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

CONTEXT: Growth hormone(GH) is an important regulator of growth and body composition. It has been shown that GH release can be promoted by intravenous as well as oral administration of various amino acids (AA), especially arginine and lysine(ARG+LYS), that are ample present in soy protein. The effects of dietary protein on GH secretion, however, are less well described. OBJECTIVE, DESIGN: In an experiment we compared the effects of oral ingestion of a mixture reflecting the amino acid composition of soy protein (AA), with oral ingestion of arginine + lysine, on GH-secretion in eight healthy women (BMI=19-25kg/m(2); 18-24years). In a second experiment, we compared oral ingestion of hydrolysed soyprotein and complete soyprotein with the AA-mixture on GH secretion in eight healthy women (BMI=19-26kg/m(2); 19-36years). Both experiments were performed in a randomized single blind cross-over design. Serum GH, insulin glucose and plasma AA were determined every 20 minutes, during 3 h in the first experiment and during 5 h 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(AUC), did not significantly differ after ingestion of the complete soyprotein, hydrolysed soyprotein or AA-mixture, but were all higher than after placebo(p<0.05). Insulin-responses(AUC) were higher after ingestion of hydrolysed soyprotein, complete soyprotein and AA-mixture, compared with placebo(p<0.05). Glucose concentrations were unaffected. CONCLUSION: Ingestion of soyprotein, either hydrolysed or intact, as well as amino acids reflecting, soyprotein stimulates GH release to a similar extent.