During fasting state insulin levels are decreased, instead of it glucagon levels are increased to supply tissues and organs with glucose. Blood glucose is supported on a stable level with the resources of liver and etc.
High level of glucagon stimulates breakdown of liver glycogen, so glycogen level is decreased.
The low levels of insulin reached during fasting stimulate lipolysis, the breakdown of fat for energy. The storage form of fat, known as triglycerides, is broken into the glycerol backbone and three fatty acid chains. So the level of free fatty acids increased. Glycerol is used for gluconeogenesis. Fatty acids may be used directly for energy by many tissues in the body, but not the brain. Ketone bodies, capable of crossing the blood-brain barrier, are produced from fatty acids for use by the brain. The two major types of ketones produced are beta hydroxybutyrate and acetoacetate, which can increase over 70 fold during fasting state.