Effects of oral ingestion of amino acids and proteins on the somatotropic axis

Context: GH is an important regulator of growth and body composition. It has been shown that GH release can be promoted by iv as well as oral administration of various amino acids (AAs), especially arginine (ARG) and lysine (LYS), which are amply present in soy protein. However, the effects of dietary protein on GH secretion are less well described. Objective and Design: In an experiment, we compared the effects of oral ingestion of a mixture reflecting the AA composition of soy protein (AA), with oral ingestion of ARG + LYS, on GH secretion in eight healthy women (body mass index 19-25 kg/m(2); age, 18-24 yr). In a second experiment, we compared oral ingestion of hydrolyzed soy protein and complete soy protein with the AA mixture on GH secretion in eight healthy women (body mass index 19-26 kg/m(2);age, 19-36 yr). Both experiments were performed in a randomized, single-blind crossover design. GH, insulin, glucose, and plasma AA were determined every 20 min, during 3 In in the first experiment and during 5 In in the second experiment. Results: Peak values of GH were higher after ingestion of the AA mixture compared with ingestion of ARG + LYS (P < 0.05). GH responses, as determined by area under the curve, did not significantly cliff er after ingestion of the complete soy protein, hydrolyzed soy protein, or AA mixture but were all higher than after placebo (P < 0.05). Insulin responses (area under the curve) were higher after ingestion of hydrolyzed soy protein, complete soy protein, and AA mixture, compared with placebo (P < 0.05). Glucose concentrations were unaffected. Conclusion: Ingestion of soy protein, either hydrolyzed or intact, as well as AAs reflecting soy protein, stimulates GH release to a similar extent.