Review

International Journal of Obesity (2012) 36, 1485–1493; doi:10.1038/ijo.2011.269; published online 17 January 2012

Corrected online: 11 December 2012

Effect of dairy consumption on weight and body composition in adults: a systematic review and meta-analysis of randomized controlled clinical trials

A S Abargouei1,2, M Janghorbani3, M Salehi-Marzijarani3 and A Esmaillzadeh1,2

  1. 1Food Security Research Center, Isfahan University of Medical Sciences, Isfahan, Iran
  2. 2Department of Community Nutrition, School of Nutrition and Food Science, Isfahan University of Medical Sciences, Isfahan, Iran
  3. 3Department of Epidemiology and Biostatistics, School of Public Health, Isfahan University of Medical Sciences, Isfahan, Iran

Correspondence: Dr A Esmaillzadeh, Department of Community Nutrition, School of Nutrition and Food Science, Isfahan University of Medical Sciences, PO Box 81745, Isfahan, Iran. E-mail: esmaillzadeh@hlth.mui.ac.ir

Received 23 August 2011; Revised 15 November 2011; Accepted 5 December 2011
Advance online publication 17 January 2012

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Abstract

BACKGROUND:

 

Although several observational and experimental studies have investigated the effect of dairy consumption on weight and body composition, results are inconsistent.

OBJECTIVE:

 

This systematic review and meta-analysis was conducted to summarize the published evidence from randomized controlled clinical trials (RCTs) regarding the effect of dairy consumption on weight, body fat mass, lean mass and waist circumference (WC) in adults.

DESIGN:

 

PubMed, ISI Web of Science, SCOPUS, Science Direct and EMBASE were searched from January 1960 to October 2011 for relevant English and non-English publications. Sixteen studies were selected for the systematic review and fourteen studies were included in meta-analysis.

RESULTS:

 

Our search led to 14, 12, 6 and 8 eligible RCTs that had data on weight, body fat mass, lean mass and WC, respectively. Overall, mean difference for the effect of dairy on body weight was −0.61kg (95% confidence interval (CI): −1.29, 0.07, P=0.08). Increased dairy intake resulted in 0.72kg (95% CI: −1.29, −0.14, P=0.01) greater reduction in fat mass, 0.58kg (95% CI: 0.18, 0.99, P<0.01) gain in lean mass and 2.19cm (95% CI: −3.42, −0.96, P-value <0.001) further reduction in WC than that in controls. Subgroup analysis revealed that increasing dairy intake without energy restriction in both intervention and control groups does not significantly affect weight, body fat mass, lean mass and WC; consumption of high-dairy weight loss diets led to 1.29kg (95% CI: −1.98, −0.6, P<0.001) greater weight loss, 1.11kg (95% CI: −1.75, −0.47, P=0.001) greater reduction in body fat mass, 0.72kg (95% CI: 0.12, 1.32, P=0.02) gain in body lean mass and 2.43cm (95% CI: −3.42, −1.44, P<0.001) additional reduction in WC compared with controls.

CONCLUSION:

 

Increased dairy consumption without energy restriction might not lead to a significant change in weight or body composition; whereas inclusion of dairy products in energy-restricted weight loss diets significantly affects weight, body fat mass, lean mass and WC compared with that in the usual weight loss diets.

This paper has been amended from an Original Article to a Review since Advance Online Publication

Keywords:

dairy; weight; body fat mass; body lean mass; waist circumference; clinical trials

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Introduction

Obesity is an important public health problem worldwide, and its prevalence is increasing in both developed and developing countries.1 At least 2.8 million adults die each year as a result of being overweight or obese. Furthermore, a World Health Organization fact sheet states that diabetes (44%), ischemic heart disease (23%) and cancer burdens (7% and 41% based on type) are attributable to overweight and obesity.1 Obesity is now an epidemic worldwide,1 and the recent increase in its prevalence suggests a strong determinant role of environmental factors in its etiology.1, 2

Dairy consumption has been extensively studied for its possible roles in body weight regulation.3 According to a report by the International Dairy Federation, per capita consumption of dairy products is higher in western countries than that in non-western countries.4 On the other hand, prevalence of overweight and obesity is higher in western populations as well.5 Although several observational and experimental studies have investigated the association between dairy consumption and weight change, the role of dairy consumption in weight loss is still to be fully explored.3, 6 Available data conflict on the effect of dairy product consumption on weight loss.3, 6, 7 The primary support for the negative association between dairy intake and obesity originated from cross-sectional studies,3 and a meta-analysis provided information on the inverse association of calcium intake (from dairy and other sources) with weight status in cross-sectional studies.3 These findings were not confirmed by cohort studies about the effect of dairy consumption;7 such that in a recent systematic review of cohort studies, eight investigations—three in children8, 9, 10 and five in adults11, 12, 13, 14, 15—showed a protective association of dairy intake against weight gain and another reported a significant protective association only in adult males who were initially overweight,16 whereas seven studies reported no effect16, 17, 18, 19, 20, 21, 22 and one reported an increased risk (among children).23 Others have reported both a decreased and increased risk of incident obesity depending on the type of dairy intake.15, 24 Owing to the heterogeneity of cohort studies, as well as inconsistent exposure and outcome measures, no meta-analysis has been done on their findings.

Well-designed clinical trials are the best way to assess the effect of dairy intake on weight and body composition. It has been postulated that dairy consumption affects weight control, but findings from randomized controlled clinical trials (RCTs) have been conflicting. Several studies have suggested the beneficial effects of dairy consumption on weight loss.25, 26, 27, 28, 29 For instance, consumption of a dairy-rich (three servings per day) weight loss diet has resulted in a 5kg additional weight loss compared with the conventional weight loss diet.27 However, several RCTs have failed to reach significant conclusions,27, 30, 31, 32 and some others have even documented their significant effects of weight gain.33, 34, 35

Although the effect of dairy consumption on body weight and composition has been reviewed extensively,3, 6, 36, 37, 38 we are aware of no meta-analysis of RCTs in this regard. Inconsistent results from RCTs might be explained by different study designs, dose and duration of intervention, variety of age groups and gender. An advantage of meta-analysis to narrative reviews is the potential to yield less biased summaries of the published findings. Therefore, the goals of this report were to conduct a systematic review and, if possible, a meta-analysis of published RCTs to summarize the evidence on the effect of dairy consumption on body weight and composition and to identify possible sources of heterogeneity between studies.

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Materials and methods

Search strategy

We searched for relevant English and non-English publications by using the online databases of PubMed, ISI Web of Science, SCOPUS, Science Direct and EMBASE for the period from January 1960 to October 2011. We also contacted experts in the field and searched reference lists of the published papers. The keywords used in our search strategy were selected from the Medical Subject Headings (MeSH) database and were included as: ‘dairy’ or ‘calcium’ or ‘milk’ or ‘yogurt’ and ‘obese’ or ‘obesity’ or ‘overweight’ or ‘fat mass’ or ‘adiposity’ or ‘adipose’ or ‘weight’ or ‘body mass index’ or ‘waist circumference (WC)’.

Eligibility criteria

To be included in the meta-analysis, a published study had to meet the following criteria: (1) original article; (2) randomized controlled trial; (3) adult human population; and (4) dairy or one of the natural dairy products as the main independent variable. When there were multiple publications from the same population or cohort, only data from the most recent report were included. Information on study design, participant characteristics, measurement of weight change and randomization was extracted independently by two reviewers (ASA and AE). Discrepancies were resolved by discussion. A total of 18 randomized controlled trials were considered for inclusion in this systematic review and meta-analysis.25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 39, 40, 41, 42, 43, 44, 45

Excluded studies

Eligible studies were read more carefully by ASA and AE for any methodological difference. We found that two papers,33, 41 which administered energy restriction, had confined the energy intake into a defined certain value in both intervention and control groups, whereas other studies restricted the energy intake of participants to the amount of 500kcal less than their computed daily energy requirement. Because restriction of energy to fixed values can directly affect weight change,33, 41 we preferred not to include the results of these two studies in the systematic review and meta-analysis; however, findings from the maintenance phase of study done by Zemel et al.41 were included in our systematic review. Besides restriction of energy intake to a fixed value, these two studies had some other distinct features. The study by Bowen et al.33 compared a high-dairy diet with a high-protein diet, whereas those we included in the meta-analysis used a high-dairy diet versus diets with the same amount of protein. Another RCT done by Zemel et al.41 was a two-phase RCT that had 3 months of energy restriction to 1200–1500kcal per day and a 6-month maintenance phase. Zemel et al.41 in their study had an intervention on physical activity, and they could not practically raise total intake of dairy and calcium (dairy consumption in intervention group: 1.2 servings per day versus 1.1 servings per day for controls). Therefore, we excluded these two papers from the meta-analysis. Further readings also revealed that a study by Eagan et al.42 was an analysis of 1-year follow-up of another intervention done by Gunther et al.32; therefore, this study was removed too. One study35 had not presented data needed for meta-analysis. We included the findings of this study in our systematic review but not in the meta-analysis. After these exclusions, 14 studies were left: 4 papers that assessed the effects of high-dairy diets (without energy restriction),30, 31, 32, 34 9 papers that assessed the effects of high-dairy weight loss diets (with energy restriction)25, 26, 28, 29, 39, 40, 43, 44, 45 and 1 study with two phases (with and without restriction)27 for meta-analysis. All mentioned papers had data on weight, 12 papers had data on fat mass,25, 26, 27, 28, 29, 30, 31, 32, 39, 43, 44, 45 6 studies reported data for lean mass26, 27, 28, 30, 32, 45 and 8 papers contained data for WC.25, 26, 27, 28, 31, 40, 43, 45

Data extraction

We extracted data on publication (the first author's last name, year of publication and country of population studied), number of individuals in intervention and control groups, duration of intervention, age, gender, mean and s.d. of change in weight, fat mass, lean mass and WC. Two studies had not reported mean and s.d. of weight change,34, 35, 40 but presented means at baseline and after intervention. We calculated mean difference for these studies by the use of post-intervention data and included these papers in our meta-analysis. Another study by Baran et al.35 that had not reported mean and s.d. of weight change was included in the systematic review but not in the meta-analysis. One of the above-mentioned studies that was done by Barr et al.34 had reported their results in men and women separately. Therefore, we included the findings of this study as two separate studies in the meta-analysis.34 Two studies had not reported s.d. of mean changes in anthropometric measures but reported P-value for comparison of means between intervention and control groups.27, 30 Therefore, we computed the s.d. for these studies and used them in the meta-analysis. For five studies26, 27, 28, 29, 45 that had reported s.e.m., we calculated s.d.s. One recently published study had not reported mean change for weight, fat mass and WC. These data were taken by contacting authors.45

Statistical analysis

The mean difference and s.d. of changes in anthropometric measures (including weight, fat mass, lean mass and WC) was used for the meta-analysis. Summary mean estimates with their corresponding s.d.s were derived by the method of DerSimonian and Laird46 by using random effects model, which incorporates between-study variability. Meta-regression and subgroup analyses were performed to check for the specific source of heterogeneity. Between subgroups heterogeneity was evaluated using a fixed effect model. Statistical heterogeneity between studies was evaluated with Cochran's Q test.47 Sensitivity analysis was used to explore the extent to which inferences might depend on a particular study or group of studies. Publication bias was assessed by visual inspection of funnel plots.48 In these funnel plots, the difference in mean change of weight and other indices were displayed against the inverse of the square of the standard error (a measure of the precision of the studies). Formal statistical assessment of funnel plot asymmetry was done with Egger's regression asymmetry test and adjusted rank correlation test.49 Reported P-values are from the intercept of the regression analysis, which provides a measure of asymmetry. In addition, Begg's adjusted rank correlation test was used.49 Statistical analyses were carried out by the use of Stata, version 11.2 (Stata Corp., College Station, TX, USA). P-values that were less than 0.05 were considered statistically significant.

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Results

In total, for the 14 studies included in the meta-analysis, 883 adults aged 18–85 years were enrolled. Our preliminary analysis indicated slightly greater weight loss among those with high dairy intake compared with those with low dairy intake (−0.61kg (95% confidence interval (CI): −1.29, 0.07, P=0.08)). However, there was a significant heterogeneity between studies (P=0.04; Figure 1). To find the source of variation, we categorized the studies into two major groups: those that administered energy restriction (almost 500kcal per day less than the estimated energy requirement) for both intervention and control groups,25, 26, 28, 29, 39, 40, 43, 44, 45 and those that did not use an energy restriction.30, 31, 32, 34 One study27 that had two phases of energy restriction and weight maintenance (without energy restriction) was considered in both categories.

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Forest plot of randomized controlled trials illustrating weighted mean difference in weight change between the dairy-supplemented and control groups for all eligible studies as well as for subgroup analysis based on energy restriction. For all the studies combined, slightly greater weight loss was seen among those with high dairy intake compared with those with low dairy intake (P-value for heterogeneity=0.04, Q test, I2=41.2% and τ2=0.64). Meta-analysis of studies that administered high dairy intake without energy restriction showed no significant effect of dairy intake on weight change (P-value for heterogeneity=0.67, Q test, I2=0% and τ2=0.0). For studies that administered energy restriction, we found the significant effect of dairy intake on weight loss (P-value for heterogeneity=0.32, Q test, I2=12.5% and τ2=0.15).

Full figure and legend (102K)

Characteristics of RCTs (without energy restriction) included in the systematic review are presented in Table 1. Two out of seven papers in this category had no reported data on weight change and its s.d.; therefore, we did not include them in meta-analysis.35, 41 For five studies27, 30, 31, 32, 34 included in meta-analysis, participants’ age ranged between 18 and 85 years. Duration of intervention varied between 21 and 48 weeks. In these studies, increasing calcium intake by 400–850mg per day via dairy products in intervention group was compared with control subjects who had maintained their habitual diet. None of these individual studies found a statistically significant effect of dairy consumption on weight. Among a total of 453 subjects, the pooled standard difference in mean weight change was 0.33kg (95% CI: −0.35, 1.00, P=0.34; Figure 1). No heterogeneity was found between studies included in this subgroup (P=0.67).


Characteristics of nine RCTs that administered high-dairy weight loss diets (with energy restriction) and included in this systematic review are presented in Table 2. Participants’ age in these studies ranged between 18 and 70 years. Duration of intervention varied between 8 and 48 weeks. In these RCTs, additional calcium intake of 550–1000mg per day via dairy products in intervention group was compared with control subjects who consumed 290–800mg calcium daily. Total energy intake had been restricted to 500 kcal less than the requirement for both groups. Among a total of 430 subjects, the pooled mean difference in mean weight change was −1.29kg (95% CI: −1.98, −0.6, P<0.001; Figure 1). Heterogeneity was not significant between studies (P=0.33).


Meta-analysis on 12 eligible studies25, 26, 27, 28, 29, 30, 31, 32, 39, 43, 44, 45 that presented data on fat mass showed that among a total of 638 adults aged 18–70 years, the pooled estimated standard difference was −0.72kg (95% CI: −1.29, −0.14, P=0.01), with a significant between-study heterogeneity (P=0.007) (Figure 2). Subgroup analysis based on energy restriction demonstrated that for nine RCTs with energy restriction,25, 26, 27, 28, 29, 39, 43, 44, 45 the pooled standard difference in means among a total of 331 subjects was −1.11kg (95% CI: −1.75, −0.47, P=0.001). No significant heterogeneity was found (P=0.33). When we considered data from four RCTs27, 30, 31, 32 that did not administer energy restriction, we observed that among a total of 253 subjects, the pooled standard difference in means was −0.16kg (95% CI: −0.97, 0.66, P=0.71), with a significant between-study heterogeneity (P=0.02). We could not find the source of this heterogeneity even after further subgroup analyses based on sex, age group and meta-regression.

Figure 2.
Figure 2 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Forest plot of randomized controlled trials illustrating weighted mean difference in fat mass between the dairy-supplemented and control groups for all eligible studies as well as for subgroup analysis based on energy restriction. For all the eligible studies combined, the significant effect of dairy intake on reducing fat mass was observed (P-value for heterogeneity <0.01, Q test, I2=56.1% and τ2=0.51). For nine RCTs with energy restriction, the effect was also significant (P-value for heterogeneity=0.33, Q test, I2=12.0% and τ2=0.11). However, data from four RCTs that did not administer energy restriction indicated no significant effect of dairy intake on body fat mass (P-value for heterogeneity=0.02, Q test, I2=70.7% and τ2=0.46).

Full figure and legend (97K)

We found a significant effect of dairy intake on increasing lean body mass in the whole data set26, 27, 28, 30, 32, 45 (the pooled estimated standard difference: 0.58kg; 95% CI: 0.18, 0.99, P<0.01; P-value for heterogeneity=0.07); subgroup analysis based on energy restriction showed that the effect is significant for four RCTs26, 27, 28, 45 that administered energy restriction (0.72kg; 95% CI: 0.12, 1.32, P=0.02, P-value for heterogeneity=0.06) and nonsignificant for three RCTs27, 30, 32 without energy restriction (0.35kg; 95% CI: −0.15, 0.86, P=0.17, P-value for heterogeneity=0.29; Figure 3).

Figure 3.
Figure 3 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Forest plot of randomized controlled trials illustrating weighted mean difference in lean body mass between the dairy-supplemented and control groups for all eligible studies as well as for subgroup analysis based on energy restriction. For all the studies combined, the significant effect of dairy consumption on lean body mass was seen (P-value for heterogeneity=0.07, Q test, I2=48.9% and τ2=0.13). For four RCTs that administered energy restriction, high dairy intake had resulted in a greater gain in lean body mass compared with that in the control group (P-value for heterogeneity=0.06, Q test, I2=59.7% and τ2=0.21). Such a finding was not obtained for three RCTs that had not administered energy restriction (P-value for heterogeneity=0.29, Q test, I2=19.5% and τ2=0.05).

Full figure and legend (68K)

Eight studies25, 26, 27, 28, 31, 40, 43, 45 had reported data on WC. For all studies combined, we found the pooled estimated standard difference of −2.19cm (95% CI: −3.42, −0.96, P<0.001, P-value for heterogeneity=0.03; Figure 4). When the subgroup analysis was performed based on energy restriction, we observed that for seven RCTs25, 26, 27, 28, 40, 43, 45 that administered energy restriction in a total of 289 subjects, high dairy intake had resulted in a greater reduction in WC compared with that in control group (the pooled standard difference in means: −2.43cm; 95% CI: −3.42, −1.44, P<0.001, P-value for heterogeneity=0.60). Such a finding was not obtained for two RCTs27, 31 that had not administered energy restriction (−2.68cm; 95% CI: −8.02, 2.66, P=0.32, P-value for heterogeneity=0.01).

Figure 4.
Figure 4 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Forest plot of randomized controlled trials illustrating weighted mean difference in WC between the dairy-supplemented and control groups for all eligible studies as well as for subgroup analysis based on energy restriction. For all the studies combined, the significant effect of dairy consumption on WC was seen (P-value for heterogeneity=0.03, Q test, I2=53.4% and τ2=1.63). For seven RCTs that administered energy restriction, high dairy intake had resulted in a greater reduction in WC compared with that in the control group (P-value for heterogeneity=0.60, Q test, I2=0.0% and τ2=0.0). Such a finding was not obtained for two RCTs that had not administered energy restriction (P-value for heterogeneity=0.01, Q test, I2=84.7% and τ2=12.8).

Full figure and legend (79K)

In a sensitivity analysis, we found that the effect of dairy foods on body weight and fat mass was not substantially modified by the result of a certain study. However, owing to the small number of studies (without energy restriction) that presented data for WC and lean mass, excluding each study could change the overall effect size. We did not find any evidence of publication bias for studies assessing dairy's effect on weight (P=0.68, Egger's test) and lean body mass (P=0.32), despite a slight asymmetry in Begg's funnel plot (Figure 5). With regard to body fat and WC, we reached a significant publication bias by funnel plot and Egger's test (P=0.03 and P=0.01, respectively); however, this was not verified by Begg and Mazumdar test (P=0.47 and P=0.28, respectively).

Figure 5.
Figure 5 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Begg's funnel plots (with pseudo 95% CIs) of the difference in means (DMs) versus the s.e.s of the MDs (mean differences) for studies that assessed the effect of dairy intake on weight (a), fat mass (b), lean body mass (c) and WC (d). The horizontal line shows the pooled DMs calculated with the DerSimonian and Laird random effects model. We did not find any evidence of publication bias for studies assessing dairy's effect on weight and lean body mass. With regard to body fat and WC, we reached a significant publication bias by funnel plot; however, this was not verified by Begg and Mazumdar test.

Full figure and legend (46K)

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Discussion

We found that increasing dairy consumption to about recommended daily intakes in adults, who do not follow any calorie-restricted diet, would not affect weight, fat mass, lean body mass and WC. However, consumption of high-dairy calorie-restricted diets might result in a greater weight loss and higher reduction of WC and fat mass compared with conventional calorie-restricted diets. Our findings indicated that increased dairy intake might lead to gain in lean body mass. To the best of our knowledge, this is the first meta-analysis of RCTs assessing the effect of dairy intake on weight and body composition.

It must be kept in mind that we could not include two papers35, 41 that met our inclusion criteria but lacked required data about weight change. Baran et al.35 found that increased dairy intake would result in weight gain compared with the controls; however, as the main end point of the study was not weight change, they did not perform further analysis on weight change difference. Zemel et al.41 showed no difference in weight between high-dairy group and controls. In both above-mentioned studies35, 41 as well as in another study done by Barr et al.,34 total energy intake was significantly increased due to the inclusion of dairy products in the diet. This was not the case for four other studies27, 30, 31, 32 that administered dairy products without energy restriction. Therefore, one might conclude that dairy consumption (without calorie restriction) would result in a significant weight gain and increased body fat mass and WC due to increased energy intake. However, with the findings of all such studies combined, we did not find any evidence supporting this hypothesis. In contrast, our findings confirm the suggestions made by recent narrative reviews on the effect of dairy consumption on weight change.3, 6 Dougkas et al.3 expressed that the majority of studies found no significant weight gain by dairy consumption when energy intake was not restricted. The same conclusion was also reached by Lanou and Barnard6 when they reviewed studies that used dairy or calcium supplements as an intervention.

We found that inclusion of dairy products in weight loss diets might accelerate weight reduction. The amount of dairy products used in published papers was equivalent to the provision of 550–1000mg additional calcium for overweight or obese subjects as compared with the consumption of 200–800mg calcium in the control group. Our findings also demonstrated that consumption of high-dairy weight loss diets would result in a greater reduction in fat mass and WC and a further increase in lean body mass. These results indicate that dairy consumption would help people to lose weight from fat compartments of the body, particularly from abdominal adipose tissue, which has been reported as a main determinant of cardiovascular risk factors.50

Narrative reviews that considered the effect of increasing dairy intake along with energy restriction have reached conflicting findings. For instance, Dougkas et al.3 found five RCTs with energy restriction in their review, and reported that three studies showed no evidence,33, 43, 44 whereas two studies27, 28 found significant effect of increasing dairy consumption along with weight loss diets on weight reduction. However, the search methodology used by Dougkas et al.3 seems to be incomplete because we found 11 studies that assessed the effect of dairy-rich weight loss diets on weight change.25, 26, 27, 28, 29, 33, 39, 40, 41, 43, 44 These studies had some inconsistencies in their method of intervention that can affect the interpretation of their results. Two of the mentioned studies restricted total energy intake of their subjects to a fixed calorie33, 41 (that is, 1200–1400kcal per day); others administered an energy intake of 500kcal less than subjects’ requirements.

Several underlying mechanisms for the effect of dairy products on body weight and composition are proposed. It has been suggested that calcium supplementation can affect adipocyte lipid metabolism, fat oxidation, fatty acid absorption and postprandial fat metabolism. Furthermore, there are some findings that suggest dairy constituents other than calcium, such as conjugated linoleic acid, medium-chain fatty acids and proteins, may play a role. Evidence about the effect of dairy on appetite regulation has been accumulating recently and some of its constituents, such as dairy protein, calcium and fat, have been proposed for this kind of effect. All these possible underlying mechanisms have been extensively reviewed and discussed by Dougkas et al.3 Despite these mechanisms, we found the beneficial effects of dairy only in energy-restricted studies. Therefore, it might be hypothesized that appetite-lowering effects of dairy work more efficiently along with energy-restricted diets. This hypothesis remains to be examined in future studies.

The majority of cross-sectional studies about the relationship between dairy consumption and body weight, composition or abdominal fat accumulation in adults found a significant inverse association.51, 52, 53, 54, 55, 56, 57, 58, 59 Some studies failed to find any relationship,53, 60, 61 and one study showed inconsistent results based on the kind of dairy product.62 Cross-sectional studies cannot reach a casual relationship; therefore, to detect a causal relationship one would need to look at cohort studies. Findings from cohort studies on the association of dairy intake and weight change are inconclusive.7 So far, many RCTs have been published worldwide about the effect of increasing dairy consumption on weight change and body composition. This makes meta-analysis of their results possible. RCTs are the most appropriate study designs to show casual effects, and meta-analysis of their results can lead to a more reliable conclusion.

It must be kept in mind that some RCTs included in the current meta-analysis had small sample sizes. Furthermore, the duration of follow-up for studies that administered dairy intake with energy restriction was short, such that only a few investigations had followed the participants for more than 12 weeks. In addition, most published papers were from the United States and limited data are available from other countries, particularly from non-western populations. As our findings, particularly those for body composition indicators (fat mass, lean body mass and WC), are based on a limited number of RCTs, further investigations are required to come to a better understanding.

In conclusion, our systematic review and meta-analysis on RCTs indicated that increasing dairy consumption without energy restriction might not lead to a significant change in weight and body composition, whereas inclusion of dairy products in weight loss energy-restricted diets would result in a greater reduction of weight, fat mass and WC and gain in lean body mass compared with the conventional weight loss diets.

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Conflict of interest

The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS

ASA and AE contributed to the conception, design, statistical analyses, data interpretation and manuscript drafting. MJ and MSM contributed to the data analysis. All authors contributed to the approval of the final manuscript for submission.

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