Fasting is dependent on three types of energy metabolism: glycogen, lipid, and amino acid. As blood glucose levels fall during fasting, the pancreas secretes increased amounts of glucagon. This action also reduces insulin secretion, which in turn decreases glucose storage in the form of glycogen.
Essentially, fasting cleanses our body of toxins and forces cells into processes that are not usually stimulated when a steady stream of fuel from food is always present. When we fast, the body does not have its usual access to glucose, forcing the cells to resort to other means and materials to produce energy.