Final answer:
The liver is the primary organ that supplies glucose to the body during the fasting state by breaking down stored glycogen and through gluconeogenesis when glycogen stores are depleted.
Step-by-step explanation:
During the fasting state, known as the postabsorptive state, the body must rely on stored glycogen. The liver is the major source of glucose for the body during fasting. When blood glucose levels drop, glucagon is released and acts on the liver to stimulate the breakdown of glycogen into glucose, which is then released into the bloodstream. If liver glycogen stores become depleted, the liver can generate glucose through a process called gluconeogenesis, utilizing amino acids and other substrates. As fasting continues and glycogen stores are exhausted, the body begins to utilize fatty acids and ketone bodies as alternative fuel sources. During prolonged fasting, glucose remains crucial for the brain and red blood cells, which require glucose for energy.