Although the small intestinal mucosa is designed to transport large quantities of all nutrients to the blood, the primary nutrients utilized by the enterocyte for growth and/or maintenance are quite restricted. The major fuels for the small intestinal mucosa are amino acids (glutamine, glutamate, aspartate), whereas glucose and fatty acids are of much less importance. Many of the experiments have been performed during growth or maintenance of mucosa in small rodents, especially the rat, a model in which the adaptation of the intestinal mucosa, at least to fasting, is quite different than in humans. A special role has been suggested for glutamine as a small intestinal fuel, compared with glutamate and aspartate, but the available data do not support this view. Clinical trials of glutamine supplementation suggest that, if glutamine has a role, it may be related to functions other than those relating to small intestinal function.