Dairy products such as milk, yoghurt and cheese are nutritious sources of protein, peptides and other nutrients including calcium, vitamin D and potassium. Unfortunately consumption of dairy products may be discouraged by concern about the risk of obesity and cardiovascular disease (CVD). In Australia milk products and dishes are the major food sources of saturated fat, accounting for ~27% of total intake [1
]. Given the link between saturated fat (SFA) and CVD [2
], this may be a reason which may reflect the relatively low consumption rates of dairy products in Australia [1
]. Despite the fact that dairy foods have previously been reported to increase risk of CVD, coronary heart disease (CHD) and stroke in prospective cohort studies [2
], several observational and cross-sectional studies have revealed an inverse association between dairy product consumption and CVD [5
] and body composition, weight loss and weight gain [6
Recently Kratz and colleagues [11
] conducted a systematic literature review of observational studies investigating associations between dairy fat and cardiometabolic health. Interestingly the authors showed that 11 out of 16 studies reported inverse associations between high fat dairy intake and measures of adiposity. Similarly, a recent systematic review and meta-analysis of randomised controlled trials [12
] reported increased dairy product intake was associated with greater reductions in fat mass and WC and a greater gain in lean mass than in controls. In fact, increased dairy product consumption intake resulted in 0.72 kg (95% CI: −1.29, −0.14, p
= 0.01) greater reduction in fat mass, 2.19 cm (95% CI: −3.42, −0.96, p
-value < 0.001) further reduction in WC and 0.58 kg (95% CI: 0.18, 0.99, p <
0.01) gain in lean mass compared with controls. The authors also stated that increasing dairy product intake without energy restriction did not affect body composition but when dairy product consumption was increased as part of an energy restricted diet designed for weight loss, high dairy food consumption resulted in greater weight loss, reduction in body fat mass, WC and greater increase in lean mass compared with controls. Similarly a prospective investigation of 120,887 men and women in the Nurses Healthy Study I and II and the Health Professionals Follow-up Study showed that yoghurt consumption was inversely associated with 4-year weight change. Additionally age-adjusted linear regression identified that whole fat dairy foods were associated with 4-year weight gain whereas low fat dairy foods were associated with 4-year weight loss. Interestingly in another study [13
], higher calcium intake was associated with a lower 5-year increase of the BMI and waist circumference in men but not women. Furthermore, in a 5-year period in men only, a higher consumption of dairy foods was associated with a better metabolic profile. The mechanism by which dairy food consumption may improve body composition is not entirely clear however, it has been postulated the benefit may be in part due to calcium which is thought to reduce lipogenesis and increase lipolysis [14
]. Other studies have reported a satiating effect of dairy protein consumption [15
], while other research suggests conjugated linoleic acid, naturally produced in dairy foods, improves weight through increased fat utilization [17
], increased satiety and caloric intake [18
], however at this time the evidence is mixed in support of these hypotheses [20
]. The purpose of this retrospective study was to explore relationships between dairy product consumption and macro/micronutrients from dairy food (namely protein and calcium) and markers of adiposity within an opportunistic population of overweight or obese adults in Australia. No other published studies have explored relationships between intakes of specific dairy foods and direct measures of body composition using dual energy X-ray absorptiometry (DEXA) within an overweight/obese population. Based on current literature, we hypothesise that dairy foods will be inversely associated with markers of adiposity.
2. Materials and Methods
This study was a cross-sectional analysis of overweight/obese adults. Baseline measurements of body composition of volunteers (n =
762) who were recruited in regional and metropolitan South Australia for 11 separate dietary intervention trials between 2004 and 2007 at the Nutritional Physiology Research Centre and CSIRO Human Nutrition were compiled into one database. Information about background information on volunteers, volunteer characteristic, inclusion criteria, data collection methods has been published elsewhere [23
]. Selection criteria for these studies were that they provided baseline dietary intake data using consistent methodology as well as body composition. These studies had been approved by the Human Research Ethics Committee at the University of South Australia or CSIRO Human Experimentation Ethics Committee. All volunteers gave written informed consent prior to commencing the trials.
2.2. Assessment of Dietary Intake
Dietary intake including total energy from macro and micronutrients was estimated using a 74-item food frequency questionnaire (FFQ) [33
] which requests information relating to food choices, frequency, portion size, quantity and consumption rate of different food and beverage items. Participants who were suspected for underestimation or overestimation of daily energy intake (<4000 kJ or >17,000 kJ) were excluded [34
]. Detailed information on type and amount and cheese (hard, firm, soft, cottage, ricotta, low-fat), ice-cream, yogurt and reduced fat and full fat milk, including flavoured milk, was collected. The FFQ did not include cream consumption. The FFQ has been validated for use in human dietary intervention trials [35
]. The 74 item FFQ was validated against 3 day weighed food records to collect dietary information over one month in n =
118 men and women aged between 31 and 74. Mean energy and nutrient intakes were within ±20% difference and classified more than two thirds of the volunteers within ±1 quintile difference for all nutrients [35
Raw intake scores (total amount in g/day) were provided for each dairy food. The nutrient composition for each product was extracted from the Foodworks Professional nutritional program (Xyris, Qld, Australia) and the energy, macronutrient and micronutrient intake provided from each individual dairy product as a proportion of total daily intake were subsequently determined. Total daily milk intake from all sources was calculated and categorised into full fat or reduced fat. Total dairy product consumption was calculated by summing intakes of all dairy products.
2.3. Anthropometry and Body Composition Assessments
Body composition assessments have been described for each study elsewhere [23
]. Briefly, body height was measured to the nearest 0.1 cm with a stadiometer while the participants were barefoot. Body weight was measured to the nearest 0.05 kg with calibrated electronic digital scales while the participants were wearing light clothing and no footwear. Body composition including % body fat and abdominal fat was assessed by using dual-energy X-ray absorptiometry (DXA) (Lunar Prodigy; General Electric, Madison, WI, USA). Body mass index (BMI) was calculated as weight (in kg) divided by height2
(in m). Waist and hip circumference were measured according to the International Standards for Anthropometric Assessment to calculate waist/hip ratio (WHR) [36
]. Waist to hip ratio was calculated by dividing waist circumference (cm) with hip circumference (cm).
2.4. Statistical Analysis
SPSS software (version 17.0; SPSS Inc., Chicago, IL, USA) was used for all statistical analyses. Data were analysed to determine normality of dependent variables by assessing the residual plots of the linear regression analysis. If residual plots were normally distributed then no transformations were performed. If residual plots were not normally distributed they were log transformed and checked for normality. Linear regression was used to explore relationships between total energy intake, and macronutrient intake as well as total dietary calcium and markers of adiposity, with statistical control for age, gender and total energy intake.
Relationships between energy, protein, fat, saturated fat, carbohydrate and calcium (all in g/day) from dairy products with all adiposity measures were also analysed using linear regression. Two models were used: (1) Basic: adjusted for age, gender and total energy intake; and (2) Full: adjusted for Basic covariates + the total dietary intake of each specific macronutrient. For example, when assessing the relationship between dairy calcium and each adiposity measure, total dietary calcium was statistically controlled for.
Absolute intakes of each individual dairy product (full fat and reduced fat milk, total milk, cheese, dairy spreads, yoghurt, and ice cream), as well as total dairy intake, were analysed using the same statistical procedure. Similarly, two models of regression analysis were performed: (1) Basic: adjusted for age, gender, and total energy intake; and (2) Full: adjusted for Basic covariates + other dairy products. For example, when assessing yoghurt intakes, intakes of milk, cheese, ice-cream and dairy spreads were controlled for. The variance inflation factor (VIF) was examined to assess for multicollinearity between variables. As the VIF was <10 for all fully adjusted models, there was no evidence of multicollinearity and subsequently no variables were removed from the analyses.
Additional multivariate analyses were conducted based on the findings from the main analyses, to further explore the relationships between intakes of dairy protein, dairy calcium, and dairy food intake. p < 0.05 was considered significant unless otherwise specified.
In this cross-sectional study of overweight/obese Australian adults, dairy protein, dairy calcium, total dairy food intake, and a number of individual dairy foods (namely yoghurt and reduced fat milk) were inversely associated with a number of adiposity measures. Relationships between dairy protein and % body fat, abdominal fat, and WC remained with full adjustment for total energy intake and total dairy food intake. Yoghurt intake and total dairy food intake remained inversely associated with % body fat (both p < 0.05), with addition of dairy protein and dairy calcium to the model. These findings are consistent with our hypothesis that higher intakes of dairy foods, as well as dairy protein and calcium would be associated with lower levels of adiposity.
Our study sample were consuming more dairy products than in Australia’s National Nutrition Survey [1
], in which men and women aged 19 years and over consumed 322 g and 258 g of milk products and other dairy foods per day, respectively. Interestingly, the average consumption in our population was still less than the recommended dietary intake (RDI) for dairy products (2–3 servings per day; serving sizes milk 250 mL, yoghurt 200 g, cheese 40 g, custard 250 mL) in Australia [37
The novel aspect of our study is that we explored relationships between intakes of specific dairy food components and direct markers of adiposity as measured by DEXA. Other studies have used indirect markers of adiposity such as BMI and have not necessarily focused on consumption of dairy foods but on nutrients such as calcium. Data from the present study is in agreement with a number of other observational and dietary intervention studies which have shown beneficial effects of dairy product consumption on body composition or inverse associations with dairy product consumption and body composition [8
]. For example, in the Coronary Artery Risk Development in Young Adults (CARDIA) study a higher dairy food intake (consumed dairy foods ≥35 times per week) was associated with lower levels of obesity (BMI) in individuals who were overweight at baseline compared with lower daily intakes of dairy products. This relationship did not exist in individuals who had a BMI of <25 kg/m2
at baseline [38
]. In the present study we showed dairy protein, dairy calcium, and reduced fat milk were associated with lower BMI. Moreover, in a weight loss/weight maintenance study by Champagne et al
], increasing low fat dairy food intake was associated with significant weight loss during the weight maintenance phase (−0.17 kg per 6 months per 1-serving increase). However, Chen and colleagues [40
] conducted a meta-analysis of the effects of dairy food intake on body weight and body fat in 29 studies with n =
2101 participants and showed that dairy food consumption did not lead to a significant reduction in weight. However, where diets were energy restricted, or in studies of less than 1 year duration, dairy food consumption reduced body weight. Studies that were of ad libitum
design or of greater than 1 year duration did not result in reduced body weight following dairy food consumption. Interestingly our data showed total dairy intake, yoghurt, and dairy protein were inversely associated with % body fat, and yoghurt and dairy protein were inversely associated with abdominal fat. Furthermore a study by Rajpathak et al
] in a 12 year follow up from the US Health Professional’s Study examining relationships between dairy food and calcium intake and weight status showed in men who increased their dairy food intake the most, compared with men who decreased their dairy food intake the most, a small increase in weight (3.1 compared with 2.6 kg; p for trend = 0.001).The authors state that the association was primarily due to an increase in high-fat dairy food intake as low-fat dairy food consumption was not significantly associated with weight change. Interestingly the authors found no association with calcium intake and weight change, which does not support current evidence that calcium from dairy food is thought to be the responsible component for the benefits with reduced adiposity however this is controversial.
In the present study we showed negative associations between calcium intake and measures of adiposity which further supports previous research showing benefits of calcium supplementation on body weight [41
]. Previous research has shown that calcium may increase faecal excretion of fatty acids, including saturated fat and bile acids, minimising effects on serum cholesterol and increasing energy loss which may impact on measures of adiposity [42
] and also influence energy partitioning through lipogenesis and lipolysis. In a study by Zemel and colleagues [44
] where participants were randomised to either a low dairy food diet (<1 serving dairy products/day) or a recommended dairy food diet (>3 servings dairy products/day) subsequent to weight loss, those on the latter diet exhibited evidence of greater fat oxidation and were able to consume more energy without greater weight gain than the low dairy group. Similarly, in another study by Zemel et al
] demonstrated a 9% loss of body weight following a high calcium diet and an 11% loss of body weight on a high dairy food diet following a 6 month energy restricted diet. Despite the energy restriction, there was a greater fat loss from the trunk region on the high dairy food calcium diet, supporting the role of intracellular calcium and energy partitioning. In contrast a study by Bowen et al
] showed no difference in the amount of weight lost in an energy restricted high dairy protein diet (high calcium 2400 mg/day) vs
. high meat protein (low calcium 500 mg/day) diet.
Besides calcium, dairy products contain a range of nutrients including proteins (whey and casein), branched chain amino acids and peptides. Data from our study showed an inverse relationship with dairy protein and all adiposity measures, with control for total dietary protein. These data support previous research which has indicated that dairy protein might be the component responsible for beneficial effects on body composition [16
]. These potential effects may be related to influence on adipocyte lipid metabolism or more specifically due to increased diet induced thermogenesis and subsequently greater energy expenditure and less fat storage. Another potential mechanism related to dairy consumption and improved body composition may be around the current evidence supporting dairy protein consumption, and the role of dairy in satiety relating to weight loss and weight gain prevention [48
]. Dairy foods are predominantly casein (80%) while whey makes up 20% which have different gastric emptying rates which subsequently impacts satiety. There in fact may be other effects of dairy protein on satiety hormones such as cholecystokinin and peptide YY and hunger stimulating hormone ghrelin however the evidence is less clear. These hormones were not measured in the present study. While growing evidence of the potential beneficial effects of dairy food consumption and obesity remains controversial, data from the current study suggest the consumption of dairy products and more specifically dairy protein and calcium derived from low fat dairy foods, such as milk and yoghurt, are beneficially associated with indices of body composition in an overweight/obese population. These findings support contemporary Australian public health dietary recommendations [37
]. This is the first study that has examined this question within such a sample as prior studies have examined a wider cross section inclusive of individuals of normal BMI. Nevertheless, given that the present study was cross-sectional, we cannot allude to cause/effect and indeed clinical interventions have shown variable results in terms of ad libitum
or energy restricted diets, duration and fat content of dairy foods. In presenting these data we recognise numerous limitations associated with not only cross-sectional studies, but the available published evidence relating to the quality of collected dietary information and choice of dietary tools, use of accurate biomarkers of dairy fat intake, nutrient composition of bovine milk, statistical adjustment of potential confounders and the fact that dairy foods are consumed as part of a diet and not in isolation. Given the complexity of studying nutrition, dairy food intake and obesity it is difficult to draw conclusions without conducting long term dietary intervention trials without energy restriction to investigate whether dairy foods exert beneficial effects on body composition. We also acknowledge the difficulty in drawing data together from different studies and two clinical trial centres and its representativeness of the general population.