The influence of dairy on bone mass accrual, bone size, and fat and lean body mass in early pubertal girls and boys: A randomized controlled trial

In adults, overweight is associated with increased bone mineral density (BMD) and reduced risk of osteoporosis. However, overweight children experience fractures more often than healthy weight children. Given adequate calcium (Ca) intake, children with a higher body mass index (BMI) have been shown to have greater Ca retention than children with a lower BMI. Dairy foods are an excellent source of Ca and a good source of other essential nutrients needed to increase bone mineral content (BMC) and decrease fracture risk in children. The Dietary Guidelines for Americans recommends three cups of low fat milk or equivalent daily for these reasons. We hypothesized that children who were overweight, as compared to those who were healthy weight, would accrue more bone mass and grow larger bones making them less vulnerable to fracture if provided with three servings of dairy daily. Additionally, we hypothesized that the additional dairy would not be associated with an increase in body fat or weight while providing a positive effect on bone. We recruited 240 low dairy consuming healthy boys and girls age 8 to 15.9 y (11.8 ± 1.5 y mean ± SD) between years 2008 and 2011. Half were healthy weight (5th - 70th BMI percentile for age (%ile) and half were overweight (≥ 85th %ile). Participants were randomized within BMI category to an 18-month dairy intervention. Subjects receiving the intervention were provided with three servings of dairy daily, while those not receiving the intervention (control) consumed their habitual diet. Main outcome measures were assessed 4 times, or every 6 months, and included total body bone mineral content and density (TBBMC, TBBMD), and bone area with dual energy x-ray absorptiometry (DXA), cortical and trabecular BMD, BMC, and area at the 4% tibia with peripheral quantitative computed tomography (pQCT), anthropometric measurements, and factors related to bone turnover and skeletal growth with fasting blood and urine samples. 176 subjects completed the trial. Ca intake at baseline was not different between groups and averaged about 600 mg/d. Throughout the trial, Ca intake among the intervention groups increased to about 1500 mg/d, while the control groups increased to about 1000 mg/d. No significant differences in the change of BMD, BMC, bone area, body composition, or serum and urine factors associated with bone growth were observed between those receiving the dairy intervention and those who did not. Among overweight subjects, those receiving the intervention had a greater increase in waist circumference than those not receiving the intervention (6.6 v 2.9 cm, p < 0.05; repeated measures mixed model analysis). Increased dairy food consumption did not enhance bone mineral acquisition or body composition either within or across weight groups. This study was an effectiveness study that failed to result in improved bone gain, possibly due to calcium intake reaching adequate levels in the comparison group, too noisy of a background diet, and/or using a cohort that varied too greatly in sexual maturity so that growth swamped any diet effects. To test our hypothesis without the limitations of the current study, an efficacy study is needed using a controlled diet with a metabolic balance study in a more homogenous population, although the outcome measures associated with a metabolic balance study are not as strong as the bone gain measures of the study reported here. Our study suggests that two cups of milk or the equivalent for calcium daily is adequate for bone gain in early adolescence.