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Effects of Dietary Carbohydrate Restriction with High Protein Intake on Protein Metabolism and the Somatotropic Axis

Divisions of Kinesiology (M.P.H., S.S., J.F.H.) and Endocrinology and Metabolism (A.L.B.), University of Michigan and Veterans Affairs Medical Center, Ann Arbor, Michigan 48109

Address all correspondence and requests for reprints to: Jeffrey Horowitz, University of Michigan, Division of Kinesiology, 401 Washtenaw Avenue, Ann Arbor, Michigan 48109-2208. E-mail: jeffhoro{at}umich.edu.

Context: Alterations in dietary macronutrient intake can influence protein turnover.

Objective: The purpose of this study was to assess the influence of a low-carbohydrate/high-protein diet (LC/HP) on skeletal muscle protein synthesis and whole-body proteolysis, without the confounding influence of a negative energy balance.

Design: Nine-day dietary intervention was applied.

Setting: Subjects remained in the General Clinical Research Center throughout the 9-d study.

Participants: Eight young, healthy volunteers participated.

Intervention: Subjects ate a typical Western diet (60% carbohydrate, 30% fat, 10% protein) for 2 d, followed immediately by 7 d of an isocaloric LC/HP (5% carbohydrate, 60% fat, 35% protein).

Main Outcome Measures: Skeletal muscle fractional synthetic rate and whole-body proteolysis [leucine rate of appearance in plasma (Ra)] were measured after an overnight fast before and after 2 and 7 d of LC/HP. We also measured plasma concentrations of insulin, GH, and IGF-I.

Results: Leucine Ra was increased (P = 0.03) after 2 and 7 d of LC/HP, and muscle fractional synthetic rate was approximately 2-fold higher (P < 0.01) after 7 d of LC/HP. Fat free mass was not altered by LC/HP. Average 24-h plasma insulin concentration was 50% lower (P < 0.001) after 2 and 7 d of LC/HP, whereas GH secretion and total plasma IGF-I concentrations were unchanged with LC/HP. However, plasma free IGF-I decreased by approximately 30% after 7 d of LC/HP (P = 0.002), whereas muscle IGF-I mRNA increased about 2-fold (P = 0.05).

Conclusions: Increasing dietary protein content during a 7-d carbohydrate restricted diet stimulated muscle protein synthesis and whole-body proteolysis without a measurable change in fat free mass.

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