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REVIEW
Vegetarian Diets and Weight Reduction: a Meta-Analysis
of Randomized Controlled Trials
Ru-Yi Huang, MD, MPH
1,2,3
, Chuan-Chin Huang, ScD
4
, Frank B. Hu, MD, PhD
4,5
, and Jorge E.
Chavarro, MD, ScD
4,5
1
Department of Medical Education, Department of Family Medicine, E-Da Hospital, Kaohsiung City, Taiwan, Republic of China;
2
School of
Medicine,I-SHOU University, Kaohsiung City, Taiwan, Republic of China;
3
Department of Environmental Health, Harvard T.H. Chan School of Public
Health, Boston, MA, USA;
4
Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA, USA;
5
Department of Nutrition,
Harvard T.H. Chan School of Public Health, Boston, MA, USA.
BACKGROUND: Vegetarian diets may promote weight
loss, but evidence remains inconclusive.
METHODS: PubMed, EMBASE and UpToDate databases
were searched through September 22, 2014, and investi-
gators extracted data regarding study characteristics
and assessed study quality among selected random-
ized clinical trials. Population size, demographic (i.e.,
gender and age) and anthropometric (i.e., body mass
index) characteristics, types of interventions, follow-
up periods, and trial quality (Jadad score) were re-
corded. The net changes in body weight of subjects
were analyzed and pooled after assessing heteroge-
neity with a random effects model. Subgroup analy-
sis was performed based on type of vegetarian diet,
type of energy restriction, study population, and
follow-up period.
RESULTS: Twelve randomized controlled trials were in-
cluded, involving a total of 1151 subjects who received the
intervention over a median duration of 18 weeks. Overall,
individuals assigned to the vegetarian diet groups lost
significantly more weight than those assigned to the
non-vegetarian diet groups (weighted mean differ-
ence, −2.02 kg; 95 % confidence interval [CI]: −2.80
to −1.23). Subgroup analysis detected significant
weight reduction in subjects consuming a vegan diet
(−2.52 kg; 95 % CI: −3.02 to −1.98) and, to a lesser
extent, in those given lacto-ovo-vegetarian diets
(−1.48 kg; 95 % CI: −3.43 to 0.47). Studies on sub-
jects consuming vegetarian diets with energy restric-
tion (ER) revealed a significantly greater weight re-
duction (−2.21 kg; 95 % CI: −3.31 to −1.12) than
those without ER (−1.66 kg; 95 % CI: −2.85 to −0.48).
The weight loss for subjects with follow-up of <1 year
was greater (−2.05 kg; 95 % CI: −2.85 to −1.25) than those
with follow-up of ≥1year(−1.13 kg; 95 % CI: −2.04 to
−0.21).
CONCLUSIONS: Vegetarian diets appeared to have signif-
icant benefits on weight reduction compared to non-
vegetarian diets. Further long-term trials are needed to
investigate the effects of vegetarian diets on body weight
control.
KEY WORDS: Vegan diet; Lacto-ovo-vegetarian diet; Overweight; Obesity;
Energy restriction.
J Gen Intern Med 31(1):109–16
DOI: 10.1007/s11606-015-3390-7
© Society of General Internal Medicine 2015
INTRODUCTION
Obesity is a worldwide public health problem. Between 1980
and 2013, the proportion of overweight or obese adults in-
creased from 28.8 to 36.9 % in men and from 29.8 to 38.0 % in
women.
1
Obesity is associated with hyperlipidemia, hyperten-
sion, diabetes, cardiovascular disease, certain cancers, and all-
cause mortality.
2
It is estimated that overweight and obesity
were the cause of 3.4 million deaths and 3.8 % of disability-
adjusted life-years worldwide in 2010.
3
The prevalence of
obesity has increased sharply in the past four decades in the
United States, and two-thirds of American adults are over-
weight or obese.
4
In 1998, management of overweight or
obese individuals accounted for 9.1 % of annual U.S. medical
expenditures.
5
As part of efforts to reduce obesity and associated
morbidity, various diets for weight reduction have been
proposed. Vegetarian dietary patterns have been reported
to be associated with decreased risk of type 2 diabetes,
coronary heart disease, and all-cause mortality.
6
The two
major types of vegetarian diets are the lacto-ovo-
vegetarian diet, in which meats are avoided but con-
sumption of milk and eggs is allowed, and the vegan
diet, in which all products originating from animals are
avoided. Thus far, the results of randomized clinical
trials
7–19
investigating the affect of vegetarian diets on
weight reduction have been inconclusive, which could
be due to the diverse populations, small sample sizes,
different intervention durations, and poor adherence. As
therehavebeennostudieswithlargesamplesizes,we
performed a meta-analysis of randomized clinical trials
conducted to date in order to compare the effect of
vegetarian diets with that of non-vegetarian diets on
weight reduction in the general population. We also
investigated whether the effects of weight reduction
differed between lacto-ovo-vegetarian and vegan diets.
Electronic supplementary material The online version of this article
(doi:10.1007/s11606-015-3390-7) contains supplementary material,
which is available to authorized users.
Published online July 3, 2015
JGIM
109
METHODS
Data Sources and Searches
Adopting the Cochrane Collaboration search strategy,
20
we
searched PubMed via the NCBI Entrez system (1950 to
September 22, 2014) and EMBASE via Ovid (1988 to
September 22, 2014) for randomized controlled studies of the
effects of vegetarian diets compared to non-vegetarian diets on
weight reduction. Key words used to search for relevant publi-
cations included the following: (Bweight^AND Bvegetarian
diet^)OR(Bweight^AND Blacto-ovo-vegetarian diet^)OR
(Bweight^AND Bvegan diet^), with the scope of the search
limited to English literature. Bibliographies of identified studies
and UpToDate Database 2014 were reviewed for additional
reference. We contacted three original authors to clarify data.
Study Selection
We included studies that were randomized clinical controlled
trials solely applying vegan or lacto-ovo-vegetarian diets com-
pared to non-vegetarian diets, with the inclusion of changes in
body weight as a study parameter or where this information
could be derived by contacting the authors. We excluded non-
original publications, abstracts of conferences, and studies for
which details could not be obtained. Studies that included
other interventions such as combined physical activity and
diet interventions were also excluded, although studies where
exercise was advised with dietary intervention were included.
Data Extraction and Quality Assessment
For every eligible study, we collected information on the year of
publication, population characteristics, sample size, intervention
type, follow-up period, and weight change. The means and
standard deviations of weight change were obtained from all
studies. For articles that did not report standard deviations,
10,17,19
we imputed a change-from-baseline standard deviation using a
correlation coefficient of 0.96 estimated from trials
9,11,12,14,16
with available data based on Cochrane’sformula.
21
For three
studies
9,11,15
with repeated measurements during the follow-up
periods, we collected results at the 1-year time point. For one
study
10
that identified subjects’dietary preferences for lacto-ovo-
vegetarian diet or control diet before randomization, we
attempted to separate the preference group from the no-
preference group for comparison within each group. To explore
study quality, we used the Jadad score, which takes into consid-
eration the randomization appropriateness, blinded outcome as-
sessment, and complete description of loss to follow-up. We then
examined each component of the Jadad score as our study-level
factor to see whether it affected the heterogeneity of the results.
Data Synthesis and Analysis
Given the diversity in study design and populations, we
employed a random effects model, and we also conducted a
fixed effects model for sensitivity analysis.
22
We calculated
weighted mean differences for identical outcome measures in
the vegetarian and non-vegetarian diet groups. We assessed
heterogeneity of treatment effects in the included studies using
the I-squared statistic and the chi-square test of the q statistic.
We further evaluated study characteristics as potential sources
of heterogeneity using meta-regression including type of in-
tervention diet (vegan vs. lacto-ovo-vegetarian), type of die-
tary intervention (supplemented food vs. group session with
dieticians), inclusion of energy restriction in the intervention
diet (yes vs. no), study duration (greater vs. less than 1 year),
gender involved in studies (females alone vs. both genders),
overweight/obesity (selected individuals with body mass in-
dex [BMI] ≥25 vs. general population), population (patients
vs. healthy subjects), study quality (Jadad score high: ≥3vs.
low: < 3), description of randomization procedures (yes vs.
no), blinding (single or double vs. no) and dropout (analysis of
possible effect vs. no). We conducted subgroup analysis by
pooling effect estimates basedonimportantdomainsof
sources of bias and significant study-level factors in the
meta-regression. To address the uneven study quality, we
conducted sensitivity analyses only for high-quality studies.
Publication bias was examined by constructing a funnel plot
and was tested using the Begg
19
method. All statistical proce-
dures were conducted using Stata software (Version 12;
StataCorp LP, College Station, TX, USA). The work was not
sponsored or supported by specific institutes.
RESULTS
The literature search identified 1513 studies. After excluding
422 duplicates and 1036 articles based on title and abstract
review, 73 articles remained for full-text evaluation (Fig. 1).
We excluded 13 studies that were not original reports, five
studies where the main intervention was a physical activity
intervention, eight studies which were multiple reports of the
same population, 33 studies that did not include information
on outcomes of interest, and two studies that compared vegan
diets versus lacto-ovo-vegetarian diets. After these exclusions,
we identified 12 studies that fulfilled the inclusion criteria and
were used in the final analysis.
Study Characteristics
Table 1summarizes the characteristics of the 12 included
trials. A total of 1151 subjects were included in this analysis,
with baseline age ranging from 18 to 82 years. Three studies
targeted postmenopausal women and one study focused on
premenopausal women. For the studies that enrolled both
genders, the proportion of male subjects ranged from 13 to
52 %. The mean baseline BMI ranged from 25 to 53 kg/m
2
.
Six studies recruited overweight or obese patients, five studies
enrolled people with type 2 diabetes, and one study included
patients with rheumatoid arthritis. Vegan and lacto-ovo-
vegetarian diets were chosen as intervention diets in eight
and four studies, respectively. Among the eight trials on vegan
diets, six used low-fat (≤10 %) recipes, while one adopted
110 Huang et al: Vegetarian Diet and Weight Control JGIM
high-carbohydrate (60 %) ingredients. The design of non-
vegetarian diets varied across studies and included low-fat,
anti-diabetes, lipid-lowering, and weight reduction recipes. Of
the six studies that applied energy restriction, five applied the
restriction to both the intervention and control arms,
8,10–12,16
while one study designated 1 week of energy restriction solely
to the intervention arm.
23
Among the remaining six studies
without energy restriction in the intervention arms, one
adopted energy restriction in their control groups who were
overweight.
9
The follow-up periods among these trials ranged
from 8 weeks to 2 years. Meta-regression identified the length
of follow-up (i.e., ≥1 year vs. < 1 year) as the only study-level
factor with borderline significance (P=0.045) for the pooling
outcome (Figs. 2and 3).
Quality of Trials
Most of the randomized clinical trials reported various ap-
proaches for evaluating adherence and causes of loss to fol-
low-up. Blinding is difficult in dietary interventions, and only
two trials
13,15
adopted a blind study design. Therefore, we
categorized the 13 studies based on Jadad score
24
as either
high quality (n = 5, score ≥3) or low quality (n=7, score<3).
Meta-regression analysis revealed that randomization,
blinding, follow-up, and type of dietary intervention were
not important effect modifiers.
Weight Change
Individuals assigned to vegetarian diets lost more weight than
those assigned to control diets (weighted mean difference, −2.02
Kg; 95 % CI: −2.80 to −1.23) in interventions ranging from 9 to
74 weeks. We observed a significant heterogeneity in weight
change (P=0.001 test for heterogeneity, I
2
=62.3 %), and conse-
quently conducted subgroup analyses in terms of intervention
type, follow-up duration, and population characteristics.
Individuals randomized to vegan diets (eight studies, intervention
ranging from 12–48 weeks) lost more weight than those random-
ized to lacto-ovo-vegetarian diets (four studies, intervention rang-
ing from 9 to 74 weeks), relative to their respective counterparts
consuming control diets. Specifically, the weighted mean weight
reduction for individuals with vegan diets was −2.52 kg (95 %
CI: −3.02 to −1.98, P=0.406 for heterogeneity, subtotal
I
2
=3.0 %), whereas the corresponding weight change for lacto-
ovo-vegetarian diets was −1.48 kg; (95 % CI: −3.43 to −0.47,
P<0.001 for heterogeneity, subtotal I
2
=83.6 %).
Six trials of vegetarian diets with energy restriction in inter-
ventions ranging from 9 to 48 weeks showed greater weight loss
(−2.21 Kg; 95 % CI: −3.31 to −1.12, P=0.002 for heterogeneity,
subtotal I
2
=71.8 %) than diets in those six trials without energy
restriction (−1.66 Kg; 95 % CI: −2.85 to −0.48, P=0.115 for
heterogeneity, subtotal I
2
=43.6 %) in interventions ranging from
8 to 74 weeks. Weight loss achieved in 11 trials lasting less than
Fig. 1 Study flow diagram. RCT randomized controlled trial
111Huang et al: Vegetarian Diet and Weight ControlJGIM
Table 1 General Characteristics of Trials Comparing Vegetarian and Non-Vegetarian Diets
Study Key population characteristics Total N Age
†
Men, % BMI
†
Intervention Control Energy restriction Duration
||
(weeks)
Jadad
score
*
Mishra 2013
7
Overweight or type 2 diabetes 291 45±15 17 35±1 Low-fat vegan Habitual diet No 18 2
Kahleova 2013
8
Type 2 diabetes 74 30–70 47 25–53 Vegan Diabetes diet Yes 24 3
Barnard 2009
9
Type 2 diabetes 99 27–82 39 25–43 Low-fat vegan Diabetes diet Yes, for BMI>25
in control arm
74 3
Burke 2008
10
Sedentary, overweight 200 18–55 13 27–43 Lacto–ovo–vegetarian Low fat Yes
§
24–72 3
Turner 2007
11
Overweight, obese
postmenopausal women
62 44–73 0 26–44 Low-fat vegan NCEP diet Yes 48–96 2
Mahon 2007
12
Postmenopausal women 25 58±2 0 29±1 Lacto–ovo–vegetarian Habitual diet Yes 9 2
Gardner 2007
13
Premenopausal overweight
and obese women
155 25–50 0 27–40 Low-fat lacto–ovo–
vegetarian
Calories from carbohydrate,
protein and fat split
40/30/30
No 48 4
Barnard 2005
16
Overweight or obese
postmenopausal women
64 44–73 0 26–44 Low-fat vegan NCEP diet Yes 14 2
Dansinger 2005
15
Overweight or obese 80 22–72 52 27–42 Low-fat
lacto–ovo–vegetarian
High-fat, high-protein,
low-carbohydrate diet
No 8–48 5
Nicholson 1999
17
Type 2 diabetes 11 34–74 46 –Low-fat vegan 55–60 % of calories from
carbohydrates, < 30 %
from fat
No 12 2
Prescott 1988
19
Healthy omnivores 64 18–60 40 –High-protein lacto–
ovo–vegetarian
High protein from meat,
1.5 g/Kg/day
No 12 2
Skoldstam 1979
18
Rheumatoid arthritis 26 35–66 27 –Lacto–ovo–
vegetarian
Habitual diet Yes, 1st week
intervention
arm only
10 1–2
*Jadad score: briefly rates study quality by way of randomization, blinding, and follow-up using a 0–5 scoring system
†Ranges of value
‡Values are reported as number (%)
§
If weight < 90.5 Kg, then 1200 Kcal for women and 1500 Kcal for men; if weight > 90.5 Kg, then 1500 Kcal for women and 1800 Kcal for men
||
The duration of follow-up in each study is equal to the intervention period except for three studies: Dansinger, 8 weeks of intervention; Turner, 14 weeks of intervention; and Gardener, 8 weeks of intervention
112 Huang et al: Vegetarian Diet and Weight Control JGIM
1 year (range: 6–24 weeks) was greater (−2.05 kg; 95 % CI:
−2.85 to −1.25, P=0.001 for heterogeneity, subtotal I
2
=66.4 %)
than in the five trials following subjects for 1 year or more (range:
8–74 weeks) (−1.13 kg; 95 % CI: −2.04 to −0.21, P=0.580 for
heterogeneity, I
2
=0 %). In seven trials that included the general
population in interventions ranging from 9 to 74 weeks, vegetar-
ian diets resulted in substantially greater weight reduction
(−2.61 kg; 95 % CI: −3.54 to −1.69, P=0.02 for heterogeneity,
subtotal I
2
=61.2%)thaninfivetrialsrestrictedtooverweight
or obese individuals at baseline in interventions ranging
from8to48weeks(−1.23 kg; 95 % CI: −2.09 to −0.37,
P=0.38 for heterogeneity, I
2
=5.1 %). In sensitivity analy-
ses, the results of high-quality trials (−1.43 kg; 95 % CI:
−2.51 to −0.35 I
2
=57.8 %) and trials of low quality
(−2.51 kg; 95 % CI: −3.53 to −1.49 I
2
=60.3 %) were
heterogeneous but not significantly different.
Publication Bias
The funnel plot showed slight asymmetry in both the small and
large sample groups. Nevertheless, we found no evidence of
substantial publication bias (Begg’stestP=0.32)
(Supplementary Fig., available online).
DISCUSSION
In this meta-analysis of randomized controlled trials compar-
ing body weight changes between individuals consuming
vegetarian diets and those consuming non-vegetarian diets,
the former showed a reduction in weight of approximately
2 kg compared to the latter. Among individuals consuming
vegetarian diets, interventions with vegan diets resulted in
greater weight loss than those with lacto-ovo-vegetarian diets.
Weight loss was also greater in trials with energy restriction. In
addition, analyses indicated that intervention effects were
attenuated over time after 1 year of follow-up, but remained
worthy of integration into daily practice.
Observational studies have suggested an association be-
tween self-reported vegetarian diets and weight control.
25–27
In fact, the results from observational studies are in agreement
with our findings, but suggested an even stronger effect than
Fig. 2 Pooled weighted mean differences in weight reduction between vegetarian and non-vegetarian diets. Effects on estimated weight reduction
for each study depicted as solid squares; error bars indicate 95 % CIs. The pooled estimate of −1.99 kg (95 % CI, −2.72 to −1.25) of weight loss is
shown as the diamond. RE random effect, Weight inverse variance weight, WMD weighted mean difference, CI confidence interval
113Huang et al: Vegetarian Diet and Weight ControlJGIM
that documented in randomized trials. A prospective matched
cohort study of 116 individuals who followed strict vegetarian
diets for 3 years showed a 15-kg weight reduction compared to
the control group.
28
While this may imply that participants
with better adherence to a vegetarian diet could have more
profound weight loss, it also might also be explained by self-
selection, residual confounding, measurement error, or lack of
repeated diet and lifestyle assessment. A long-term interven-
tional study by Ornish et al. found a weight reduction of 10 kg
at1yearand8kgat5yearscomparedtobaselineinthegroup
consuming a vegetarian diet, while the control group
experienced an increase of 2 kg.
29
However, the sample size
was small, and the addition of moderate exercise could have
contributed to the substantial weight reduction in the vegetar-
ian diet group. In our meta-analysis, we avoided those issues
by including only randomized controlled trials and excluding
those that combined diet and exercise in the intervention.
For the lacto-ovo-vegetarian diets, the random effects mod-
el showed no significant effect, whereas the fixed model
yielded a small but significant effect on weight reduction.
The difference between the results of the fixed and random
effects models could be due to limited statistical power in the
A
NOTE: Weights are from random effects analysis
.
.
Overall (I-squared = 62.3%, p = 0.001)
Skoldstam (1979)
Barnard (2009)
Mahon (2007)
Subtotal (I-squared = 3.0%, p = 0.406)
Mishra (2013)
ID
1
Study
Gardner (2007)
Burke(no prefer) (2008)
Nicholson (1999)
Burke(prefer) (2008)
Subtotal (I-squared = 83.6%, p = 0.000)
Prescott (1988)
Turner (2007)
Kahleova (2013)
Dansinger (2005)
0
Barnard (2005)
-2.02 (-2.80, -1.23)
-2.00 (-3.63, -0.37)
-1.40 (-3.76, 0.96)
-4.40 (-5.58, -3.22)
-2.52 (-3.06, -1.98)
-2.84 (-3.77, -1.91)
WMD (95% CI)
-0.60 (-2.44, 1.24)
0.00 (-2.14, 2.14)
-3.40 (-10.38, 3.58)
-0.30 (-2.36, 1.76)
-1.48 (-3.43, 0.47)
-0.10 (-2.26, 2.06)
-2.50 (-4.87, -0.13)
-3.00 (-3.85, -2.15)
-2.70 (-6.13, 0.73)
-2.00 (-3.54, -0.46)
100.00
8.88
6.25
10.89
59.18
12.02
Weight
%
8.03
6.94
1.16
7.22
40.82
6.89
6.23
12.36
3.83
9.29
-2.02 (-2.80, -1.23)
-2.00 (-3.63, -0.37)
-1.40 (-3.76, 0.96)
-4.40 (-5.58, -3.22)
-2.52 (-3.06, -1.98)
-2.84 (-3.77, -1.91)
WMD (95% CI)
-0.60 (-2.44, 1.24)
0.00 (-2.14, 2.14)
-3.40 (-10.38, 3.58)
-0.30 (-2.36, 1.76)
-1.48 (-3.43, 0.47)
-0.10 (-2.26, 2.06)
-2.50 (-4.87, -0.13)
-3.00 (-3.85, -2.15)
-2.70 (-6.13, 0.73)
-2.00 (-3.54, -0.46)
100.00
8.88
6.25
10.89
59.18
12.02
Weight
%
8.03
6.94
1.16
7.22
40.82
6.89
6.23
12.36
3.83
9.29
LOV diet
Vegan diet
0-1 0 1
B
NOTE: Weights are from random effects analysis
.
.
Overall (I-squared = 62.3%, p = 0.001)
Barnard (2009)
Study
0
Prescott (1988)
Subtotal (I-squared = 43.6%, p = 0.115)
Mahon (2007)
Nicholson (1999)
Dansinger (2005)
Burke(no prefer) (2008)
1
Turner (2007)
Burke(prefer) (2008)
Mishra (2013)
Kahleova (2013)
ID
Skoldstam (1979)
Subtotal (I-squared = 71.8%, p = 0.002)
Barnard (2005)
Gardner (2007)
-2.02 (-2.80, -1.23)
-1.40 (-3.76, 0.96)
-0.10 (-2.26, 2.06)
-1.66 (-2.85, -0.48)
-4.40 (-5.58, -3.22)
-3.40 (-10.38, 3.58)
-2.70 (-6.13, 0.73)
0.00 (-2.14, 2.14)
-2.50 (-4.87, -0.13)
-0.30 (-2.36, 1.76)
-2.84 (-3.77, -1.91)
-3.00 (-3.85, -2.15)
WMD (95% CI)
-2.00 (-3.63, -0.37)
-2.21 (-3.31, -1.12)
-2.00 (-3.54, -0.46)
-0.60 (-2.44, 1.24)
100.00
6.25
%
6.89
38.18
10.89
1.16
3.83
6.94
6.23
7.22
12.02
12.36
Weight
8.88
61.82
9.29
8.03
-2.02 (-2.80, -1.23)
-1.40 (-3.76, 0.96)
-0.10 (-2.26, 2.06)
-1.66 (-2.85, -0.48)
-4.40 (-5.58, -3.22)
-3.40 (-10.38, 3.58)
-2.70 (-6.13, 0.73)
0.00 (-2.14, 2.14)
-2.50 (-4.87, -0.13)
-0.30 (-2.36, 1.76)
-2.84 (-3.77, -1.91)
-3.00 (-3.85, -2.15)
WMD (95% CI)
-2.00 (-3.63, -0.37)
-2.21 (-3.31, -1.12)
-2.00 (-3.54, -0.46)
-0.60 (-2.44, 1.24)
100.00
6.25
%
6.89
38.18
10.89
1.16
3.83
6.94
6.23
7.22
12.02
12.36
Weight
8.88
61.82
9.29
8.03
Energy Restriction
No Energy Restriction
0-1 0 1
C
NOTE: Weights are from random effects analysis
.
.
Overall (I-squared = 60.8%, p = 0.000)
Barnard (2005)
Mahon (2007)
Kahleova (2013)
Mishra (2013)
Burke(no prefer 24 week) (2008)
Barnard (22 week) (2006)
Subtotal (I-squared = 66.4%, p = 0.001)
Burke (2006)
Barnard (74 week) (2009)
Turner (96 week) (2007)
Burke(no prefer 72 week) (2008)
Prescott (1988)
Study
Burke(prefer 24 week) (2008)
Dansinger (24 week) (2005)
1
Gardner (2007)
Nicholson (1999)
Skoldstam (1979)
Subtotal (I-squared = 0.0%, p = 0.580)
Dansinger (48 week) (2005)
ID
0
Burke(prefer 72 week) (2008)
-1.82 (-2.50, -1.15)
-2.00 (-3.54, -0.46)
-4.40 (-5.58, -3.22)
-3.00 (-3.85, -2.15)
-2.84 (-3.77, -1.91)
0.40 (-1.58, 2.38)
-1.50 (-3.23, 0.23)
-2.05 (-2.85, -1.25)
-0.53 (-2.35, 1.29)
-1.40 (-3.76, 0.96)
-2.45 (-4.92, 0.02)
-1.40 (-3.53, 0.73)
-0.10 (-2.26, 2.06)
-2.90 (-4.58, -1.22)
-0.90 (-3.87, 2.07)
-0.60 (-2.44, 1.24)
-3.40 (-10.38, 3.58)
-2.00 (-3.63, -0.37)
-1.13 (-2.04, -0.21)
-2.70 (-6.13, 0.73)
WMD (95% CI)
0.10 (-1.90, 2.10)
100.00
6.75
7.90
8.96
8.72
5.48
6.16
71.95
5.90
4.56
4.33
5.10
5.02
%
6.32
3.42
5.84
0.85
6.46
28.05
2.80
Weight
5.42
-1.82 (-2.50, -1.15)
-2.00 (-3.54, -0.46)
-4.40 (-5.58, -3.22)
-3.00 (-3.85, -2.15)
-2.84 (-3.77, -1.91)
0.40 (-1.58, 2.38)
-1.50 (-3.23, 0.23)
-2.05 (-2.85, -1.25)
-0.53 (-2.35, 1.29)
-1.40 (-3.76, 0.96)
-2.45 (-4.92, 0.02)
-1.40 (-3.53, 0.73)
-0.10 (-2.26, 2.06)
-2.90 (-4.58, -1.22)
-0.90 (-3.87, 2.07)
-0.60 (-2.44, 1.24)
-3.40 (-10.38, 3.58)
-2.00 (-3.63, -0.37)
-1.13 (-2.04, -0.21)
-2.70 (-6.13, 0.73)
WMD (95% CI)
0.10 (-1.90, 2.10)
100.00
6.75
7.90
8.96
8.72
5.48
6.16
71.95
5.90
4.56
4.33
5.10
5.02
%
6.32
3.42
5.84
0.85
6.46
28.05
2.80
Weight
5.42
<1 year
>=1 year
0-1 0 1
D
NOTE: Weights are from random effects analysis
.
.
Overall (I-squared = 62.3%, p = 0.001)
Nicholson (1999)
Kahleova (2013)
Burke(no prefer) (2008)
Gardner (2007)
Barnard (2009)
Barnard (2005)
Turner (2007)
Study
Subtotal (I-squared = 5.1%, p = 0.384)
Prescott (1988)
Subtotal (I-squared = 61.2%, p = 0.017)
Mishra (2013)
Skoldstam (1979)
Burke(prefer) (2008)
Mahon (2007)
1
Dansinger (2005)
0
ID
-2.02 (-2.80, -1.23)
-3.40 (-10.38, 3.58)
-3.00 (-3.85, -2.15)
0.00 (-2.14, 2.14)
-0.60 (-2.44, 1.24)
-1.40 (-3.76, 0.96)
-2.00 (-3.54, -0.46)
-2.50 (-4.87, -0.13)
-1.23 (-2.09, -0.37)
-0.10 (-2.26, 2.06)
-2.61 (-3.54, -1.69)
-2.84 (-3.77, -1.91)
-2.00 (-3.63, -0.37)
-0.30 (-2.36, 1.76)
-4.40 (-5.58, -3.22)
-2.70 (-6.13, 0.73)
WMD (95% CI)
100.00
1.16
12.36
6.94
8.03
6.25
9.29
6.23
%
41.55
6.89
58.45
12.02
8.88
7.22
10.89
3.83
Weight
-2.02 (-2.80, -1.23)
-3.40 (-10.38, 3.58)
-3.00 (-3.85, -2.15)
0.00 (-2.14, 2.14)
-0.60 (-2.44, 1.24)
-1.40 (-3.76, 0.96)
-2.00 (-3.54, -0.46)
-2.50 (-4.87, -0.13)
-1.23 (-2.09, -0.37)
-0.10 (-2.26, 2.06)
-2.61 (-3.54, -1.69)
-2.84 (-3.77, -1.91)
-2.00 (-3.63, -0.37)
-0.30 (-2.36, 1.76)
-4.40 (-5.58, -3.22)
-2.70 (-6.13, 0.73)
WMD (95% CI)
100.00
1.16
12.36
6.94
8.03
6.25
9.29
6.23
%
41.55
6.89
58.45
12.02
8.88
7.22
10.89
3.83
Weight
General Population
Obese or Overweight
0-1 0 1
Fig. 3 Pooled weighted mean differences in weight reduction by subgroup. Effects on estimated weight reduction for each study depicted as solid
squares; error bars indicate 95 % CIs. The pooled estimate of weight loss is shown as the diamond. RE random effect, Weight inverse variance
weight, WMD weighted mean difference, CI confidence interval. A. Vegan diets vs. lacto-ovo-vegetarian (LOV) diets. Vegan diets were defined
as avoiding all animal products, whereas lacto-ovo-vegetarian diets avoided meat but consumption of milk and eggs was allowed. B. Energy
restriction vs. no energy restriction group. C. Follow-up < 1 year vs. ≥1 year. D. General population vs. overweight or obese population.
Normal represents general population including normal-weight, overweight and obese individuals
114 Huang et al: Vegetarian Diet and Weight Control JGIM
lacto-ovo-vegetarian diets subgroup (number of studies = 5) in
the random effects model. Therefore, more studies are needed
to examine the effects of lacto-ovo-vegetarian diets. As the
total energy intake was not adjusted in the studies that assigned
meals to two groups,
8,17,19
it is not clear whether the different
weight reduction effects between vegan and lacto-ovo-
vegetarian diets was due to the discrepant total energy intake
in the two dietary patterns.
A possible mechanism underlying the effect of vegetarian
diets on weight reduction may be the abundant intake of whole
grains, fruits, and vegetables. Whole-grain products and veg-
etable generally have low glycemicindex values,and fruits are
rich in fiber, antioxidants, phytochemicals, and minerals.
2
Viscous fiber, around 20 to 50 % in whole-grain products,
could delay gastric emptying and intestinal absorption.
30
Several prospective studies have reported an inverse associa-
tion between fiber consumption and weight loss. The
Coronary Artery Risk Development in Young Adults
(CARDIA) study identified a significant association between
dietary fiber intake and lower body weight at baseline in both
whites and blacks.
31
In the Nurses’Health Study (NHS),
women who had greatly increased their fiber intake reported
a mean weight gain that was 1.52 kg less than that in women
who reported a small increase in fiber intake.
32
He et al.
33
further found that women in the NHS with the largest increase
in fruit and vegetable intake had a 28 % lower risk of major
weight gain (≥25 kg) than those with lowest intake. A meta-
analysis that included 20 small randomized controlled trials
showed that soluble fiber supplementation was not efficacious
for enhancing weight loss,
34
although this result cannot be
generalized to long-term effects of fiber from foods on body
weight.
To the best of our knowledge, this study is the first meta-
analysis to examine the effect of a vegetarian diet on weight
loss relative to other diets (e.g., the American Diabetes
Association-recommended diet, the diet supported by the
National Cholesterol Education Program, and the Atkins diet).
We conducted an extensive literature search, thus diminishing
the possibility of publication bias. However, we found signif-
icant heterogeneity among trials, which may have been largely
due to different study designs, the variety of vegetarian diets,
the presence or absence of energy restriction, suboptimal study
quality as reflected in the lack of blindness in 11 out of 13
studies, a wide range of available dietary adherence from 51 to
83 %, and the intervention strategy (e.g., provided food or
dietician instruction-based). Although we did comprehensive
subgroup analysis, the reliability of results might be limited by
the relatively small sizes of the subgroups. While this meta-
analysis provides evidence that vegetarian diets are more
effective than non-vegetarian diets for weight loss, multiple
gaps in the literature remain. For example, while our analysis
suggested attenuation of effects over time, in agreement with
the wider literature on dietary interventions for weight loss,
most of the trials included in the meta-analysis lasted less than
12 months, and none lasted more than 18 months. Therefore,
the long-term effects of vegetarian diets on body weight
remain unsettled. The short duration of these trials also limits
the available information on other clinically relevant outcomes
such as cardiovascular morbidity and cardiovascular risk fac-
tors. Hence, further intervention trials are warranted to explore
the long-term effects of vegetarian diets on weight loss and
clinical outcomes.
CONCLUSIONS
In summary, vegetarian diets, and vegan diets in particular,
appear to have beneficial effects on weight reduction.
However, these benefits were attenuated over time. Longer-
term intervention trials are needed to investigate the effect of
vegetarian diets on weight control and cardiometabolic risk.
Acknowledgments: Dr. Chung Hsieh, Department of Epidemiology,
Harvard School of Public Health, provided conceptual advice, and Dr.
Stephanie Smith-Warner provided nutritional knowledge and statisti-
cal advice.
Support: This work was supported by NIH grants P30DK46200 and
1U54CA155626.
Conflict of Interest: The authors declare that they have no conflicts of
interest.
Corresponding Author: Jorge E. Chavarro, MD, ScD; Department of
Nutrition, Harvard T.H. Chan School of Public Health, Building 2, 3rd
Floor, 655 Hungtinton Avenue, Boston, MA 02115, USA
(e-mail: jchavarr@hsph.harvard.edu).
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