Where is the majority of glucose derived from in the body during fasting?

During fasting, the majority of glucose in the body is derived from glycogen stores. Glycogen is a polysaccharide that serves as a form of energy storage and is primarily found in the liver and muscles. When blood glucose levels drop, such as during periods of fasting, the body mobilizes stored glycogen through a process called glycogenolysis. In the liver, glycogen is broken down into glucose and released into the bloodstream to maintain blood glucose levels and provide energy for tissues, especially the brain and red blood cells, which rely heavily on glucose for their energy needs.

While the body does matter from other sources, such as the breakdown of fat stores and the gluconeogenesis process (which utilizes amino acids and other precursors), the rapid and immediate source of glucose during fasting is the glycogen that has been stored prior. Thus, glycogen breakdown is a critical and efficient way to ensure a continued supply of glucose during times when dietary intake is not occurring.