In adults, dietary protein seems to induce weight loss and dairy proteins may be insulinotropic. However, the effect of milk proteins in adolescents is unclear. The objective was to test whether milk and milk proteins reduce body weight, waist circumference, homeostatic model assessment, plasma insulin, and insulin secretion estimated as the plasma C-peptide concentration in overweight adolescents. Overweight adolescents (n = 203) aged 12-15 y with a BMI of 25.4 ± 2.3 kg/m(2) (mean ± SD) were randomized to 1 L/d of skim milk, whey, casein, or water for 12 wk. All milk drinks contained 35 g protein/L. Before randomization, a subgroup of adolescents (n = 32) was studied for 12 wk before the intervention began as a pretest control group. The effects of the milk-based test drinks were compared with baseline (wk 0), the water group, and the pretest control group. Diet and physical activity were registered. Outcomes were BMI-for-age Z-scores (BAZs), waist circumference, plasma insulin, homeostatic model assessment, and plasma C-peptide. We found no change in BAZ in the pretest control and water groups, whereas it was greater at 12 wk in the skim milk, whey, and casein groups compared with baseline and with the water and pretest control groups. The plasma C-peptide concentration increased from baseline to wk 12 in the whey and casein groups and increments were greater than in the pretest control (P < 0.02). There were no significant changes in plasma C-peptide in the skim milk or water group. These data suggest that high intakes of skim milk, whey, and casein increase BAZs in overweight adolescents and that whey and casein increase insulin secretion. Whether the effect on body weight is primary or secondary to the increased insulin secretion remains to be elucidated.

Being overweight is associated with vascular abnormalities, which are important in the development of atherosclerosis. However, little is known about dietary and lifestyle determinants of vascular function in overweight children. In adults, dietary protein and milk intake are associated with reduced blood pressure and reduced risk of metabolic syndrome. This study examined the associations between dietary protein, milk intake, physical activity, and adiposity on arterial stiffness in overweight children. In a cross-sectional study, overweight children with habitual milk intakes ≤ 250 mL/d were examined by DXA scans, pedometer counts, anthropometry, and metabolic variables. Dietary intake was registered for 4 d. The outcomes were arterial stiffness measured by pulse wave velocity (PWV) (n = 182) and augmentation index (Aix) (n = 183). The PWV (mean ± SD) was 4.78 ± 0.72 m/s and the Aix was -0.77 ± 9.44%. In multivariate models, the android fat:gynoid fat and android fat:body fat ratios were positively associated with PWV (β = 1.49 and β = 10.3, both P < 0.05) and Aix (β = 28.3, P < 0.01 and β = 153, P < 0.05), whereas the gynoid fat:body fat ratio was negatively associated with the Aix (β = -134; P < 0.001). Protein intake (percentage energy) was positively associated with PWV (β = 0.05; P < 0.01). Milk intake (L/d) tended to be negatively associated with PWV (β = -0.64; P = 0.05). Pedometer counts were negatively associated with the Aix; however, the association became nonsignificant after controlling for HOMA, which was positively associated with the Aix (β = 0.95; P < 0.01). In conclusion, central adiposity and protein intake are associated with increased arterial stiffness measured as PWV in overweight children independent of blood pressure and heart rate. The effect of protein intake may be caused by meat, because the milk intake was low.

The plasma and urine metabolome of 192 overweight 12–15-year-old adolescents (BMI of 25.4 ± 2.3 kg/m2) were examined in order to elucidate gender, pubertal development measured as Tanner stage, physical activity measured as number of steps taken daily, and intra-/interindividual differences affecting the metabolome detected by proton NMR spectroscopy. Higher urinary excretion of citrate, creatinine, hippurate, and phenylacetylglutamine and higher plasma level of phosphatidylcholine and unsaturated lipid were found for girls compared with boys. The results suggest that gender differences in the metabolome are being commenced already in childhood. The relationship between Tanner stage and the metabolome showed that pubertal development stage was positively related to urinary creatinine excretion and negatively related to urinary citrate content. No relations between physical activity and the metabolome could be identified. The present study for the first time provides comprehensive information about associations between the metabolome and gender, pubertal development, and physical activity in overweight adolescents, which is an important subject group to approach in the prevention of obesity and life-style related diseases. While this study is preliminary, these results may have the potential to translate into clinical applicability upon further investigations; if biomarkers for Tanner stage can be established, these might be used for identification of individuals susceptible to an early pubertal development.

Consumption of cow's milk is recommended in many countries. Observational and intervention studies show that cow's milk most likely has a positive influence on growth in children. The strongest evidence comes from observational studies and intervention studies in low-income countries, but there are also observational studies from high-income countries showing positive associations between milk intake and growth. Milk seems thus to have a specific stimulating effect on linear growth, not only in developing countries with high rates of malnutrition, but also in industrialized countries. However, it is not known which components in milk stimulate growth. Possible components are proteins, minerals, vitamins or combinations of these. Cow's milk proteins have a high protein quality, and whey has a slightly higher quality than casein, according to some indices based on amino acid composition. Studies, mainly from sport medicine, have suggested that whey protein also has the potential to increase muscle mass. Whether whey improves body composition to a larger extent than other milk proteins is not clear. The mechanism behind a possible growth-stimulating effect of milk and milk components is likely to be through a stimulation of insulin-like growth factor-I synthesis and maybe insulin secretion. In conclusion, there is strong evidence that milk stimulates linear growth. The mechanism is not yet clear, and more intervention studies are needed to understand which components in milk are responsible for the growth stimulation. The effects of milk on linear growth and adult height may have both positive and negative long-term implications.

Vitamin D status is negatively associated with blood lipids, body mass index and waist circumference in infants where nearly all received vitamin D supplements. Whether this has long-term health effects remains to be elucidated.