Low-protein dietary intake results in reduced plasma IGF-I levels and skeletal muscle fiber atrophy in elderly women
The effects of protein restriction without energy deficit on IGF-I levels, skeletal muscle size and function were evaluated in 12 healthy sedentary elderly women over a 10-week period. Subjects were randomly assigned to one of two weight maintaining diets containing 0.45 or 0.92 g protein per kg body weight per day. These levels were estimated to be approximately one-half the RDA and the full RDA, respectively. To maintain body weight, energy intake was increased by 15±2% (p<0.0001) and 3±0.4% (NS), in the 1/2-RDA and RDA groups, respectively. Repeated measures analyses of covariance with the change in energy intake as a covariate were performed. There were no differences in baseline measures. There was a significant time effect (p<0.01), and time by diet interaction (p<0.0001) in the plasma IGF-I levels. Plasma IGF-I levels decreased by 26% in the 1/2-RDA group while they increased by 18% in the RDA group. There was a significant time by diet interaction (p<0.02) in the cross-sectional area of type I muscle fibers. In the 1/2-RDA group, type I mean area decreased by 11.4% (p<0.04) compared to a 15% increase (p<0.03) in the RDA group. Although the changes in type II fibers did not reveal statistical significance they were associated with the changes in type I fibers (r= 0.83, p<0.001). The results of this study indicate that protein restriction with adequate energy intake is associated with reduced plasma IGF-I levels as well as skeletal muscle atrophy. These data parallel the whole-body losses of lean body mass and functional capacity previously reported.
Castaneda, C; Gordon, PL; Fielding, RA; Evans, WJ; Crim, MC
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