Figure 1: Glucose is necessary not only to supply energy for the central nervous system but also to produce pyruvate that can be transformed into oxaloacetate. Oxaloacetate must be maintained at a level sufficient to allow citric acid cycle function (i.e., the condensation between acetyl-CoA and oxaloacetate). Oxaloacetate is unstable and must be refurnished (this kind of reactions is called anaplerotic). The main way to produce oxaloacetate is from pyruvate that derives from glucose. In mammals pyruvate cannot be produced from acetyl-CoA as shown in the figure.