A low-protein diet (8 vs. 20%) administered during pregnancy affects the structure and function of the endocrine pancreas of the offspring. At 21.5 days of gestation, we reported a reduction of cell proliferation, islet size, islet vascularization, and pancreatic insulin content. In this study, we demonstrated an impairment of insulin secretion of these fetal islets when stimulated in vitro with amino acids such as arginine and leucine. If the offspring is kept on the same low-protein diet during suckling, weaning, and adulthood, fasting insulin levels remain low in the presence of normal blood glucose levels. Glucose tolerance at 70 days is impaired, with lower insulin response. In addition, permanent functional damage seems to be induced in utero by a low-protein diet, because a normal diet given from birth to adulthood does not restore normal insulin response after a glucose challenge. Our experimental results stress the impact of a balanced diet with qualitative and quantitative amino acid composition for the fetal endocrine pancreas to develop normally, without lasting functional and structural consequences in adulthood.