Vegetarian diet and type 2 diabetes

VegetarianNutritionJournal
Open Access Freely Available Online
Vegetariandietandtype2diabetes
Ju‐ShengZheng,FengleiWang,DuoLi
DepartmentofFoodScienceandNutrition,ZhejiangUniversity
Abstract
Type2diabetescontinuestobeamostcommonchronicdiseaseintheworldandtheprevalenceof
type2diabeteshaskeptincreasingworldwide,especiallyindevelopingcountries.Avegetariandietis
suggestedtobeinverselyassociatedwithriskoftype2diabetesinmanyobservationalstudies,yet
prospectivecohortstudiesarestilllimited.Forpatientswithtype2diabetes,clinicaltrialssuggesta
greater improvement of insulin sensitivity or other glycemic traits when adopting a vegetarian diet
thanaconventionaldiabeticdiet.However,long‐termclinicaltrialsarestillwarrantedtoconfirmand
replicatethepreviousfindings.Sofar,evidenceisstillweakfortherecommendationofavegetarian
diettopreventtype2diabetesortomanageglycemiccontrolindiabeticpatients.
Keywords:vegetarian,type2diabetes,glycemiccontrol,prospectivecohort,clinicaltrial
Editor:Susianto
ReceivedMay6,2014;AcceptedMay9,2014;PublishedJuly,2014
Copyright:2014Zhengetal.Thisisanopen‐accessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedthe
originalauthorandsourcearecredited.
CompetingInterests:Theauthorshavedeclaredthatnocompetinginterestsexist.
Correspondingaddress:DuoLi,Ph.D.DepartmentofFoodScienceandNutrition,ZhejiangUniversity 866YuhangtangRoad,Hangzhou,China,310058 Phone:+86‐571‐88982024 Fax:+86‐571‐88982024 E‐mail:[email protected] ThisstudywasfundedbytheNationalNaturalScienceFoundationofChina(81273054);bythePh.D.Programs
FoundationofMinistryofEducationofChina(20120101110107)
Introduction
Type 2 diabetes is one of the most common
chronicdiseasesandtheprevalenceofdiabetes
among adults was estimated to be 6.5% (285
million) in 2010 and 7.7% (439 million) in
2030 (1). Dietary and lifestyle intervention is
considered to be a most effective strategy for
the prevention or treatment of type 2 diabetes
(2). Vegetarian diet is becoming increasingly
popular during the past decade, and Food Pyr‐
amid for vegetarian has been developed by
somescientists(3).Therearevarietiesofvege‐
tarianism, including vegan, ovovegetarian, lac‐
tovegetarian, ovo‐lactovegetarian, raw vegan,
Su vegetarian, fruitarian, pesco‐vegetarian and
semi‐vegetarian. Each type of vegetarianism
has its own characteristics, including or ex‐
cluding certain foods (4). Vegetarian diet, com‐
paredwithanomnivorousdiet,arerichinfiber,
magnesium, Fe3+, folic acid, vitamins E and C
andn‐6polyunsaturatedfattyacid,phytochem‐
icals and antioxidants, but low in total fat, Fe2+,
zinc, vitamins A, B12, and D (5) and especially
n‐3 polyunsaturated fatty acids (6). Although
both the advantages and disadvantages of a
vegetarian diet are documented, epidemiologi‐
calstudiesconsistentlysupportthatvegetarian
dietisassociatedwithlowerriskoftype2dia‐
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Open Access Freely Available Online
betesandotherchronicdiseases(4,7).Inaddi‐
tion, promising results have been reported by
severalclinicaltrialsforthetreatmentoftype2
diabetesbyalow‐fatvegetariandiet(8).
Vegetariandietandriskoftype2diabetes
Prospective cohort studies have suggested a
remarkably protective association between a
vegetarian diet andtype 2 diabetes. In theAd‐
ventistHealth Study‐2, prevalence oftype2 di‐
abetes is much lower in vegans (2.9%), lac‐
to‐ovo
(3.2%),
pesco
(4.8%)
or
semi‐vegetarians
(6.1%)
than
in
non‐vegetarians(7.6%)(9).Aftertwo‐yearfol‐
low‐up of the Adventist Health Study‐2, either
vegans (OR=0.381; 95%CI: 0.236‐0.617), lac‐
to‐ovovegetarians
(OR=0.618;
95%CI:
0.503‐0.760) or semi‐vegetarians (OR=0.486;
95%CI: 0.312‐0.755) have significantly lower
risk of type 2 diabetes compared with
non‐vegetarians (10). In another cohort study,
involving8410participantsandfollowedupfor
17years(11),long‐termnon‐vegetariandietis
positively associated with 38% higher risk of
type 2 diabetes (OR=1.38; 95%CI: 1.06‐1.80)
compared with long‐term vegetarian diet. In
theTzuChiHealthStudy(12),vegetariandietis
inversely associated with 51% lower risk of
type2diabetes(OR=0.49;95%CI:0.28‐0.89)in
men, 74% lower risk of type 2 diabetes
(OR=0.26;
95%CI:
0.06‐1.21)
in
pre‐menopausalwomenand75%lowerriskof
type 2 diabetes in menopausal women
(OR=0.25;95%CI:0.15‐0.42).Inaddition,other
observational studies also suggest that vege‐
tarians, compared with omnivores, are associ‐
ated with improved insulin sensitivity in dif‐
ferentpopulations(13‐15). Vegetariandietandglycemiccontrol india‐
beticpatients
Clinicaltrialsintype2diabeticpatientsindicate
agreaterimprovementofinsulinsensitivityor
other glycemic traits in a vegetarian diet than
VegetarianNutritionJournal
conventionaldiabeticdiet(16‐19). Inanearly16‐daytrial(20),20leanmenwith
type 2 diabetes receive control diets for an av‐
erageof7daysfollowedbyalowfat,highcar‐
bohydrate, near vegetarian diet for an average
of 16 days. Fasting and 3‐hour postprandial
plasmaglucoseislowerinmostpatientsonthe
near vegetarian diet than on the control diet.
Anothersmall12‐weekrandomizedpilotstudy
(19),involving13type2diabeticpatients,sug‐
gests that participants in the vegan diet group
show 28% mean reduction of fasting glucose,
comparedwith12%reductioninaconvention‐
aldietgroup.Inaddition,medicationuseisre‐
duced or discontinued in some participants in
thevegangroup,whilenopatientinthecontrol
groupreducemedicationuse. In a 22‐week randomized trial, 99 individual
with type 2 diabetes are assigned to a low‐fat
vegandietoradietfollowingtheAmericanDi‐
abetes Association guidelines (16). The trial
showsthatalow‐fatvegandiethasgreaterim‐
provementinglycemicandlipidcontrolintype
2 diabeticpatients. Following thesamepartici‐
pants for 74 weeks, the low‐fat vegan diet still
appearstoimproveglycemiccontrolmorethan
theconventionaldiabetesdiet(17).Recently,in
a24‐weekrandomizedparalleltrial,74patients
withtype2diabetesareassignedtoavegetar‐
ian diet or a conventional diabetic diet, with
bothdietbeingisocaloricandcalorierestricted
(18). The calorie‐restricted vegetarian diet
showsgreaterimprovementofinsulinsensitiv‐
itythantheconventionaldiabeticdietafterthe
intervention. Mechanismbehindvegetarianandglycemic
control
Thereareseveralpotentialmechanismsforthe
beneficial effects of a vegetarian diet on glyce‐
mic control or type 2 diabetes. First of all, a
vegetarian diet contributes to better weight
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VegetarianNutritionJournal
Open Access Freely Available Online
control. As obesity is an important risk factor
forinsulin resistance and diabetes (21), a veg‐
etarian diet may benefit glycemic control and
prevent risk of type 2 diabetes through reduc‐
ing body weight. Second, low in saturated fat
intake(22)andhighfiberintake(23)foraveg‐
etariandietmayberelatedtoimprovedinsulin
sensitivity. Third, a vegetarian diet is usually a
diet with low glycemic index. Low glycemic in‐
dex, compared with high glycemic index, is as‐
sociated with 40% reduced risk of type 2 dia‐
betes (24). Other mechanism may include
change in intramyocellular lipid accumulation
(8).
Conclusion
Results from observational studies and ran‐
domized trials consistently support that a veg‐
etariandietisprotectivelyassociatedwithrisk
oftype2diabetesandimproveglycemiccontrol
in type 2 diabetic patients. However, prospec‐
tive cohort studies or long‐term randomized
clinical trials are still limited, and effects of a
vegetarian diet on patients from different eth‐
nicities are still to be explored. Therefore, the
available evidence for a recommendation of a
vegetarian diet for diabetic patients is weak.
More large prospective cohorts and long‐term
randomizedtrialsarewarrantedtoconfirmthe
beneficial effect of a vegetarian diet on type 2
diabetes or glycemic control in diabetic pa‐
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