Dairy products such as milk, cheese and yoghurt are an important source of essential micronutrients including calcium, riboflavin, phosphorus, potassium, magnesium, zinc, vitamin A and vitamin B₁₂ [1]. They also provide a combination of protein, carbohydrate, and fat. Nutrients from dairy products are well known for their role in building and maintaining strong bones [2], although meta-analyses of different populations show varied results in the area of dairy and bone health. Increased dietary calcium and dairy products have been shown to significantly improve total bone mineral content in children with initially low intakes [3], and positive benefits of calcium on bone mineralization in children and adolescents have been found in nine out of ten randomized controlled trials in a meta-analysis [4]. Despite this, the latter analysis found no consistent benefit when all types of studies were considered [4], and an analysis examining milk and hip fracture risk in adults was also inconclusive [5]. However dairy nutrients are also reported to be important for immune and nervous systems, red blood cell production, eyesight, muscle and nerve function, skin maintenance and wound healing [6]. In terms of chronic disease, higher dairy intakes are also associated with a reduction in risk for all cause deaths, cardiovascular disease and diabetes [7]. Hence, dairy products are recommended as part of a healthy diet in childhood, adolescence [8] and adulthood [9].

Adequate intake of nutrients found in dairy products, such as calcium, is particularly important in adolescence as this is a critical time for growth and accumulation of bone mass [10,11], with 40% of total bone mass development occurring during this period [12]. Bone turnover is high and bone accrual lags due to increases in height and weight, with peak bone mass usually attained by 20 years of age [13,14]. This increase in nutrient requirements for growth coincides with increasing autonomy of the adolescent to make their own food and drink choices [15], and the establishment of eating patterns that track into adulthood [16]. Increasing independence, peer acceptance, more time spent at school or other activities, and self image contribute to food choices made at this time, setting the scene for long term health outcomes [17,18].

Despite the importance of dairy nutrients, the 2007 Australian National Children’s Nutrition and Physical Activity Survey indicated the majority of Australian adolescents do not consume the recommended intakes of some of these nutrients, in particular calcium [19]. Dieting is one factor that has been strongly associated with low consumption of dairy foods [20]. Girls appear to be at a higher risk than boys of not meeting their dietary calcium requirements, and consumption of dairy foods is often reported as low in adolescent groups [19,21,22,23]. Analysis of dairy products from the Australian National Children’s Nutrition and Physical Activity Survey indicated that milk products and dishes contributed between 32% and 60% of dietary calcium across childhood and adolescence, highlighting their importance in the diet [24]. To our knowledge, there is a lack of Australian data following the same individuals through adolescence evaluating how nutrient intakes and dairy product consumption change over time.

This study tracks the dietary intakes of dairy products and associated nutrients of adolescents participating in the Western Australian Pregnancy Cohort (Raine) Study. The same subjects were followed from 14 to 17 years of age. We hypothesize that intake of dairy products and associated nutrients decreases from 14 to 17 years, and that greater decreases are associated with female gender, lower socio-economic status and higher weight category.

Results of our study indicate that on average, adolescents decrease their dairy intake from 14 to 17 years. As a consequence, our findings suggest that nutrient intakes are less than optimal during the important adolescent growth period. Based on an average serve of dairy contributing 300 mg of calcium, from our study results we estimate that dairy products contribute 65% of calcium intake for girls and 68% for boys at 14 years, decreasing slightly to 61% for girls and 65% for boys at 17 years. The majority of our adolescents did not meet the RDIs for calcium and magnesium at 14 years, and this situation worsened as they became older. Suboptimal calcium intake has been previously identified in Australian adolescents [21,37], with girls showing lower intakes than boys [21]. Our results are also similar to a longitudinal study conducted in the United States, which showed that teenagers reduced their calcium intake from the ages of 16 to 21, particularly females [38].

These findings are of concern for both genders, as adequate calcium intake during adolescence is important for bone growth, development and for reducing the risk of bone fractures during adolescence, as well as laying the foundation for bone health later in life [10,39,40,41]. Females in particular are at risk of osteoporosis and hip fracture, due to the loss of the bone protective factor oestrogen during menopause and older age [1,39]. In addition to calcium being of concern, magnesium is also an important nutrient as a cofactor for over 300 enzymes involved in metabolism of food and synthesis of metabolic products [42]. Associations have been found between magnesium deficiency and insulin resistance in obese children [43], and there are links with osteoporosis [11,44] and diabetes [42,45]. The link with insulin resistance and diabetes is thought to be related to the use of magnesium in carbohydrate metabolism [42]. Magnesium is thought to contribute to bone health through affects on parathyroid hormone secretion, with low intakes of magnesium leading to lower parathyroid hormone and a decrease in serum 1,25 dihydroxy vitamin D3, resulting in altered hydroxyapitite crystal formation and impaired bone health [42]. In our cohort, mean intakes for dairy products fell short of the recommended three serves per day [33] for girls at both age groups, with boys closer to but still less than three. Our figures of between 14% and 22% of girls and 37% and 40% of boys meeting recommendations supports previously reported figures of 17% for girls and 38% of boys in the Australian adolescent population overall [46]. Depending upon an individual’s activity level and energy requirements, some adolescents, particularly boys, may benefit from up to five serves of dairy per day [33].

We observed in our study that lower milk consumption was responsible for the largest decrease as the adolescents grew older. Total dairy intake, and that of most dairy products, decreased from 14 to 17 years for both boys and girls. The exceptions were cheese and butter, which increased significantly over this period.

To compare our results with other Australian research, the 1995 National Nutrition Survey reported cheese intake along with butter/dairy fat intake was higher in 16–18 years old compared with 12–15 years old [47], although the total dairy category of milk products and dishes was also shown to be higher in the older age group. The National Nutrition Survey is an older study and therefore it is possible there has been some change in habits and eating patterns since the time of data collection. The more recent Child and Adolescent Physical Activity and Nutrition Survey of Western Australia surveyed primary and secondary school students in 2003 and 2008, and found the proportion of students consuming milk products and dishes decreased from primary to secondary school, and from 2003 to 2008, significantly so for girls [21]. These results are similar to our findings, although this survey used a different group of adolescents at each time point (2003 and 2008), whereas our study followed the same adolescents. The 2007 Australian National Children’s Nutrition and Physical Activity Survey also showed gender differences. Total milk intake was lower in girls in the 14–16 year age group compared to the 9–13 age group, but the opposite was reported for boys [24]. This survey also showed mean intake of flavoured milk was higher in older boys, similar to the trend we observed in our adolescents. This trend may be due in part to marketing campaigns targeting the male audience, and the increasing overall popularity of flavoured milk (usually fat reduced), particularly iced coffee [48]. We found in the Raine Study that boys from the low SES areas were the only group to report a mean increase in dairy intake. We postulate that boys from lower SES backgrounds may be more likely to drop out of school and enter apprenticeships as blue collar workers [49], and tradesmen may have more of a culture of consumption of flavored milks like iced coffee. An Australian product development company refers to market research describing the primary consumers of iced coffee as stalwart tradesman who would consume several cartons daily, often referring to it as “daytime beer” [50].

In terms of predicting change in dairy intake from 14 to 17 years, our results showed that weight category was a significant predictor. Adolescents who were overweight at 14 years were more likely to have decreased their dairy intake at 17 years, by an average of 190 g/day. Decreasing dairy intake may be a result of trying to lose weight, particularly in girls [20,51]. The category of underweight boys was the only weight category to increase consumption from 14 years, and this may reflect trying to build body mass or a lack of concern over weight gain related to dairy intake, whether this stigma is justified or not. Recent evidence suggests that it is not justified, with a review of prospective cohort studies suggesting that the consumption of dairy foods may actually reduce the risk of overweight, rather than increase [52]. The review found that three out of ten studies in children and adolescents showed protection against weight gain, with one suggesting an increased risk and six studies showing no association. In adults, a meta-analysis of randomized controlled trials found an overall negative association between weight gain and dairy intake which bordered significance, with the combination of increased dairy with energy restriction significantly associated with a decrease in weight [53].

Our study is unique as it follows the same large population based sample of adolescents from 14 years to 17 years to allow matched pairing of subjects for analysis of changes in dairy consumption over time. This study also provides valuable information for further studies of calcium intake and bone status in the same cohort. The FFQ method for analysing dietary intake has the advantage of being cost effective given the large subject group size and with a relatively low subject burden. A potential limitation of the CSIRO FFQ was that quantity information was generated from frequency data in relation to standard serve sizes. This required the respondent to consider their usual serve size in comparison to the standard given and then adjust the reported frequency accordingly, which may increase the risk of estimation error. Although the accuracy of FFQs can be of concern [54], the FFQ used has been validated in our Raine Study group with a three day food record [55]. The power of analyses investigating intakes by BMI and SES categories were limited by small numbers in some groups. Although Raine Study adolescents participating in dietary studies tend to have older mothers, higher family income or a lower BMI than the non-respondents [56], families who were involved in the study were more likely to be of middle to lower socioeconomic status initially [57], which may improve the generalisation of our results.

Our findings identify a discernible gap between reported consumption and recommended nutrient and dairy product intake over the adolescent period, particularly for girls. Possible barriers for adolescents consuming dairy products may include lactose intolerance, changes in eating patterns with a changing lifestyle (for example skipping breakfast), and being conscious of their weight and body image. Increasing intake of dairy products will help improve intakes of a range of important nutrients, particularly calcium and magnesium that were especially lacking compared to recommended intakes in our subject group. A recent review identified taste exposure and prompting practice as important strategies utilised in effective interventions to increase calcium or dairy intake, along with concentrated delivery across a variety of settings [58].

Our results suggest that both girls and boys are reducing their intake of dairy foods at a time when calcium and other nutrient requirements are increased. Furthermore, the majority of adolescents are not meeting their calcium requirements at early adolescence, and this continues into later adolescence.Adolescence is considered an important time for physiological growth and prevention of future chronic disease such as osteoporosis [11,44] and diabetes [45]. Based on our findings, public health messages for this age group may benefit from promotion of a calcium rich food at every meal, such as dairy products. Non-dairy foods that are good sources of calcium include sardines and salmon (with bones), broccoli, nuts and seeds, and products fortified with calcium such as breakfast cereals, and soy beverages. It may be more beneficial to obtain calcium from food sources than supplements, due to suggestions that high dosage supplements may have health consequences such as kidney stones and artery calcification which could lead to heart disease [59]. Future research investigating reasons behind the decline we have observed in our population would help further inform public health campaigns addressed at improving healthy eating during adolescence.