Effects of dairy intake on body weight and fat: A meta-analysis of randomized controlled trials

Article (PDF Available)inAmerican Journal of Clinical Nutrition 96(4):735-47 · August 2012with33 Reads
DOI: 10.3945/ajcn.112.037119 · Source: PubMed
Abstract
Some intervention studies have suggested that dairy products may influence body weight, but the results remain controversial. We identified and quantified the effects of dairy consumption on body weight and fat mass from randomized controlled trials (RCTs). We conducted a comprehensive search of PubMed and EMBASE databases (to April 2012) of English reports of RCTs regarding dairy consumption on body weight, body fat, or body weight and body fat in adults. The results across studies were pooled by using a random-effects meta-analysis. Twenty-nine RCTs were included with a total of 2101 participants. Overall, consumption of dairy products did not result in a significant reduction in weight (-0.14 kg; 95% CI: -0.66, 0.38 kg; I(2) = 86.3%). In subgroup analysis, consumption of dairy products reduced body weight in the context of energy restriction or short-term intervention (<1 y) trials but had the opposite effect in ad libitum dietary interventions or long-term trials (≥1 y). Twenty-two RCTs that reported results on body fat showed a modest reduction in the dairy group (-0.45 kg; 95% CI: -0.79, -0.11 kg; I(2) = 70.9%), and further stratified analysis indicated significant beneficial effects of dairy intervention on body fat in energy-restricted or short-term trials but not in long-term or ad libitum studies. This meta-analysis does not support the beneficial effect of increasing dairy consumption on body weight and fat loss in long-term studies or studies without energy restriction. However, dairy products may have modest benefits in facilitating weight loss in short-term or energy-restricted RCTs.
See corresponding editorial on page 687.
Effects of dairy intake on body weight and fat: a meta-analysis of
randomized controlled trials
1–4
Mu Chen, An Pan, Vasanti S Malik, and Frank B Hu
ABSTRACT
Background: Some intervention studies have suggested that dairy
products may influence body weight, but the results remain contro-
versial.
Objective: We identified and quantified the ef fects of dairy consump-
tion on body weight and fat mass from randomized controlled trials
(RCTs).
Design: We conducted a comprehensi v e search of PubMed and EM-
B ASE databases (to April 2012 ) of English reports of RCTs reg arding
dairy con sumption on body weight, body fat, or body weight and body
fat in adults. The results across studies were pooled by using a random-
effects meta-analysis.
Results: Twenty-nine RCTs were included with a total of 2101
participants. Overall, consumption of dairy products did not result
in a significant reduction in weight (20.14 kg; 95% CI: 20.66, 0.38
kg; I
2
= 86.3%). In subgroup analysis, consumption of dairy prod-
ucts reduced body weight in the context of energy restriction or short-
term intervention (,1 y) trials but had the opposite effect in ad
libitum dietary interventions or long-term trials ($1 y). Twenty-two
RCTs that reported results on body fat showed a modest reduction in
the dairy group (20.45 kg; 95% CI: 20.79, 20.11 kg; I
2
= 70.9%),
and further stratified analysis indicated significant beneficial effects of
dairy intervention on body fat in energy-restricted or short-term trials
but not in long-term or ad libitum studies.
Conclusions: This meta-analysis does not support the beneficial
effect of increasing dairy consumption on body weight and fat loss
in long-term studies or studies without energy restriction. However,
dairy products may have modest benefits in facilitating weight loss
in short-term or energy-restricted RCTs. Am J Clin Nutr
2012;96:735–47.
INTRODUCTION
The prevalence of overweight and obesity has increased
dramatically in the United States and around the world (1, 2).
Thus, weight control is a national and global priority. It has been
postulated that the consumption of dairy products may facilitate
body weight and fat loss (3) because dairy products contain cal-
cium, protein (casein and whey), and other bioactive compounds
that may favorably affect energy balance.
Many randomized controlled trials (RCTs) have been con-
ducted to determine the effect of dairy foods on weight loss and
body composition. Howe v er, results have been inconsistent (4–9).
Discrepancies in results might be due to small sample sizes, in-
sufficient study durations, differences in study design, and the
diversity of study populations. Thus, a meta-analysis is needed to
increase the statistical power and enhance the precision of estimates
across multiple modest-sized trials. Recently, a meta-analysis (10)
on this topic was published, but a number of eligible studies were
not included, and the result from one trial was repeatedly used.
These methodologic issues may have led to biased results, and
incomplete study selection may have impaired the statistical
power to detect influential factors on the pooled estimates, such as
study duration. Therefore, to achieve a more precise estimation of
effects across trials, we performed a systematic review and meta-
analysis on RCTs to evaluate whether increasing the consump-
tion of dairy products could promote weight loss.
METHODS
Data sources and searches
This meta-analysis was conducted after a review protocol (11).
We searched PubMed (http://ww.nlm.nih.gov/pubs/factsheets/
pubmed.html) and EMBASE (http://www.embase.com) data-
bases for clinical trials published from January 1966 to April
2012 that described the effects of dairy products on body weight
and composition in adults. We specified 2 comprehensive search
themes. The first theme identified relevant terms for dairy by
combining exploded versions of the Medical Subject Headings
terms dairy products, dairy, milk, calcium, cheese, and yogurt
and corresponding key words in titles and abstracts. The second
theme identified terms related to body weight by combining weight
loss, weight r eduction, weight change, body fat,oradiposity and
corresponding key words in titles and abstracts. The third theme
identified terms related to randomized controlled trials by com-
1
From the Departments o f Nutrition (MC, AP, VSM, and FBH) and
Epidemiology (FBH), Harvard School of Public Health, Boston, MA; the
Channing Division of Network Medicine, Department of Medicine, Brigham
and Women’s Hospital and Harvard Medical School, Boston, MA (FBH); and
Saw Swee Hock School of Public Health, National University of Singapore,
Singapore (AP).
2
The funder had no role in the study design, the data collection and analysis,
the decision to publish, or the preparation of the manuscript.
3
Supported by the NIH (grants DK58845, P30 DK46200, U54CA155626,
and HL60712).
4
Address correspondence and reprint requests to FB Hu, Department of
Nutrition, Harvard School of Public Health, 655 Huntington Avenue, Boston,
MA 02115. E-mail: frank.hu@channing.harvard.edu.
Received February 16, 2012. Accepted for publication July 17, 2012.
First published online August 29, 2012; doi: 10.3945/ajcn.112.037119.
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Supplemental Material can be found at:
bining the Medical Subject Headings term intervention studies and
corresponding key words in titles and abstracts. Additional articles
were identified from the reference lists of included studies and
relev ant review s. See Supplemental data” in the online issue for
a detailed search strategy.
Study selection
We included RCTs with either a parallel or a crossover design
that were conducted in adults aged $18 y. Dairy products should
have been used as the main intervention but not as part of a multi-
component dietary supplement in either experimental or control
groups, the intervention in control groups was not soy milk, and
participants had to consume dairy products for $4 wk. Furthermore,
data on changes of body weight or fat mass in both experimental and
control groups were extracted from the report or obtained from the
authors. We included English-language articles only .
Data extraction and quality assessment
We extracted the following information from each study:
authors, publication year, geographic location, funding sources,
sample size and attrition, intervention and control re gimens, study
duration, study design (crossover or parallel), a nalysis strategy
(intent-to-treat or per-protocol analysis), and participa nt in-
formation (age, sex, and baseline intake). We used the Jadad score to
assess study quality (12). Trials scored one point for each item
addressed in the study design, including random assignment,
blinding, description of withdr awals and dropouts, methods of
random assignment, and double-blinding status, which generated
a scale from 0 to 5. Higher numbers represented the better quality of
a given study. Because double blinding was almost not possible in
this type of trial, studies with a Jadad score $3 were defined as high
quality, and the rest of the studies were classified as low quality.
Most of the RCTs showed the intervention dose in servings per
day. In 6 studies (13–18) in which dairy interventions were not
shown in servings per day, we standardized units to servings per
day, which was equivalent to 240 mL or 8 oz in volume or 452 mg
Ca or 8.4 g protein in nutrient content per day according to the
USD A national nutrient database for standard reference (19). In 2
studies (17, 20), participants were randomly assigned to a control
group or medium- or high-dairy groups; thus, we combined me-
dium- and high-dairy groups as the intervention arm with relevant
data provided by the authors.
We extracted the means and SDs of changes from baseline to
endpoint (both intervention and control arms) from all studies.
SDs were calculated from SEs or CIs when necessary; for articles
with missing SDs for measurements of change (4, 13, 16),
change-from-baseline SDs were imputed by using the correlation
coefficient method referenced in the Cochrane Handbook for
Systematic Reviews of Interventions (21). We used the average
correlation coefficient of 0.96 between baseline and endpoint
measurements, which was estimated from all other included studies
with available data. A sensitivity analysis indicated that results were
not substantially alter ed by removing the 3 studie s with imput ed
values.
Meta-analysis and statistical analysis
The estimate of the principal effect was defined as the mean
difference (net change in kilograms) in body weight and body fat
mass between participants assigned to dairy products and par-
ticipants assigned to control regimens. Cochran’s Q test was
conducted to test the statistical heterogeneity of treatment ef-
fects between studies (P , 0.1). We also examined the I
2
sta-
tistic, and I
2
. 50% was considered to indicate a significant
heterogeneity across trials (22). Results were presented by using
the random-effects model because high heterogeneities were
shown in most cases, and we repeated the analysis by fixed-
effect models if there was a low heterogeneity ( I
2
, 50%) (21).
Potential publication bias was examined by using funnel plots in
which SEMs of the studies were plotted against their corre-
sponding effect sizes (21). Meta-regression was implemented to
examine characteristics of studies that were hypothesized to
influence observed treatment effects (23), including energy re-
striction (yes or no), intervention dose (,3or$3 servings/d), study
duration (,1or$1 y), baseline BMI (in kg/m
2
; ,25 or $25),
funding sources, study quality (high or low), and sex (me n,
women, or both). Sensitivity analyses were also conducted to
evaluate the impact of individual studies on overall pooled esti-
mates and heterogeneity. The meta-analysis was performed with
STATA software (version 11.0; StataCorp).
RESULTS
Results of literature search
The literature search yielded 2028 citations (1106 from
PubMed and 1526 from EMBASE with 604 duplicate records);
among the citations, 139 articles remained for detailed full-text
evaluation after title and abstract screening. We excluded 110
studies on the basis of our study selection criteria, and details of
the study flow are depicted in Figure 1. One study (24) was
excluded because participants were on a severe energy-deficit diet
by reducing 7 MJ/d (1673 kcal/d) from their preintervention en-
ergy amounts. Finally, 29 citations met the inclusion criteria and
were included in the meta-analysis.
Characteristics of studies
Primary characteristics of the 29 included trials are shown in
Table 1. A total of 2441 participants were randomly assigned to
intervention or control regimens in these trials with a completion
rate of 84.4% (n = 2060). The sample size varied from 20 to 265
participants with a mean age range from 20.0 to 62.0 y (median:
41.4 y). The mean baseline BMI ranged from 20.2 to 43.0, and
24 trials enrolled overweight or obese participants. Of the 29
trials, 14 trials were conducted exclusively in women (5 trials
conducted in postmenopausal women, 8 trials conducted in
premenopausal women, and 1 trial that did not mention menopausal
status), 1 trial was conducted in men, and the other 14 trials
were conducted in both sexes, with 2 trials that reported results
separately by sex (4, 14).
Intervention regimens were diverse. Low-fat fluid milk was
evaluated in 7 trials (4, 17, 25–29); skimmed-milk powder was
used in 2 trials (30, 31); yogurt was used in 2 trials (7, 32);
yogurt, cheese, and milk were freely chosen as the intervention
in 11 trials (6, 8, 9, 14, 17, 33–38); a yogurt and milk com-
bination was used in 2 trials (5, 24, 39); and the remaining 5
studies (13, 15, 16, 20, 40) did not specify the intervention
regimen. The inter vention dose (total dairy intake) varied from
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1 to 6.5 servings/d. A habitual diet or a calorie-restricted diet
with lower dairy consumption was chosen as the control reg-
imen in most studies. An isocaloric sucrose beverage or fruit
juice was provided to participants in control groups in 2 studies
(32, 39).
The trials lasted from 1 to 24 mo, with a median duration of 4
mo. Most of the trials used parallel study designs, and 2 trials (15,
39) used crossover designs. The quality of selected trials was
div erse, with 19 stu dies classified as high quality (Jadad score $3)
and 10 studies classified as low quality.
In 17 weight-loss trials, participants were instructed to reduce
their daily energy intakes to a certain amount below their energy
requirements. Caloric requirements were estimated individually
on the basis of resting metabolic rates and physical activity levels,
and e nergy deficits from 100 to 600 kcal/d were subtracted
from estimate d caloric requirements in 16 of 17 studies. The
remaining one study (14) administrated isocaloric low-energy
diet plans for all participants regardless of t heir individual
energy requirements. Participants were instructed to keep
physical activity levels constant in the majority of tria ls except
in 2 studies, one of which (27) had a 2-by-2 f actorial design that
included an exercise component, and in the other study (28),
milk was given in addition to exercise, whereas participants in
the control arm received only an exercise intervention. Most
studies were conducted in the United States (n = 17), and other
studies were conducted in Europe (n = 2), Canada (n =3),
Australia (n =2),Mexico(n =1),Asia(n = 3), and Puerto
Rico (n =1).
FIGURE 1. Summary of evidence search and selection. Searched databases included PubMed (http://ww.nlm.nih.gov/pubs/factsheets/pubmed.html) and
EMBASE (http://www.embase.com). RCT, randomized controlled trial.
META-ANALYSIS: DAIRY ON WEIGHT AND BODY FAT 737
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TABLE 1
Characteristics of included studies
1
First author, year of
publication (reference)
Enrollment
(completers) Age
2
Men BMI
3
Intervention
Baseline
intake
Intervention
dose
4
Duration
Jadad
score Design Energy controlControl group Dairy group
n y % kg/m
2
Servings/d Servings/d mo kcal/d
Baran, 1990 (13) 59 (37) 30–42 0 NA Usual diet Dairy supplement of 500–600 mg Ca/d 1.22 1.22 36 3 RP No
Barr, 2000 (3) 205 (200) 55–85 35.5 25.9 Usual diet Three 8-oz servings of skim or
1% fluid milk to the usual diet
5
,1.5 2.25 3 3 RP No
Bowen, 2005 (14) 69 (50) 20–65 40 35.0 Mixed diet containing
500 mg Ca/d
High-dairy diet containing 2400 mg Ca/d NA 4.2 4 3 RP First 12 wk ER
+4wkEB
Buchowski, 2010 (15) 40 (34) 21–50 29.4 32.0 Nondairy Ca:
500 mg/1500mg
Dairy Ca: 500 mg/1500 mg NA 2.21 3 4 RC 100–200 ER
Chee, 2003 (30) 200 (173) 50–65 0 24.0 Habitual diet Extra 50 g high-calcium skimmed-milk
powder with 400 mL H
2
O
,2 1.67 24 3 RP No
Faghih, 2011 (25) 50 (42) 20–50 0 30.4 Restricted diet 3 servings (220 mL) low-fat milk (1.5%) NA 3 2 2 RP 500 ER
Ghadirian, 1995 (16) 265 (158) 50–90 0 NA Dairy-free diet Dairy food diet providing 30 g protein #1.72 3.57 1 2 RP No
Gilbert, 2011 (26) 41 (25) 25–50 0 33.0 1000 mg/d Ca and
energy-equivalent
placebo
Milk supplement (1% fat) #1.77 2.37 6 4 RP 600 ER
Gunther, 2005a (17) 155 (135) 18–30 0 22.5 Habitual diet Medium dairy containing 1050 mg Ca/d
or high dairy containing 1350 mg Ca/d
#1.77 2.65 12 3 RP No
Gunther, 2005b (18) 26 (19) 18–26 0 22.0 Habitual diet Nonfat or low-fat milk supplement
containing 1000–1400 mg/d
#1.77 2.65 12 2 RP No
Harvey-Berino, 2005 (33) 53 (44) 18–60 7.5 30.0 ,2 dairy servings/d 3–4 dairy servings/d ,1 2.5 12 4 RP 500 ER
Josse, 2011 (20) 55 (46) 19–45 0 32.0 0–1 serving/d 3–4 and 6–7 servings dairy products/d low 5.24 4 3 RP 500 ER
Kukuljan, 2009 (27) 180 (175) 50–79 100 27.5 Habitual diet 400 mL reduced-fat (w1%) milk NA 1.67 12 3 RP No
Lau, 2001 (31) 200 (185) 55–59 0 24.0 Habitual diet Extra 50 g high-calcium, low-fat,
low-lactose milk powder with 400 mL
H
2
O (1200 mg Ca)
NA 1.67 24 2 RP No
Manios, 2009 (5) 82 (75) 55–65 0 30.5 Habitual diet 3 portions of low-fat dairy products,
milk, and yogurt
NA 3 12 2 RP No
Palacios, 2011 (6) 20 (16) 22–50 18.8 38.3 Habitual diet 4 daily servings of dairy products 1.55 4 5.25 4 RP No
Rosado, 2011 (29) 93 (69) 25–45 0 34.8 An energy-restricted diet
with no intake of milk
Additional 3 servings of low-fat milk ,3 3 4 3 RP 500 ER
Stancliffe, 2011 (36) 40 (40) 22–52 47.5 36.7 ,0.5 dairy serving
providing 600 mg Ca/d
.3.5 dairy servings containing
1200 mg Ca/d
NA 3 3 3 RP 0
Thomas, 2010 (40) 35 (29) 29–45 0 29.1 Low-calcium diet 3 d/wk High-dairy-based calcium diet
($1200 mg/d), 3 d/wk
#1 3 4 3 RP 250 ER
Thomas, 2011 (32) 35 (29) 29–45 0 29.1 6-oz isoenergetic sucrose
beverage 3 d/wk
5
6-oz serving of fat-free yogurt
supplement 3 d/wk
5
NA 0.64 4 2 RP 249 ER
Thompson, 2005 (34) 59 (48) 25–70 13.3 35.0 ,2 dairy servings/d 4 dairy servings ($2 fluid milk)/d NA 3 12 2 RP 500 ER
Van Loan, 2011 (37) 78 (71) 20–50 NR 32.5 ,1 dairy serving
providing 460 mg Ca/d
.4 dairy servings containing
1339 mg Ca/d
#1 3 3 3 RP 500 ER
van Meijl, 2010 (39) 40 (35) 18–70 28.6 32.0 600 mL fruit juice and
43 g fruit biscuits/d
500 mL low-fat (1.5%, wt/wt) milk and
150 g low-fat (1.5%, wt/wt) yogurt
,2 2.67 1 1 RC No
(Continued)
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TABLE 1 (Continued)
First author, year of
publication (reference)
Enrollment
(completers) Age
2
Men BMI
3
Intervention
Baseline
intake
Intervention
dose
4
Duration
Jadad
score Design Energy controlControl group Dairy group
Wagner, 2007 (28) NA (30) 19–53 0 33.0 Placebo capsules Extra dairy servings of 1% milk
containing 800 mg Ca/d
NA 2.76 3 3 RP 500 ER exercise
3 times/wk
Wennersberg, 2009 (38) 121 (113) 30–65 32.7 30.0 Habitual diet 3–5 dairy servings NA 4 6 2 RP No
Zemel, 2004 (9) 28 (21) 18–60 15.6 34.9 ,1 dairy serving
containing 500 mg
Ca/d and a daily
placebo supplement
3 dairy servings containing 1200 mg
Ca/d, with placebo
1.2 2.5 6 3 RP 500 ER
Zemel, 2005a (35)
6
39 (34) 26–55 32.4 34.5 ,1 dairy serving
containing 500 mg Ca/d
3 dairy servings containing 1200 mg
Ca/d, at least 1 fluid milk
NA 2.5 6 3 RP No
Zemel, 2005a (35)
6
36 (29) 26–55 13.8 35.5 ,1 low-fat dairy
serving containing
500 mg Ca/d
3 dairy servings containing 1200 mg
Ca/d, at least 1 fluid milk
NA 2.5 6 3 RP 500 ER
Zemel, 2005b (7) 38 (34) 18–50 20.6 33.5 ,1 dairy serving
providing 500 mg Ca/d
Three 6 oz fat-free yogurt,
1100 mg Ca/d
5
1.2 2.5 3 3 RP 500 ER
Zemel, 2009 (8) 70 (64) 18–35 22.8 29.1 ,1 dairy serving
providing 500 mg Ca/d
3 dairy servings containing
1200 mg Ca/d
1.33 2.5 3 1 RP 500 ER
1
EB, energy balance; ER, energy restriction; NA, not applicable; RC, randomized crossover design; RP, randomized parallel design.
2
All values are ranges.
3
All values are means of baseline BMI.
4
Difference of total dairy intake between dairy intervention and control groups.
5
Eight ounces = 236.6 mL.
6
Zemel, 2005a (35) is listed twice because one subtrial was energy restricted and the other subtrial was for weight maintenance.
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Changes in weight and body fat
In 5 studies (26, 28, 29, 33, 34), data were analyzed by using
the intention-to-treat principle, whereas the other studies ana-
lyzed data in completers only. For this reason, a total of 2101
participants in 29 studies were included in the analysis of weight
change, and 1536 participants in 22 studies were included in the
analysis of changes in body fat.
There was no significant difference in body weight changes
between the dairy intervention and control groups (20.14 kg;
95% CI: 20.66, 0.38 kg; Figure 2A); however, a significant
reduction that favored dairy products in body fat was shown
(20.45 kg; 95% CI: 20.79, 20.11 kg; Figure 2B). A significant
heterogeneity was observed for both body weight (I
2
= 86.3%)
and body fat (I
2
= 70.9%).
Subgroup analysis
Meta-regression analysis showed that energy restriction (P =
0.008) and study duration (P = 0.008) significantly i nfluenced
pooled estima tes. Therefore, we conducted subgroup analyses
according to these var iables.
A significant reduction in body weight in the dairy group was
shown in studies that imposed energy restriction (20.79 kg; 95%
CI: 21.35, 20.23 kg; I
2
= 38.5%; Figure 3A). On the contrary,
in studies without energy restriction, no significant effect of
dairy intervention was observed (0.39 kg; 95% CI: 20.36, 1.13
kg; I
2
= 89.7%). Similarly, body fat declined significantly in
energy-restricted trials ( 20.94 kg; 95% CI: 21.53, 20.34 kg;
I
2
= 59.2%; Figure 3B) but not in studies without energy re-
striction (20.12 kg; 95% CI: 20.71, 0.46 kg; I
2
= 81.9%).
FIGURE 2. Net change (95% CI) in body weight (A) and fat (B) associated with dairy interventions expressed as the change (kg) during the intervention
with dairy products minus the change during control regimen. Horizontal lines denote the 95% CIs; solid diamonds represent the point estimate of each study.
The open diamond represents the pooled estimate of the intervention effect. The dashed line denotes the point estimate of the pooled result. ID, identification;
WMD, weighted mean difference.
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We also classified studies according to duration as short term
(,1 y duration) or long term ($1 y duration; Figure 4). Dairy
products significantly reduced body weight in the short-term
interventions (20.47 kg; 95% CI: 20.90, 20.03 kg; I
2
= 59.2%)
but moderately increased weight gain in long-term interventions
(0.66 kg; 95% CI: 20.14, 1.46 kg; I
2
= 80.7%). Furthermore,
a significant reduction in body fat in the dairy group was shown
in short-term studies (20.91 kg; 95% CI: 21.43, 20.38 kg; I
2
=
70.3%), whereas no significant change was observed in long-
term trials (0.32 kg; 95% CI: 20.17, 0.81 kg; I
2
= 59.5%).
Stratified analysis by both energy restriction and duration
indicated that dairy products significantly facilitated weight loss
in short-term trials with energy restriction, whereas a marginally
significant weight gain was observed in long-term trials without
energy restriction. For body fat, a significant r eduction was
shown in short-term studie s with energy restriction, whereas
a marginally significant weight gain was observed in long-term
trials without energy restriction. No significant diffe rence was
observed in weight or body fat change between dairy and
control groups in short-term RCTs without energy restriction or
long-term energy-restricted RCTs. See Figure 1 under “Sup-
plemental data” in the online issue for forest plots.
Sensitivity analysis
We conducted several sensitivity analyses by excluding 1 RCT
that provided isoenergetic low-energy diet plans (14), 2 RCTs
(27, 28) that included a physical activity–intervention compo-
nent, 3 RCTs (3, 13, 16) for which change-from-baseline SDs
were imputed, 3 RCTs of ,3-mo duration (16, 25, 39), and 10
RCTs for which the Jadad score was ,3, respectively. For all
analyses, results were not substantially changed (data not shown).
We also carried out sensitivity analyses to explore heterogeneity.
After excluding the study of Stancliffe et al (36) from short-term
trials without energy restriction, the heterogeneity in that subgroup
decreased dramatically (I
2
= 78.4–0.0%), and a similar impact of
the study of Manios et al (5) was observed on the heterogeneity of
long-term studies without energy restriction (I
2
= 63.4–3.0%).
Publication bias
On the basis of a funnel plot (see Figure 2 under “Supple-
mental data” in the online issue) and Begg’s test, no significant
publication bias was shown in the meta-analysis of body weight
(P = 0.44) or body fat (P = 0.19) changes.
FIGURE 2. (Continued)
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DISCUSSION
This meta-analysis does not support the beneficial effect of
increasing dairy consumption on body weight and fat loss in long-
term studies ($1 y) or studies without ener gy restriction. However,
dairy products may have modest benefits in facilitating weight loss
in short-term or energy-restricted RCTs.
The dairy–weight loss hypothesis has also been investigated
in a number of prospective epidemiologic studies. In a review
(41) of 9 prospective cohort studies on dairy consumption and
overweight and obesity in adults, the authors conclud ed that these
studies provide evidence of a suggestive but not consistent pro-
tective effect of dairy consumption on risk of overweight and
obesity. It should be noted that prospective cohort studies can
be susceptible t o residual or unmeasured c onfounding by in-
dividual dietary components. Specifically, milk consumption
has often been associated with a better overall dietary profile
(42) and i nversely associated with the consumption of sugar-
sweetened beverages, especially soda and fruit juice. A recent
study (43) in 120,877 US adults from 3 large cohorts showed
a null association between changes in consumption of most
dairy foods a nd long-term weight gains, whereas the increase
of yogurt intakes was inversely associated with weight gains.
Among studies included in our meta-analysis, only 2 trials used
yogurt as the intervention; one 3-mo RCT in 34 participants in-
dicated a significant benefit of 3 servings fat-free yogurt/d on weight
loss (body weight: 20.71 kg; 95% CI: 21.41, 20.01 kg; body
FIGURE 3. Net change (95% CI) by energy restriction in body weight (A) and fat (B) associated with dairy interventions expressed as the change (kg)
during the intervention with dairy products minus the change during the control regimen. Horizontal lines denote the 95% CIs; solid diamonds represent the
point estimate of each study. The open diamond represents the pooled estimate of the intervention effect. The dashed line denotes the point estimate of the
pooled result. ID, identification; WMD, weighted mean difference.
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fat: 20.68 kg; 95% CI: 21.37, 0.01 kg) (7), and the other
2-mo RCT in 29 individuals reported a nonsignificant weight
reduction of 3 servings fat-free yogurt/wk (body weight: 20.38
kg; 95% CI: 21.12, 0.35 kg; body fat: 20.32 kg; 95% CI: 21.06,
0.41 kg) (32). Long-term, high-quality RCTs are warranted to
further assess the effect of yogurt consumption on weight loss.
A meta-analysis (10) of RCTs on dairy intervention and weight
loss was recently published, but it included only 14 RCTs with
883 participants compared with 29 RCTs with 2101 participants
in our analysis. We have enrolled all RCTs included in the previous
meta-analysis except one study (44), which was a substudy of
another RCT (8), and the 2 publications were repeatedly used in the
previous meta-analysis; in addition, we were able to obtain ad-
ditional data from investigators for 2 studies (6, 20) in which
imputed data had been used in the previous meta-analysis. Our
literature search yielded 16 more RCTs that were not included
in the previous meta-analysis, and these RCTs were deemed
eligible for the meta-ana lysis on the basi s of inclusion criteria.
Therefore, our meta-analysis provides a more comprehensive
view of the curre nt literature. Because of the difference in t he
total number of included studies, the study weight from each
study was significantly changed. In particular, study weights of
several RCTs (7–9, 34–36) from Zemel’s group were much
lower in our meta-analysis than i n the previous meta-analysis.
Therefore, our results were stable and less influenced by in-
dividual studies. Overall, our analysis showed somewhat dif-
ferent results for weight change (20.14 kg; 95% CI: 20.66,
0.38 kg) than the previous meta -analysis showed (20.61 kg;
95% CI: 21.29, 0.07 kg). In a ddition, the larger number of
studies included in our meta-analysis enabled us to identify
a significant heterogeneity according to trial dura tion in ad-
dition to energy restriction that was reported in the previous
meta-analysis. Although hypocaloric interventions and short-
term trials tended to induce favorable effects of dairy on
weight loss, ad libitum interventions and long-term trials showed
less weight loss in dairy-intervention than in control groups. A
long-term and sustained weight loss is of greater public health and
clinical significance than a short-term weight loss is, and the
FIGURE 3. (Continued)
META-ANALYSIS: DAIRY ON WEIGHT AND BODY FAT 743
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results of this meta-analysis do not support increasing dairy con-
sumption as an effectiv e way for long-term weight control.
The reasons for the heterogeneity of effects in different
subgroups are not entirely clear. One possible re ason might
have been either differential compliance with the intervention
protocol between s hort and long trials or its relation with
energy restriction. Unfortunately, most studies did not provide
adequate da ta on compliance. In addition, an extra dairy intake
in ad libit um d ietary intervention might have led to an increased
energy intake, which would have resulted in weight gains or
have offset the potential protective effect of the dairy intervention.
For example, in the study of Barr (3), the total energy intake from
two 3-d dietary records showed that participants assigned to the
dairy intervention group had w100-kcal/d higher calorie intakes
during the study period than at baseline, whereas no substantial
change in the mean energy intake was observed in indi viduals in
the control arm. In contrast, caloric requirements were estimated
individually in most energy-restricted trials on the basis of resting
metabolic rate s and physical activity levels, a nd thus, energy
intakes were better controlled. Therefore, the potential bene-
fits of dairy on body weight and body fat in energy-restriction
interventions could be interpreted as the effects of the sub-
stitution of dairy products for certain other foods.
There are several postulated mechanisms for the effect of dairy
products on body weight and fat. An increased calcium intake
may be beneficial for weight loss because a high calcium intake
FIGURE 4. Net change (95% CI) by study duration (short-term: ,1 y; long-term: $1 y) in body weight (A) and fat (B) associated with dairy interventions
expressed as the change (kg) during the intervention with dairy products minus the change during control regimens. Horizontal lines denote the 95% CIs; solid
diamonds represent the point estimate of each study. The open diamond represents the pooled estimate of the intervention effect. The dashed line denotes the
point estimate of the pooled result. ID, identification; WMD, weighted mean difference.
744 CHEN ET AL
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can reduce lipogenesis and stimulate lipolysis, probably via the
suppressing formation of 1,25-dihydroxyvitamin D and secretion
of parathyroid hormone or calciotropic hormones (45). Also,
calcium has been shown to combine with fatty acids in the in-
testine to form insoluble soaps, which leads to reduced absorption
of fat (46). Several RCTs have been conducted to investigate the
effects of calcium supplementation on body weight, and a recent
meta-analysis (47) of 7 calcium supplementation trials showed
a small but significant reduction in body weight that favored calcium
ov er a placebo (mean difference: 20.74 kg; 95% CI: 21.00, 20.48
kg). Ho wev er , the largest study (48) included in the meta-analysis
did not fi nd a significant ef fect of 2-y intervention using 1500-mg
Ca supplements/d on body weight or fat mass in overweight or
obese adults. Therefore, whether calc ium supplementation has
a l ong-term effect on body weight remains unclear. Except for
calcium, other dairy constituents have also been proposed to
facilitate weight and body fa t loss. For example, whey protein
may have some beneficial effects on muscle sparing and lipid
metabolism (49, 50), and conjugated linolenic acid may reg-
ulate adipogenesis, inflammation, and lipid metabolism (51).
Taken together, future research is needed to f urther illustrate
potential mechanisms of dairy products on body weight rele-
vant to energy restriction and inte rvention duration.
Several issues warrant additional discussion. First, the effects
of dairy on body weight and fat were not uniform because of the
substantial heterogeneity in individual studies that were due to
differences in study populations, study designs, and intervention
methods and durations. Second, most of the studies did not use
a double-blinded design because of practical reasons. Indeed, we
showed that the weight- and fat-reducing effects of dairy were
related to energy restriction and study duration. However, trials
that imposed energy restrictions were small in size and relatively
short in duration. In addition, influences of other factors such as
the quality of products and amounts of specific bioactive com-
ponents and their bioavailability could not be fully determined
because of a lack of information in most existing studies. Efforts
should be made to include a detailed nutrient content and com-
pliance rate in future studies.
FIGURE 4. (Continued)
META-ANALYSIS: DAIRY ON WEIGHT AND BODY FAT 745
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In conclusion, our meta-analysis does not support the bene-
ficial effect of increasing dairy consumption on body weight and
fat management in long-term studies or studies without energy
restriction. However, dairy products may have modest benefits in
facilitating weight loss when energy is restricted, but this effect
seems to be short and not sustainable.
The authors thank Jo-Anne Gilbert, Leonie van Meijl, Cristina Palacios,
and Andrea Josse for providing their unpublished data. We appreciate Eric
Ding for his valuable comments.
The authors’ responsibilities were as follows—MC, AP, VSM, and FBH:
designed the research; MC and AP: performed the systematic review and
analyzed data; AP: reviewed data; FH: had primary responsibility for the
final content of the manuscript; and all authors: wrote, reviewed, and ap-
proved the final manuscript. None of the authors had a conflict of interest.
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    • "The review included the only meta-analysis [39] and a prospective cohort [50] on the subject. Although a proposed mechanism for breakfast and breakfast cereal's role in weight management is needed, greater milk intakes reported with cereal may be a contributing factor, as greater dairy intakes have been inversely associated with weight status [63,64]. It is necessary to investigate the impact of breakfast choice and perhaps type of cereal (higher sugar versus lower sugar, high fibre, wholegrain), on anthropometric measures in longer-term trials. "
    [Show abstract] [Hide abstract] ABSTRACT: Recent data on breakfast consumption among Australian children are limited. This study examined the impact of breakfast skipping and breakfast type (cereal or non-cereal) on nutrient intakes, likelihood of meeting nutrient targets and anthropometric measures. A secondary analysis of two 24-h recall data from the 2007 Australian National Children’s Nutrition and Physical Activity Survey was conducted (2–16 years; n = 4487) to identify (a) breakfast skippers and (b) breakfast consumers, with breakfast consumers further sub-divided into (i) non-cereal and (ii) cereal consumers. Only 4% skipped breakfast and 59% of skippers were 14–16 years. Breakfast consumers had significantly higher intakes of calcium and folate, and significantly lower intakes of total fat than breakfast skippers. Cereal consumers were more likely to meet targets and consume significantly higher fibre, calcium, iron, had significantly higher intakes of folate, total sugars and carbohydrate, and significantly lower intakes of total fat and sodium than non-cereal consumers. The prevalence of overweight was lower among breakfast consumers compared to skippers, and among cereal consumers compared to-cereal consumers (p < 0.001), while no significant differences were observed for mean body mass index (BMI), BMI z-score, waist circumference and physical activity level across the categories. Breakfast and particularly breakfast cereal consumption contributes important nutrients to children’s diets.
    Full-text · Article · Aug 2016
    • "Given the current obesity epidemic, the increase in energy intake is a potential concern. However, there is a large body of evidence indicating that dairy consumption is not associated with weight gain, and adequate dairy consumption my benefit weight loss during energy restriction, at least in studies lasting one year or less [34][35][36]. These data suggest that recommended amounts of dairy can be incorporated into a healthy eating pattern. "
    [Show abstract] [Hide abstract] ABSTRACT: Diets rich in plant foods and lower in animal-based products have garnered increased attention among researchers, dietitians and health professionals in recent years for their potential to, not only improve health, but also to lessen the environmental impact. However, the potential effects of increasing plant-based foods at the expense of animal-based foods on macro- and micronutrient nutrient adequacy in the U.S. diet is unknown. In addition, dairy foods are consistently under consumed, thus the impact of increased dairy on nutrient adequacy is important to measure. Accordingly, the objective of this study was to use national survey data to model three different dietary scenarios to assess the effects of increasing plant-based foods or dairy foods on macronutrient intake and nutrient adequacy. Data from the National Health and Nutrition Examination Survey (NHANES) 2007-2010 for persons two years and older (n = 17,387) were used in all the analyses. Comparisons were made of usual intake of macronutrients and shortfall nutrients of three dietary scenarios that increased intakes by 100%: (i) plant-based foods; (ii) protein-rich plant-based foods (i.e., legumes, nuts, seeds, soy); and (iii) milk, cheese and yogurt. Scenarios (i) and (ii) had commensurate reductions in animal product intake. In both children (2-18 years) and adults (≥19 years), the percent not meeting the Estimated Average Requirement (EAR) decreased for vitamin C, magnesium, vitamin E, folate and iron when plant-based foods were increased. However the percent not meeting the EAR increased for calcium, protein, vitamin A, and vitamin D in this scenario. Doubling protein-rich plant-based foods had no effect on nutrient intake because they were consumed in very low quantities in the baseline diet. The dairy model reduced the percent not meeting the EAR for calcium, vitamin A, vitamin D, magnesium, and protein, while sodium and saturated fat levels increased. Our modeling shows that increasing plant-based foods could lead to unintended dietary outcomes without simultaneous changes in the types and amounts of plant foods currently consumed. Increasing dairy foods, which are currently under-consumed, could assist in improving the intakes of many nutrients of concern.
    Full-text · Article · Jul 2016
    • "As the majority of participants in individual studies were females further research in males is required to investigate sex effects. The results are in agreement with recently published meta-analysis showing greater weight and fat mass loss in the context of energy restriction [9,10] and reduced lean mass loss with dairy intake compared to control [10]. The current meta-analysis advances this prior research by including two trials missed in the previous meta-analyses [45,57], including more recently published trials [41,50] and trials using dairy protein supplements [21,42,[47][48][49]and targeting the specific age group of 18–50 years. "
    [Show abstract] [Hide abstract] ABSTRACT: Background/aims: A meta-analysis of randomized controlled trials (RCTs) was performed to investigate the effects of dairy food or supplements during energy restriction on body weight and composition in 18-50-year-old. Methods: RCTs ≥ 4 weeks comparing the effect of dairy consumption (whole food or supplements) with control diets lower in dairy during energy restriction on body weight, fat and lean mass were identified by searching MEDLINE, EMBASE, Pubmed, Cochrane Central and World Health Organization International Clinical Trials Registry Platform (WHO ICTRP) until March 2016. Reports were identified and critically appraised in duplicate. Data were pooled using random-effects meta-analysis. Chi²- and I²-statistics indicated heterogeneity. Dose effect was assessed using meta-regression analysis. GRADE guidelines were used to rate the quality (QR) of the evidence considering risk of bias, inconsistency, indirectness, imprecision, publication bias and effect estimates. Results: 27 RCTs were reviewed. Participants consumed between 2 and 4 standard servings/day of dairy food or 20-84 g/day of whey protein compared to low dairy control diets, over a median of 16 weeks. A greater reduction in body weight (-1.16 kg [-1.66, -0.66 kg], p < 0.001, I² = 11%, QR = high, n = 644) and body fat mass (-1.49 kg [-2.06, -0.92 kg], p < 0.001, I² = 21%, n = 521, QR = high) were found in studies largely including women (90% women). These effects were absent in studies that imposed resistance training (QR = low-moderate). Dairy intake resulted in smaller loss of lean mass (all trials pooled: 0.36 kg [0.01, 0.71 kg], p = 0.04, I² = 64%, n = 651, QR = moderate). No between study dose-response effects were seen. Conclusions: Increased dairy intake as part of energy restricted diets resulted in greater loss in bodyweight and fat mass while attenuating lean mass loss in 18-50-year-old adults. Further research in males is needed to investigate sex effects.
    Full-text · Article · Jul 2016
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