Scope of the review

The present review aims to provide an overview of the totality of the available evidence taken from cross-sectional studies, prospective longitudinal studies and intervention studies that have reported on the associations between intakes of milk and other dairy products, and obesity and indicators of adiposity, in children. This critical review also aims to explore whether any associations identified related equally to all milk and other dairy products. It also provides context for these findings, specifically in terms of the mechanisms by which milk and other dairy products might make an impact on obesity and indicators of adiposity in children and adolescents.

Differences by type of milk and other dairy products

The majority of prospective studies have examined only milk; thus the impact of ‘type’ of dairy products on the associations between dairy product consumption and body composition remains largely unknown.

With respect to type of milk and the effects of added sugar on health, Fayet-Moore⁽ Reference Fayet-Moore ^{34 )} recently reviewed the literature related to the role of flavoured or sweetened milk in the diet and weight status of children. Although flavoured milk has similar fat and protein content to white milk, it provides approximately 60 kcal (250 kJ) more per serving portion than plain milk due to added sugars⁽ Reference Noel, Ness and Northstone ^{35 )}. Two prospective studies have examined the relationship between milk and measures of adiposity, by differentiating between white milk and flavoured milk⁽ Reference Noel, Ness and Northstone ^{35 ,} Reference Vanselow, Pereira and Neumark-Sztainer ^{36 )}. A beneficial association of white milk with body weight was reported in the Project EAT (Eating Among Teens) by Vanselow et al. ⁽ Reference Vanselow, Pereira and Neumark-Sztainer ^{36 )}. The authors showed that plain milk and not flavoured milk consumption assessed by FFQ was inversely related to weight gain (self-assessed) over the 5-year follow-up (P=0·05 v. P=0·31) without displaying a dose–response relationship in 2294 adolescents and independent of their overweight status. A more recent study employed more accurate measures of dietary intake and body composition by using dietary weighed records and dual-energy X-ray absorptiometry in 2270 children at the age of 10 years⁽ Reference Noel, Ness and Northstone ^{35 )}. Results showed that the decrease in percentage body fat was less in overweight/obese children who consumed flavoured milk (17 %) than non-consumers of flavoured milk (83 %) (–0·16 (95 % CI –3·8, 3·5) v. –3·4 (95 % CI –6·5, –0·42) %; P=0·02). However, there was no association between flavoured milk consumption at 10 years and change in percentage body fat from 11 to 13 years in normal-weight children. It is worth noting that this was the only study that has examined whether the subjects’ BMI at baseline could influence the association between dairy product consumption and body weight or abdominal obesity during childhood or adolescence.

Overall, although the body of literature on flavoured milk is limited, according to the review by Fayet-Moore⁽ Reference Fayet-Moore ^{34 )} consumption of flavoured milk is not related to weight gain or increased body fat among normal-weight children. However, there are conflicting and inconsistent results among overweight and obese children; thus randomised intervention studies are warranted to examine the effect of flavoured milk consumption on BMI and adiposity. Given the popularity of flavoured milk as the preferred dairy product amongst children, controlled studies with better description of exposure and outcome variables are needed.

Low-fat v. full-fat dairy products

Given the scientific interest on the issue of dairy fat and its contribution to the development of obesity due its high energy content⁽ Reference Kratz, Baars and Guyenet ^{37 )}, we explored the data to assess whether there was any evidence of differential associations of low- and full-fat milk with weight status. Five of the six longitudinal studies examined the relationship between low- or full-fat milk and other dairy products on obesity and indicators of adiposity as a primary outcome of their analysis⁽ Reference Noel, Ness and Northstone ^{38 –} Reference Dubois, Diasparra and Bogl ^{43 )}. Although evidence is limited, data suggest that replacing whole milk with reduced-fat milk does not lead to an inverse or positive association between milk and weight gain or prevention of overweight in early childhood. For instance, in 10- to 13-year-olds, Noel et al. ⁽ Reference Noel, Ness and Northstone ^{38 )} showed that there was no significant difference between low-fat or skimmed milk v. whole milk at 10 years and change in percentage body fat (P>0·09) at 11 and 13 years. In preschool children, Huh et al. ⁽ Reference Huh, Rifas-Shiman and Rich-Edwards ^{39 )} reported inverse association between whole milk, but not low-fat milk, consumption at age 2 years and BMI z score (β=–0·09 (95 % CI –0·16, –0·01); P=0·02) at age 3 years in 852 children, yet there was no association between either whole or low-fat milk with incidence of overweight at age 3 years. Similarly, Scharf et al. ⁽ Reference Scharf, Demmer and DeBoer ^{40 )} showed in a longitudinal analysis examining 10 700 children that milk consumption at age 2 years, whether whole or low-fat, was not related to changes in BMI z score at the age of 4 years (P=0·6). However, children who were normal weight at baseline and consistently drank 1 % fat or skimmed milk at both 2 and 4 years of age had an increased risk of becoming overweight or obese during this time interval (OR 1·57 (95 % CI 1·03, 2·42); P<0·05). This contradicts the current American Academy of Pediatrics recommendation in favour of low-fat milk consumption at children ≥2 years⁽ Reference Gidding, Dennison and Birch ^{44 )}, yet the results could be related to residual confounders that have not been assessed in their analysis such as other food items, total energy intake and physical activity. Berkey et al. ⁽ Reference Berkey, Rockett and Willett ^{41 )} evaluated the association between annual milk consumption and BMI changes and found that in 12 829 children aged 9–14 years, those drinking ≥3 servings of milk per d had a significantly higher increase in BMI over 4 years, compared with those drinking >1 and ≤2 servings per d. Particularly, high consumption of 1 % milk for boys and skimmed milk for girls had more consistent positive associations with annual BMI gain than did whole or 2 % milk (P<0·05). However, adjustment for multiple variables revealed that total energy intake mediated those associations, contributing to the heavier weight status.

Can dietary calcium reduce body weight?

As dairy products represent the major source of dietary Ca, a few studies reported on the association between dietary Ca and adiposity, without a clear distinction between dairy and non-dairy Ca sources. Nevertheless, as far as Ca is concerned, three studies suggested an inverse relationship between dietary Ca consumption and prevalence of obesity and adiposity in children⁽ Reference Carruth and Skinner ^{45 –} Reference Dixon, Pellizzon and Jawad ^{47 )}. Skinner et al. ⁽ Reference Skinner, Bounds and Carruth ^{46 )} suggested that increasing the Ca intake with 240 ml skimmed milk or yoghurt daily would reduce children’s body fat by 0·4 %, which in the longer term could reduce the risk of obesity in later childhood, adolescence or adulthood. However, several studies, conducted mostly in girls, showed that dietary Ca over the years was not associated with BMI z scores, percentage body fat or trunk fat during adolescence⁽ Reference Phillips, Bandini and Cyr ^{33 ,} Reference Fisher, Mitchell and Smiciklas-Wright ^{48 –} Reference Rockell, Williams and Taylor ^{50 )}. Very few studies have evaluated the relationship of dietary Ca and adiposity according to sex or ethnicity⁽ Reference Carruth and Skinner ^{45 ,} Reference Dixon, Pellizzon and Jawad ^{47 ,} Reference Striegel-Moore, Thompson and Affenito ^{51 )} and differences in terms of inherent sex characteristics, growth, pubertal status and sexual maturation⁽ Reference Wang ^{52 )} add complexity to the impact of sex on Ca–adiposity associations.

Differences by age of child

Hasnain et al. ⁽ Reference Hasnain, Singer and Bradlee ^{53 )} conducted the longest study using data from the Framingham Children’s Study, in which 103 boys and girls aged 3–5 years old were enrolled and followed annually for 12 years. Prospective analysis between milk and percentage body fat (dual-energy X-ray absorptiometry) indicated that children in the highest tertile of milk consumption (411 ml/d) had 7·3 % lower percentage body fat compared with the lowest (115 ml/d; 30 v. 22·6 %; P=0·0095) at ages 15–17 years. However, DeBoer et al. ⁽ Reference DeBoer, Agard and Scharf ^{54 )} recently found that in 4-year-old children, drinking ≥4 servings of milk daily had a higher risk of becoming overweight compared with those with an intake of <1 serving daily at age 4 years (OR 1·159 (95 % CI 1·02, 1·32); P=0·021), yet the positive association was no longer present at 5 years (OR 1·094 (95 % CI 0·92, 1·31); P=0·318). As the authors suggested, this minor adverse association of excess milk consumption with weight status could be age-dependent, with more pronounced association of milk consumption with height over time contrary to its association with weight gain.

Discussion of longitudinal studies

As summarised in Table 1, there are considerable differences in the exposure and outcome variables and population characteristics, with seventeen studies examining specifically the association between milk consumption and indicators of body weight or adiposity. Data from most prospective studies included in the present review showed that there is a null association between increased milk (n 9) or dairy product (n 5) consumption and measures of adiposity or body weight. However, there was high variation on the definition and inclusion of dairy foods and type of milks in the studies. For instance, considering the type of milk or fat content, a few studies followed the United States Department of Agriculture definition^{( 15 )} which includes white milk (cow, sheep or goat), while other studies also included sweetened and flavoured, evaporated, condensed, formula, reduced-fat, full-fat or non-fat milk. Similarly, there were notable differences among the seven studies⁽ Reference Rangan, Flood and Denyer ^{31 –} Reference Phillips, Bandini and Cyr ^{33 ,} Reference Huh, Rifas-Shiman and Rich-Edwards ^{39 ,} Reference Bigornia, LaValley and Moore ^{42 ,} Reference Dixon, Pellizzon and Jawad ^{47 ,} Reference Moore, Singer and Qureshi ^{55 )} that investigated total dairy products, with most including any type of yoghurt or cultured milk drinks and natural or processed cheese and some others including dairy desserts such as ice-cream, milkshakes, custard creams and puddings in addition to milk, yoghurt and cheese. However, there were no differences in adiposity outcomes between the studies that included dairy desserts and those that did not. Furthermore, four of them⁽ Reference Garden, Marks and Almqvist ^{32 ,} Reference Phillips, Bandini and Cyr ^{33 ,} Reference Huh, Rifas-Shiman and Rich-Edwards ^{39 ,} Reference Bigornia, LaValley and Moore ^{42 )} reported the portion or percentage energy from milk or other type of dairy products. Based on their reports, milk contributed approximately 50–70 % of the total dairy product consumption followed by yoghurt and cheese, particularly in the highest quartiles or groups of dairy product consumption. Although milk, cheese and yoghurt share several functional properties, differences in their absorption kinetics, macro- and micronutrient content and level of bioactive components could lead to different metabolic effects⁽ Reference Charney ^{56 )}. Thus, disaggregating the effects of individual types of dairy product consumption from general dairy products in large-scale epidemiological studies seems prudent.

Few longitudinal studies directly compared body weight or adiposity outcomes related to reduced-fat v. full-fat milk or dairy products and results suggest no significant differences between reduced-fat compared with full-fat milk or total dairy products and body weight or adiposity measures⁽ Reference Phillips, Bandini and Cyr ^{33 ,} Reference Noel, Ness and Northstone ^{38 –} Reference Dubois, Diasparra and Bogl ^{43 )}. Most of these studies were conducted in the US population and results could have been influenced by reporting bias or residual confounding due to the cultural perception related to fat consumption or other unmeasured associated dietary and health behaviours⁽ Reference Liu, Choi and Ford ^{57 ,} Reference Margolis, Wei and de Boer ^{58 )}. Additionally, the range or form that dairy food is consumed might differ, given that in the USA, dairy fat is usually consumed as ice-cream or dairy desserts, whilst in Europe plain cheeses or unsweetened yoghurt are the major source of full-fat dairy products⁽ Reference Kratz, Baars and Guyenet ^{37 )}. However, given the limited data, it makes it difficult to draw firm conclusions about whether reduced-fat or full-fat milk and other dairy product consumption may be more beneficial.

Most of the observational studies assessed milk and other dairy product consumption by FFQ, which, compared with food records⁽ Reference Sempos ^{59 )}, are not the most accurate dietary assessment tool for certain foods, particularly foods that are perceived as unhealthy such as whole milk and cream⁽ Reference Salvini, Hunter and Sampson ^{60 )}. Another acknowledged source of bias in any nutrition research is the over- and under-reporting of dietary intakes, which could lead to gross misclassification of full-fat dairy product intake, particularly among overweight adolescents⁽ Reference Huang, Roberts and Howarth ^{61 )}. The accuracy of energy reporting may decline as children become older⁽ Reference Bandini, Must and Cyr ^{62 )}; thus unbiased estimates of intake are more likely to occur when the dietary reporting is conducted by parents in early childhood (before the age of 6 years)⁽ Reference Moore, Singer and Qureshi ^{55 )}. Although there is some evidence that milk consumption in childhood tracks over time⁽ Reference Moore, Singer and Qureshi ^{55 ,} Reference Singer, Moore and Garrahie ^{63 ,} Reference Boulton, Magarey and Cockington ^{64 )}, a limitation within the longitudinal studies reviewed in the present study is the lack of regular assessment of dairy product and dietary intake throughout childhood and adolescence. The patterns with regard to the type of milk and other dairy product consumption might not be stable over time, especially with the introduction and higher availability of reduced-fat dairy products over the last 25 years.

In addition to diversity in describing the type or fat content of milk and other dairy products, there was variation on the definition and reporting of dairy food serving size, with some studies using the term ‘serves per day’, ‘portions per day’, ‘grams per day’ or ‘times per day’. Similarly, considerable variation was found in the reporting of outcome variables related to weight status and adiposity measures, making it difficult to compare research findings. Future studies in this research area should include a set of well-defined exposure and outcome variables, measured ideally by health professionals to minimise error and reported in a consistent format. For instance, milk, cheese and yoghurt and the type of those dairy foods and fat content reported in g per d as an absolute serving size should be defined. Change in weight in children and adolescents should be reported as BMI z score, body weight, percentage body fat, percentage lean body mass and waist circumference.

Adjustment for important confounding factors such as energy intake, diet quality, physical activity levels, baseline BMI, sex, race and socio-economic status were inconsistent and varied among the studies, making it difficult to interpret and compare the results across study cohorts. For instance, twenty-six of the thirty-one studies reviewed included data on both boys and girls, and only sixteen adjusted for sex. Similarly, five⁽ Reference Noel, Ness and Northstone ^{35 ,} Reference Noel, Ness and Northstone ^{38 ,} Reference Berkey, Rockett and Willett ^{41 ,} Reference Bigornia, LaValley and Moore ^{42 ,} Reference Fisher, Mitchell and Smiciklas-Wright ^{48 )} of the fifteen studies, which analysed data in school-age children and adolescents, adjusted for pubertal growth development. This is important given that children and adolescents’ body size and composition are influenced by the sex and pubertal growth rate, for example, change in height⁽ Reference Petit ^{65 )}, with notable differences in growth rate in the period from early childhood to adolescence. Thus, assumptions that the body composition measures would be independent of pubertal growth stage are invalid and both observational and controlled randomised trials need to consider the influence of pubertal development on the effect of milk and other dairy product consumption on indicators of adiposity. Furthermore, adjustment for energy intake needs to be carefully considered given the high degree of association between increased energy intake and adiposity⁽ Reference Willett, Howe and Kushi ^{66 )}. As Berkey et al. ⁽ Reference Berkey, Rockett and Willett ^{41 )} highlighted, the positive association between milk and annual BMI gain was mediated by the total energy intake and not necessarily by the dairy products per se. Given that dairy products could influence body weight through mediating appetite, energy intake and therefore energy regulation as described in the mechanistic section below, analyses should be performed with and without energy intake in the adjustment models. Energy intake was reported in thirteen studies and five studies reported food groups or beverages or macronutrients as indicators of dietary intake in their adjustment models. Thus, inclusion and appropriate control of potential confounders is essential in analyses of all studies.

Sample size and duration varied significantly among the studies, with studies with a protective association between milk and other dairy products and obesity having a smaller sample size (nineteen studies n<1000, of which eight showed inverse association) relative to the studies reported an adverse association between increased milk and dairy product consumption and obesity (twelve studies n>1000, of which two showed an adverse association). One-third of the studies were funded by the dairy or private industry and five studies showed results favourable for industry compared with the four out of twenty-one public-funded studies, which demonstrated a favourable effect for dairy foods and obesity risk. It is also worth noting that twenty-six out of the thirty-one studies were performed in English-speaking nations (USA (n 16), Australia (n 4), UK (n 4) and Canada (n 2)); thus results might not be generalisable to all developed and developing countries.

Overall, the majority of prospective studies showed a null relationship between increased dairy product consumption and measures of adiposity or body weight in children and adolescents with high relative to low or no dairy product consumption. Although data are limited, they suggest that replacing whole milk with reduced-fat milk does not lead to an inverse or positive association between milk and weight gain or prevention of overweight in early childhood.

What do studies that examined bone health reveal about body weight?

The majority of the studies that were primarily designed to examine bone health failed to detect differences among the treatments⁽ Reference Chan, Hoffman and McMurry ^{67 –} Reference Vogel, Martin and McCabe ^{74 )}. For example, Chan et al. ⁽ Reference Chan, Hoffman and McMurry ^{67 )} conducted a 12-month randomised parallel study in pubertal girls and found no differences in body weight, lean body mass and fat mass between the high dairy Ca group (1200 mg Ca/d) and the control group (usual diet). Likewise, Cadogan et al. ⁽ Reference Cadogan, Eastell and Jones ^{68 )} reported no effect of whole or reduced-fat milk consumption (568 ml/d) on body composition in 12-year-old girls for 18 months, and Merrilees et al. ⁽ Reference Merrilees, Smart and Gilchrist ^{69 )} found no effect with the provision of 1000 mg Ca/d from dairy foods over 2 years on body weight, body lean mass and fat mass in post-pubertal teenage girls compared with those girls on their usual diet. Similar neutral effects were observed in an 18-month randomised parallel study in 9- to 10-year-old Chinese children, between the 80 g (equivalent to 250 ml whole milk) or 40 g milk powder supplementation group and the control group (usual diet)⁽ Reference Lau, Lynn and Chan ^{72 )}. A more recent 18-month randomised parallel study showed that 240 healthy-weight and overweight 8- to 16-year-old boys and girls when assigned to either receive three servings of dairy products/d (1500 mg Ca/d) or maintain their usual diet (1000 mg Ca/d) had no differences in body weight and fat gain⁽ Reference Vogel, Martin and McCabe ^{74 )}. In contrast, one study conducted in 757 Chinese girls with low habitual Ca diet and high prevalence of vitamin D deficiency showed weight gain and enhanced growth⁽ Reference Du, Zhu and Trube ^{75 )}. Du et al. ⁽ Reference Du, Zhu and Trube ^{75 )} conducted the largest school-milk intervention trial, where the 10-year-old girls were randomly assigned to a group that received 330 ml of Ca-fortified milk, a group that received 330 ml of Ca-fortified milk plus vitamin D or a control group (habitual diet). There were no differences in body height and weight between the three groups over the 24-month study period, yet milk consumption substantially increased height, sitting height and body weight when expressed by mean percentage changes from baseline compared with the control.

Dairy products v. dietary/supplemental calcium

Although the effect of exclusively supplemental Ca on body composition is out of the scope of the present review, it is worth mentioning the only study that was designed to examine the effects of both dairy products and Ca supplements (calcium carbonate tablets) on bone mass accrual and body composition, therefore allowing a direct comparison and an important insight into the purported bioactive components of dairy products⁽ Reference Cheng, Lyytikäinen and Kröger ^{71 )}. In this 2-year randomised trial in 10- to 12-year-old girls, findings indicated no differences in body weight, height, lean tissue and fat mass between the cheese (1000 mg/d), the supplemental Ca (1000 mg/d), the placebo and the reference group (habitual high Ca intake >900 mg/d). The authors suggested that the lack of differences in body composition could have been masked by the diverse growth pattern in the girls. In concordance with the findings based on bone health studies, Lappe et al. ⁽ Reference Lappe, Rafferty and Davies ^{76 )} showed that 9-year-old girls in the high Ca diet group (1656 (sd 191) mg Ca/d) compared with the control group (usual diet: 961 (sd 191) mg Ca/d) gained similar body weight (34 v. 33 %; P>0·05), lean mass (31 v. 31 %; P>0·05) and fat mass (38 v. 33 %; P>0·05) despite the nearly two-fold increase in dairy product consumption in the treatment group over 2 years. Based on a recent meta-analysis of combined results from nine studies including both observational and randomised clinical trials⁽ Reference Li, Fan and Lu ^{77 )}, Ca intake (derived from Ca-rich foods, dairy products and Ca supplements) was inversely related to body weight gain in children and adolescents (mean –0·26 (95 % CI –0·41,–0·11) kg; P<0·001).

Differences by type of milk and other dairy products

Most intervention trials, like observational studies, have examined only milk or total dairy products, with no studies having addressed the effect of ‘type’ of dairy products on childhood and adolescent obesity. Arnberg et al. ⁽ Reference Arnberg, Mølgaard and Michaelsen ^{78 )} conducted a 3-month randomised parallel study (the Milk Components and Metabolic Syndrome (MoMS) study) in overweight subjects aged 12–15 years and showed that adolescents in the skimmed milk (1 litre/d), whey- and casein-based drink (35 g protein/l per d) supplementation groups increased BMI-for-age z scores compared with the control group (water intake) and pre-test control subgroup (3 months before intervention). However, given that puberty is a period of rapid growth, BMI is not the most representative and adequate index of body composition. Thus, Larnkjaer et al. ⁽ Reference Larnkjaer, Arnberg and Michaelsen ^{79 )} in a follow-up analysis of this study demonstrated that milk-based drinks increased both body fat and lean mass. Although milk-based drinks improved body composition by increasing lean mass and maintaining fat mass relative to pre-test control, they had a less favourable effect on body composition than water intake. However, this could be attributable to the replacement of one energy drink for a non-energy beverage such as water. Replacement of one energy drink for another was examined in a 16-week study, which included an intervention with increasing flavoured milk, whilst almost eliminating sugar-sweetened beverage consumption by delivering milk beverages to the homes of overweight and obese 8- to 10-year-old children⁽ Reference Albala, Ebbeling and Cifuentes ^{80 )}. Although no differences in percentage of body fat were observed, milk consumption increased the accretion of lean body mass compared with the control (usual diet). Of all the randomised intervention studies reviewed, the recent study by Lappe et al.⁽ Reference Lappe, McMahon and Laughlin ^{81 )} is the first study to report the effects of dairy food on body weight and fat in adolescents. They enrolled 13- to 14-year-old girls with BMI between the 50th to 98th percentiles and with habitual low Ca intake (<600 mg/d) and assigned them to either the intervention dairy food group (reduced-fat milk and yoghurt, ≥1200 mg Ca/d) or the control group (usual diet) for 12 months. There were no differences in weight and fat gain between the dairy food and control groups. In an explanatory analysis, they found that the effect of reduced-fat milk did not differ from yoghurt on change in percentage of body fat or weight, yet due to the nature of the intervention incorporating both dairy products, the effect of each type of dairy product remains to be explored.

Interventions aimed to manage childhood overweight and obesity, which included advice on different milk and other dairy products

Two studies included healthy eating counselling as part of their weight management intervention⁽ Reference St-Onge, Goree and Gower ^{82 ,} Reference Kelishadi, Zemel and Hashemipour ^{83 )}. Findings from a 16-week randomised parallel trial in 8- to 9-year-old overweight children, indicated that high milk (4 × 236 ml/d) compared with low milk (1×236 ml/d) consumption tended to decrease visceral adipose tissue (–0·18 (sem 0·08) v. –0·11 (sem 0·05) litres; P>0·05), despite the fact that there was no significant change in body weight or body composition⁽ Reference St-Onge, Goree and Gower ^{82 )}. As authors suggested, a larger sample size and a longer duration might have been needed to adequately examine and detect body composition changes between the milk groups. Furthermore, a randomised controlled trial in 120 obese and prepubescent children examined the effect of a dairy product-rich diet on long-term weight management⁽ Reference Kelishadi, Zemel and Hashemipour ^{83 )}. The children were randomly assigned to a dairy product-rich diet group (>800 mgCa/d, without any energy restriction), energy-restricted diet or control group and all of them had six healthy lifestyle family sessions once per month during the 6-month intervention. BMI standard deviation scores and waist circumference were decreased significantly in all groups after the 6 months. A follow-up was done twice yearly for 3 years and a significant increase was observed in BMI and waist circumference given the period of rapid growth. However, the increase was significantly lower in the dairy product group compared with the other two groups suggesting that in addition to lifestyle modifications, a dairy food-rich diet could be a potent strategy for weight management in young overweight children.

Similarly, a recent 38-month study in 7- to 10-year-old children⁽ Reference Weber, Stark and Ittenbach ^{73 )} included a behavioural modification and nutritional education intervention to gradually increase dietary Ca intake to 1500 mg/d compared with the usual-care group (a single 45 min nutrition session). Results showed an increase in Ca intake in the intervention group and an inverse association between dietary Ca intake and gain in BMI and fat mass irrespective of treatment group.

Although several studies have examined the effect of milk or dairy product consumption on alterations in body weight and composition under energy restriction in adults⁽ Reference Chen, Pan and Malik ^{84 ,} Reference Abargouei, Janghorbani and Salehi-Marzijarani ^{85 )}, there is only one study which explored the relationship between dairy product consumption and BMI or fat mass in overweight and obese children during energy restriction⁽ Reference Ghayour-Mobarhan, Sahebkar and Vakili ^{86 )}. In this 12-week study, 120 overweight and obese children on an energy-restricted regimen (500 kcal/d (2090 kJ/d) below requirement) were assigned to two, three or four servings of dairy products/d. There were significant reductions in BMI, body weight and body fat, yet these reductions were due to the energy deficit and independent of the effect of increasing dairy product consumption.

Discussion of intervention studies

Examining Table 2, many aspects of the study methodology used in the included studies differed considerably, notably the length of the intervention (ranging from 3 months to 2 years) and the number of participants (for example, twenty-eight to 757), with half of the studies using dairy products and the other half milk consumption as part of their intervention. Data from the majority of the studies reviewed show that there is a neutral effect of dairy product (n 10) and milk (n 9) consumption on body weight and body composition in children and adolescents. More specifically, all the studies that examined the impact of a dairy food-rich diet relative to habitual diet on indicators of adiposity showed neutral effect and three studies⁽ Reference Du, Zhu and Trube ^{75 ,} Reference Arnberg, Mølgaard and Michaelsen ^{78 ,} Reference Larnkjaer, Arnberg and Michaelsen ^{79 )} that used milk consumption as the intervention showed an increase in body weight. However, as pointed out in the description of those studies, the increase in body weight was accompanied by an increase in lean mass⁽ Reference Arnberg, Mølgaard and Michaelsen ^{78 ,} Reference Larnkjaer, Arnberg and Michaelsen ^{79 )} or overall growth⁽ Reference Du, Zhu and Trube ^{75 )}. Specifically, in the study by Du et al. ⁽ Reference Du, Zhu and Trube ^{75 )}, as the largest school-milk intervention trial, the 757 included girls were healthy but on a persistently low-Ca diet and had a high prevalence of vitamin D deficiency⁽ Reference Du, Greenfield and Fraser ^{87 )}, thus representing a population that is not comparable with most of the intervention studies examined.

It is also noteworthy that there are no studies with body composition or any other indicators of obesity that have included intervention with both regular or reduced-fat milk or other type of dairy products, thereby not allowing a direct comparison. The processing of dairy foods often involves removal of whey protein from cheese, or addition of ingredients such as sugar in flavoured milk, yoghurt and ice-creams⁽ Reference Fellows and Hampton ^{88 )}. It can also lead to a number of alterations or biochemical changes in milk constituents, macronutrients and bioactive factors through such as insulin-like growth factor-1 (IGF-1)⁽ Reference Kang, Kim and Imm ^{89 )}. Thus, using total dairy product consumption might not be a suitable variable to elucidate the underlying impact of milk and other type of dairy products on adiposity indicators. Among the randomised controlled trials reviewed that included total dairy products, most included milk, cheese and yoghurt with a few studies including additionally dairy desserts⁽ Reference Merrilees, Smart and Gilchrist ^{69 )} or any curd products⁽ Reference Kelishadi, Zemel and Hashemipour ^{83 )}. However, there were no reports in the percentage of milk or other dairy products contribution to the total dairy product consumption, which does not allow examination of any differential effects. Thus, disaggregation of dairy products, such as reduced-fat compared with full-fat milk, fermented products such as yoghurt and cheese and in general those that are encouraged by the dietary guidelines as opposed to those that should be consumed sporadically (i.e. ice-cream, milkshakes), is essential in the analysis of children and adolescent obesity.

Dairy food serving sizes were adequately defined in most of the intervention studies with feasible quantification of milk and total dairy product consumption with the observed outcomes. However, variation in the definition of serving size/portions and high, low or habitual milk or dairy product consumption among the studies is apparent, which makes it very difficult, if not inappropriate, to compare results across studies conducted in different location and populations. For instance, while in Finland a habitual diet contains approximately 900 mg Ca/d⁽ Reference Cheng, Lyytikäinen and Kröger ^{71 )}, in the USA population this is reported to contain between 600 and 700 mg Ca/d⁽ Reference Vogel, Martin and McCabe ^{74 ,} Reference Lappe, McMahon and Laughlin ^{81 ,} Reference Weaver, Campbell and Teegarden ^{90 )}. There were also variations in the way the outcome variable is reported within studies, with most studies reporting body weight, BMI, lean body mass, percentage body fat while very few reported BMI z score, waist circumference or waist:height ratio, which have been suggested as more appropriate for children and adolescents and important early markers for the development of chronic diseases⁽ Reference Maffeis, Pietrobelli and Grezzani ^{91 ,} Reference Savva, Tornaritis and Savva ^{92 )}.

The majority of the included studies were predominately conducted in girls (n 7) and in school-aged children and adolescents. There were no differences in the outcome variables related to sex comparing the studies that included only girls and those that included both sexes (n 12). However, sex was adjusted in seven out of twelve studies and rate of growth (pubertal or Tanner stage) adjusted in only four⁽ Reference Lau, Lynn and Chan ^{72 ,} Reference Vogel, Martin and McCabe ^{74 ,} Reference Arnberg, Mølgaard and Michaelsen ^{78 ,} Reference Larnkjaer, Arnberg and Michaelsen ^{79 )} out of eighteen studies conducted in school-aged children and adolescents (age 8–18 years).

It is important to recognise that many of the intervention studies (thirteen out of twenty) included had a non-intervention control group, reported as usual or habitual diet often low in milk or dairy products, as the comparator to increased milk and total dairy food intervention group. Although this information allows separating the effects of treatment group on measures of adiposity, it does not provide insight on whether the effects observed are attributable to the increased milk or dairy product consumption per se, or to the concomitant manipulation of other food groups. Only one study utilised an isoenergetic replacement food such as fruit juice⁽ Reference Volek, Gómez and Scheett ^{70 )} as a comparator to milk and another study increased the consumption of milk by decreasing the consumption of sugar-sweetened beverages⁽ Reference Albala, Ebbeling and Cifuentes ^{80 )}, therefore separating the effect of milk. Both of those studies showed a neutral effect of increased milk consumption compared with fruit juice or usual diet on lean and fat mass⁽ Reference Volek, Gómez and Scheett ^{70 ,} Reference Albala, Ebbeling and Cifuentes ^{80 )}. Future studies should try to include both positive (i.e. isoenergetic food or beverages) and negative comparators (i.e. water or no milk group) to increase the ability to draw robust conclusions with regard to the treatment effect.

Although intervention studies are highly diverse considering the duration, location or sample size (Table 2), there are no clear patterns or trends identified between the interventions of milk or other dairy products that influence body weight and indicators of adiposity positively, neutrally or negatively. Additionally, half of the intervention studies were funded by the dairy or food industry and results showed that the source of funding did not affect the results.

Taken together, data from the majority of the intervention studies reviewed show that there is a neutral effect of dairy product and mainly milk consumption compared with the habitual diet on body weight and body composition in children and adolescents. Inclusion of milk and other dairy products as part of an energy-restricted diet did not enhance, but neither did it adversely affect, BMI or weight loss. However, given the limited data available from studies conducted under conditions of energy restriction, no conclusions can be drawn.

Evidence from intervention studies of the effect of dairy product intake on appetite and energy intake regulation

Given that obesity is characterised by an imbalance between energy intake and expenditure, understanding the factors or dietary components that could regulate appetite and energy intake has been a substantial focus in recent years. Although several studies have examined the effect of milk and other dairy product consumption as whole foods on appetite and subsequent meal energy intake in adults⁽ Reference Dove, Hodgson and Puddey ^{153 –} Reference Onvani, Haghighatdoost and Surkan ^{156 )}, there is a limited number of studies conducted in children⁽ Reference Brindal, Baird and Slater ^{157 –} Reference Green, Stevenson and Rumbold ^{161 )}. Brindal et al. ⁽ Reference Brindal, Baird and Slater ^{157 )} showed that there was no difference in subsequent appetite or energy intake at the ad libitum lunch 3 h after the consumption of a full milk beverage compared with isoenergetic half milk/glucose and glucose beverages in 10- to 12-year-old children. In contrast, Mehrabani et al. ⁽ Reference Mehrabani, Safavi and Asemi ^{158 )} recently showed that 10- to 12-year-old obese boys who consumed a fixed content breakfast with low-fat milk for two consecutive days had lower subjective appetite ratings and subsequent energy intake at lunch 5 h after, compared with apple juice and water (1010 kcal (4226 kJ), 1059 kcal (4431 kJ) and 1236 kcal (5171 kJ), respectively; P<0·001). Furthermore, Green et al. ⁽ Reference Green, Stevenson and Rumbold ^{161 )} showed that acute mid-morning milk consumption as a snack reduced ad libitum energy intake compared with an isoenergetic and isovolumetric serving of fruit juice in adolescent boys. Differences in subjects’ characteristics (obese boys v. normal-weight boys and girls) and time interval between breakfast or snack and the ad libitum lunch could explain the discrepancy in results between the studies. Furthermore, Mehrabani et al. ⁽ Reference Mehrabani, Safavi and Asemi ^{158 )} provided one serving of fruit between the breakfast and lunch which could have assisted in prolonging the feelings of satisfaction up to 5 h, whilst Brindal et al. ⁽ Reference Brindal, Baird and Slater ^{157 )} used an interval of 3 h where appetite ratings have returned to baseline values. Despite the modest decrease in energy intake by 50 kcal (209 kJ) between milk and apple juice, the authors suggested it could help with energy control and maintaining a healthy weight in the longer term.

Free-living setting outcomes

The majority of the studies have investigated appetite and energy intake as the primary endpoints within a laboratory setting but some of the studies, which included assessment of energy intake as part of body weight measurements, could shed some light on the effect of dairy foods on energy intake in a free-living environment. For instance, Lappe et al. ⁽ Reference Lappe, Rafferty and Davies ^{76 )} showed that girls consuming a Ca-rich diet were not ingesting more energy relative to those on their habitual diet and Albala et al. ⁽ Reference Albala, Ebbeling and Cifuentes ^{80 )} reported decreased energy intake by the concurrent replacement of sugar-sweetened beverages with milk consumption compared with control (–91 v. 9·7 kcal/d (–381 v. 41 kJ/d); P=0·03). Similarly, Andersen et al. ⁽ Reference Andersen, Arnberg and Trolle ^{162 )} recently revisited the MoMS intervention study, investigating the effects of increased water or milk-based drinks on total energy intake and dietary patterns in overweight adolescents. Although water intake reduced (–236 kcal/d (–987 kJ/d) while milk-based drinks did not alter total energy intake during the 12-week intervention compared with the background diet (baseline), both water and milk-based drinks had a favourable effect on the diet by decreasing the consumption of convenience foods, including sugar-sweetened beverages. This is particularly important given that there were no other restrictions in the diet and the milk-based drinks were added as extra intake, implying a compensatory effect of those on energy balance and reflecting positive dietary changes in usual life and free-living behaviour. Finally Green et al. ⁽ Reference Green, Stevenson and Rumbold ^{161 )} recently showed no statistically significant differences between chronic (28 d) milk and fruit juice consumption as a mid-morning snack on change in total daily energy intake. However, with the group consuming milk as a mid-morning snack, total daily energy intake declined over the 28 d (P=0·013); no such change was observed in the group consuming fruit juice.

Effect of type of milk or dairy product on satiety and energy intake

Considering the impact of the type of dairy foods with regard to the fat content on satiation and energy intake, Kling et al. ⁽ Reference Kling, Roe and Sanchez ^{159 )} recently showed that there was a compensatory effect of full-fat relative to low-fat milk consumption when served with lunch on preschoolers’ food intake, although there were no differences in total meal energy intake (lunch + milk). However, there was a sex effect; lunch intake was reduced by 43 (sem 8) kcal (180 (sem 33) kJ) (P=0·001) after full-fat milk without altering total meal energy intake for boys, whereas in girls there was no reduction in lunch intake, which increased total meal energy intake by 24 (sem 10) kcal (100 (sem 42) kJ) (P=0·03). However, this was an acute study and it remains unknown whether energy compensation persists over the whole day or in intakes over the long term. There is one study which examined the effect of replacing regular-fat with reduced- or low-fat dairy products on dietary intakes and health outcomes⁽ Reference Hendrie and Golley ^{163 )}. In this 24-week cluster randomised controlled trial, parents and their 4- to 13-year-old children were randomly assigned to behavioural nutrition education to change to reduced-fat dairy products (intervention group) or reduce screen time (control group). The results indicated that there was a 74 % decrease in regular-fat dairy product intake in the intervention group, which led to a reduction of 3·3 percentage points in saturated fat compared with the control group at week 24. However, there was no difference in total energy intake (–195 (95 % CI –772, 383) kJ; P=0·504) or BMI z score and waist circumference between the two groups. The authors in a secondary analysis of this trial tried to address whether there was compensation or energy adjustment by incorporating other foods in the diet when lower-energy-dense dairy products were introduced⁽ Reference Golley and Hendrie ^{164 )}. The results showed that replacing regular- with reduced-fat dairy foods did not adversely have an impact on children’s overall food intake by revealing no differences in food group contribution to energy intake between the intervention and the control group at week 24. The foods that contained excess fat, added sugar and/or salt had the largest contribution to children’s total energy intake (27 %) and saturated fat intake (33 %), highlighting that reduction of energy-dense foods is necessary to achieve better control of saturated fat and total energy intake.

In general, inclusion of any dairy products into weight-maintenance diets did not lead to a higher overall energy intake, implying a partial compensatory effect of milk on energy balance. Further, adequately designed studies are needed to examine if habitual consumption of milk and other dairy products make an impact on appetite regulation and consequently energy intake in children and adolescents.