homocysteine
metabolism,andthemyelinationandfunctionofthecentralnervoussystem[289].
VitaminB12deficiencyisnotuncommonamongtheelderlyandunsupplementedvegans.
Itcanmanifestitselfwithconsequentialhematologicalandneurologicalchanges.
Typically,themeandietaryintakeofvitaminB12ofvegansfallswellbelowtheDRI,while
thatoflactoovovegetariansmaybemarginal,dependingontheuseofdairyproducts
[246,290,291].VegansmustobtaintheirvitaminB12eitherfromregularuseofvitamin
B12fortifiedfoods,suchasfortifiedplantbasedbeverages,fortifiedbreakfastcereals,
fortifiedvegetarianmeatanalogs,orfromaregularvitaminB12supplement.Unfortified
plantfoodssuchasfermentedsoyfoods,leafyvegetables,seaweeds,mushrooms,and
algae(includingspirulina)donotcontainsignificantamountsofactivevitaminB12to
providedailyneeds[292].Furthermore,anumberofmedicationscanimpairthe
absorptionorutilizationofB12.VitaminB12appearstobeacofactorinvolvedinthe
productionofnitricoxide[293],whichwouldhaveimportantimplicationsforvascular
andimmunehealth.
About50%ofdietaryB12isnormallyabsorbedviailealreceptors,mediatedbythe
intrinsicfactor,aglycoproteinfromthestomach.Theilealreceptorsbecomesaturated
with1.5to2μgofB12,limitingfurtherabsorption[216].Wheningestinglargedosesof
supplementalB12,about1%ofthedoseisabsorbedbypassivediffusionacrossthe
intestinaltract[216].Dailyneedscanbeadequatelymetinnonpregnant,nonlactating
peoplebyconsuminga500μgB12supplementatleastthreetimesaweek.VitaminB12
iswellabsorbedfromeithersublingualorchewabletablets.Whilethemethylcobalamin
supplementistoutedasthemoreeffectiveformofB12,itsbioavailabilityisnotsuperior
tothatofcyanocobalamin,whichisthemorestableandmostcommonlyusedformofB12
infortifiedfoodsandmanysupplements[294,295].
AdeficiencyofvitaminB12maytakeyearstodevelopinadults,asmostoftheB12
secretedintothegutviathebilegetsreabsorbed,thusconservingthebodystores[216].
Therefore,aregularconsumptionofadequateB12isimportanttoavoidasubclinical
deficiencythatcangoundetectedforyears.Anelevatedserummethylmalonicacid
(MMA)levelisareliableindicatorofB12deficiency[216],whiletheserumB12levelisan
insensitiveindicatorofB12status.WhileserumB12levelsbetween148and221pmol/L
(200300pg/mL)areconsideredborderlineB12deficiency[296],someindividualswith
B12valuesinthisrangemanifestneuropsychiatricproblemsandmemoryloss[297].Asa
goodpreventativemeasure,allvegansshouldannuallychecktheirB12status.
9.6.VitaminD
VitaminDfacilitatescalciumabsorptionfromthegut,regulatesbonemineralization,
cellgrowth,anddifferentiation.Itsotherrolesincludereductionofinflammationand
modulationofneuromuscularandimmunefunction[298].Becausecutaneousproduction
ofvitaminDfromsunlightexposureisnotadequate(especiallyintheelderly,dark
skinnedindividuals,andheavysunscreenusers)tomeetnutritionneedsinpopulations
livinginhighlatitudes,especiallyduringthewintermonths,regularfoodand
supplementsourcesarenecessary.FoodscontainlimitedamountsofvitaminD,so
supplementsareoftenneededtomeetneeds.Dependinguponone’sage,geographical
location,dietarypreferences,andbodyweight,adailysupplementaldoseof10–50μg
Nutrients2021,13,414414of30
(400to2000IU)ofvitaminDmaybeneededtoachieveoptimalserumlevelsof25
hydroxyvitaminD(25(OH)D)yearround[299].
Onestudyfoundno