Effects of dietary carbohydrate restriction with high protein intake on protein metabolism and the somatotropic axis

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.