Resistance Training and Milk-Substitution Enhance Body Composition and Bone Health in Adolescent Girls

Background: Increased soft-drink consumption has contributed to poor calcium intake with 90% of adolescent girls consuming less than the RDA for calcium.Purpose/objectives: The purpose of this investigation was to determine the independent and additive effects of two interventions (milk and resistance training) on nutrient adequacy, body composition, and bone health in adolescent girls.Methods: The experimental design consisted of four experimental groups of adolescent girls 14-17 years of age: (1) Milk + resistance training [MRT]; n = 15; (2) Resistance training only [RT]; n = 15; (3) Milk only [M] n = 20; (4) Control [C] n = 16. A few significant differences were observed at baseline between the groups for subject characteristics. Testing was performed pre and post-12 week training period for all groups. Milk was provided (3, 8 oz servings) for both the MRT and the M groups. The MRT group and the RT groups performed a supervised periodized resistance training program consisting of supervised one-hour exercise sessions 3 d/wk (M, W, F) for 12 wk. Baseline dietary data was collected utilizing the NUT-P-FFQ and/or a 120 item FFQ developed by the Fred Hutchinson Cancer Research Center (Seattle, Washington). Body composition was measured in the morning after an overnight fast using dual-energy X-ray absorptiometry (DXA) with a total body scanner (Prodigy^TM, Lunar Corporation, Madison, WI). A whole body scan for bone density and lumbar spine scans were performed on all subjects. Maximal strength of the upper and lower body was assessed via a one-repetition maximum (1-RM) squat and bench press exercise protocols. Significance was set at P ≤ 0.05.Results: Significant differences in nutrient intakes between groups generally reflected the nutrient composition of milk with greater intakes of protein and improved nutrient adequacy for several B vitamins, vitamin A, vitamin D, calcium, magnesium, phosphorus, potassium, and zinc. Mean calcium intake was 758 and 1581 mg/d, in the non-milk and milk groups, respectively, with 100% of girls in the milk groups consuming > RDA of 1300 mg/d. There were no effects of milk on body composition or muscle performance, but resistance training had a main effect and significantly increased body mass, lean body mass, muscle strength, and muscle endurance. There was a main effect of milk and resistance training on several measures of bone mineral density (BMD). Changes in whole body BMD in the M, RT, MRT, and CON were 0.45, 0.52, 1.32, and -0.19%, respectively (P < 0.01).Conclusions: Over the course of 12 weeks the effects of 1300 mg/d of calcium in the form of fluid milk combined with a heavy resistance training program resulted in the additive effects of greater nutrient adequacy and BMD in adolescent girls. While further studies are needed, combining increased milk consumption with resistance training appears to optimize bone health in adolescent girls.