The significance of normal variations in dietary and plasma nutrient content to brain metabolism and function began to receive examination in the past decade. It is now clear that the brain is much more sensitive to variations in nutrient supply than previously thought. Indeed, it seems likely that the diet-induced plasma fluctuations in nutrients, either as a result of their cofactor roles or as neurotransmitter precursors, are important components of feedback systems assisting the brain in controlling many of its functions. This discovery has suggested new approaches to understanding mechanisms controlling brain function and to treatment of diseases of the brain.