It takes considerable energy to fuel the "energetically expensive" reactions of gluconeogenesis. Why, then, does the body have such a process?

Answer 1

Although most cells can use glucose and fatty acids for energy, the brain and central nervous system use glucose preferentially, and red blood cells use glucose exclusively. To ensure that these and other tissues have a continual supply of glucose, small amounts of glucose are stored as glycogen in the liver and skeletal muscle. However, when glycogen stores are depleted, noncarbohydrate molecules are transformed into glucose by various anabolic pathways. This process, called gluconeogenesis, occurs primarily within liver cells and, to a lesser extent, kidney cells. During periods of starvation, gluconeogenesis provides an important source of glucose to cells that depend on it as their major or sole source of energy.