Increasing milk affects bone mineral density in adolescent boys - Benefit of Milk vs Juice

Nutrition Research Newsletter, Nov, 2003 by J Volek, A Gomez, T Scheet, M Sharman, D French, M Rubin, N Ratamess, M McGuigan, W Kraemer

Changing beverage consumption patterns in children has contributed to poor calcium intake in addition to inadequate intake of several other nutrients. Over half of young adults consume less than one serving of milk/day, well below the recommended three servings/day, with no trends of improved intake. The optimal intake of dietary calcium needed for optimal bone mineral accrual during periods of rapid growth and physical exercise is a debatable issue. A large body of literature has shown that both calcium and physical exercise have a favorable effect on bone. Resistance training may also have a favorable effect on bone because young male power lifters and Olympic lifters have significantly greater bone mineral density (BMD) compared with age-matched controls, and resistance training is effect in improving muscle strength and muscle size in youth. The primary purpose of the present study was to examine the effects of increasing milk intake on bone health in response to resistance training in adolescent boys.

A two-group prospective study design involving 12 weeks of resistance training was used to examine the effects of increasing milk consumption on bone responses in adolescent boys. Anthropometric measures, body composition, bone density, dietary intakes, and performance measures were assessed at zero, six, and 12 weeks. Subjects who consumed three servings of fluid milk or more than or equal to 1, 500 mg of calcium/day were excluded. The final sample consisted of 28 healthy boys aged 13 to 17 years, who were matched on physical characteristics and performance measures, and then randomly assigned to consume three servings of 1% fluid milk (n=14) or unfortified apple juice alternated with grape juice (n=14) group. Milk and juice were provided to subjects, weekly, at their exercise training sessions, and consumed in addition to their habitual diet. On exercise days, two servings were consumed in addition to their workout in the presence of a member of the investigation team, and the remaining serving was consumed at another time, which was recorded on log sheets.

The resistance exercise program consisted of supervised one-hour exercise sessions three days/week (Monday, Wednesday, Friday) for 12 weeks. The program consisted of varying training loads within each week of training as well as increasing intensity with concomitant decreasing volume over the 12 weeks to optimize strength and power performance gains. Body mass was measured on an electronic scale. Seven skin folds and three circumference measurements were serially obtained in duplicate from the right side by the same investigator. A whole body scan was performed using dual-energy x-ray absorption (DXA) with a total body scanner.

Compared with the juice group, the milk group recorded significantly higher intakes of protein, fat, vitamins A and D, riboflavin, calcium, phosphorus, and magnesium, and lower intakes of carbohydrate and vitamin C. The only variable that changed differently between the milk and juice groups was whole-body BMD. The milk group had a two-fold greater increase over the juice group. As a group, there was a significant relationship between vitamin D intake and the absolute change in BMC. No significant relations were observed among nutrient intake and changes in bone. Maximal strength significantly increased in both groups after six weeks, and further increased after week 12.

This was the first study to examine the effects of increasing milk and juice consumption during resistance training in adolescent boys. A primary finding indicated that 12 weeks of resistance training increased whole body BMD to a greater extent in boys who added to their diet three additional servings of milk/day compared with a juice group. The increase in BMD was two-fold greater in the milk compared to the juice group The most likely reason for the greater BMD response is the additional calcium and/or vitamin D consumed by the milk group.

J Volek, A Gomez, T Scheet, M Sharman, D French, M Rubin, N Ratamess, M McGuigan, W Kraemer. Increasing fluid milk favorably affects bone mineral density responses to resistance training in adolescent boys. JADA 103(10): 1353-1356 (October 2003) [Correspondence: Jeff S. Volek, PhD, RD, Assistant Professor, Department of Kinesiology, 2095 Hillside Roar Unit 1110, University of Connecticut, Storrs, CT 06269-1110. E-mail: jvolek@uconnnvm.edu]