Fuel Homeostasis and the Fed to Fasted Switch

After a carbohydrate-containing meal, rising blood glucose stimulates pancreatic beta cells to release insulin. Insulin increases glucose uptake in muscle and fat by moving GLUT4 transporters to the cell surface. Liver glucose entry does not depend on GLUT4. Insulin promotes glucose use and storage in the liver.

In the fed state, liver and muscle store glucose as glycogen, a chain of glucose units. The liver can convert surplus carbohydrate into fat. Fat tissue stores triglycerides, made from fatty acids and glycerol. Insulin promotes these storage processes and reduces the release of stored fat. Fuel is also used immediately; not every nutrient goes into storage.

Between meals, insulin falls and glucagon from pancreatic alpha cells helps sustain liver glucose output. The liver breaks down glycogen and makes glucose by gluconeogenesis. Lactate, glycerol and glucogenic amino acids can supply carbon for new glucose. Gluconeogenesis contributes before glycogen runs out. Rising glucose can reduce glucagon release through negative feedback.

Insulin and glucagon use different receptors and signaling pathways. Their changing balance shifts liver enzyme activity toward storage or glucose production. Insulin favors glycogen synthesis; glucagon favors glycogen breakdown. This coordination limits wasteful cycling. It does not require both pathways to run at full speed or keep glucose perfectly constant. Adrenaline and cortisol also influence fuel availability.

Muscle uses its glycogen locally. It lacks glucose-6-phosphatase for converting glycogen-derived glucose-6-phosphate into free glucose for export. Lower insulin during fasting permits fat tissue to release fatty acids and glycerol. Many tissues burn fatty acids; glycerol can supply carbon for glucose synthesis. The liver supplies blood glucose, and the kidneys also contribute, especially in prolonged fasting.

Glycogen breakdown, fat use and gluconeogenesis overlap. Protein breakdown can supply amino acids for glucose synthesis early in a fast, while fat stores remain. As fasting continues, liver glycogen contributes less and can become depleted. Glucose synthesis continues while many tissues use more fatty acids. There is no fixed glycogen-then-fat-then-protein order.

Over several days, the liver makes more ketone bodies from fatty acids. The brain uses more ketones, reducing but not ending its glucose need. Red blood cells still require glucose because they lack mitochondria. Reduced glucose demand can lower protein breakdown for glucose synthesis. Protein use continues and can increase when fat reserves become severely depleted.