Protein metabolism of growing animals is greatly affected by quantity and nutritional quality of dietary proteins. When animals are fed diets that contain enough proteins of good nutritional quality, they grow well. However, if they are fed diets deficient in protein or in some essential amino acids, their growth rate is markedly depressed. In this paper, we review the response of plasma insulin-like growth factor-I (IGF-I) to quantity and nutritional quality of dietary proteins. The sound correlation between plasma IGF-I concentration and the gain or loss of body proteins under various nutritional conditions suggests that the plasma IGF-I most possibly regulates the growth rate of animals or the rate of whole body protein synthesis. The quantity and nutritional quality of dietary proteins also regulates plasma concentration of IGF-binding proteins (IGFBPs). The changes in plasma concentration of IGFBPs presumably modifies the activity of IGF-I to regulate whole body protein synthesis. Molecular mechanisms of the changes in plasma concentrations of IGF-I and IGFBPs as affected by dietary proteins are also reviewed.
