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ORIGINAL RESEARCH COMMUNICATION |
1 From the Applied Physiology Laboratory (TLPB and MDV), the Reproductive Physiology Laboratory (JAC), and the Ethel A Martin Program in Human Nutrition (BLS and TLB), South Dakota State University, Brookings, SD
Background: Exercise is beneficial for bone when adequate nutrition is provided. The role of protein consumption in bone health, however, is controversial.
Objective: The objective was to ascertain the effect of high protein intake on insulin-like growth factor I (IGF-I) and markers of bone turnover during 6 mo of exercise training.
Design: Fifty-one subjects aged 18–25 y (28 men, 23 women) received a protein supplement (42 g protein, 24 g carbohydrate, 2 g fat) or a carbohydrate supplement (70 g carbohydrate) twice daily. Exercise consisted of alternating resistance training and running 5 times/wk. Plasma concentrations of IGF-I, insulin-like growth factor-binding protein 3, serum bone alkaline phosphatase, and urinary N-telopeptide collagen crosslink (NTx) concentrations were measured at 0, 3, and 6 mo after 24 h without exercise and a 12-h fast.
Results: Three-day diet records indicated no difference in energy intake between the groups. Average protein intakes after supplementation began in the protein and carbohydrate groups were 2.2 ± 0.1 and 1.1 ± 0.1 g/kg, respectively (P < 0.001). The increase in plasma IGF-I was greater in the protein group than in the carbohydrate group (time x supplement interaction, P = 0.01). There were no significant changes over time or significant differences by supplement in plasma insulin-like growth factor-binding protein 3 (44 and 40 kDa). Serum bone alkaline phosphatase increased significantly over time (P = 0.04) and tended to be higher in the protein group than in the carbohydrate group (P = 0.06). NTx concentrations changed over time (time and time squared; P < 0.01 for both) and were greater in the protein group than in the carbohydrate group (P = 0.04). Men had higher NTx concentrations than did women (74.6 ± 3.4 and 60.0 ± 3.8 nmol/mmol creatinine; P = 0.005).
Conclusion: Protein supplementation during a strength and conditioning program resulted in changes in IGF-I concentrations.
Key Words: Protein • bone alkaline phosphatase • collagen crosslinks • exercise • insulin-like growth factor-binding proteins
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