Context: GH is an important regulator of growth and body composition. We previously showed that GH release can be promoted by oral ingestion of soy protein; it is not known, however, whether these somatotropic effects of soy protein are also present when soy protein is ingested as part of a complete meal.
Objective/design: We compared the effects of oral ingestion of soy protein alone with the effects of a meal containing the same amount of soy protein on GH secretion in six healthy women (body mass index 19-26 kg/m(2), 19-36 years), in a randomized crossover design. During the whole experiment, serum GH, insulin, and glucose were determined every 20 min.
Results: GH responses as determined by area under the curve (AUC) and peak values were lower after ingestion of the meal, in comparison with GH responses after the soy protein consumption alone (P<0.05), and did not differ from the placebo. Glucose and insulin responses, both determined as AUC and peak values, were higher after ingestion of the meal, compared with those after ingestion of the protein drink or the placebo (P<0.05).
Conclusion: The somatotropic effect of soy protein is reduced and delayed when soy protein is ingested as part of a complete meal. Dietary carbohydrates, by increasing serum levels of glucose and insulin concentration, as well as dietary fat, may have interfered with the somatotropic effects of soy protein.