--- title: "Fuel Homeostasis and the Fed to Fasted Switch" description: "After a carbohydrate-containing meal, insulin promotes fuel use and storage. During fasting, lower insulin and relatively greater glucagon action favor liver glucose production and fuel mobilization. " canonical: https://lightmysky.com/learn/science/fuel-homeostasis-and-the-fed-to-fasted-switch-mt_zZrXwbWqw- source: https://lightmysky.com/learn/science/fuel-homeostasis-and-the-fed-to-fasted-switch-mt_zZrXwbWqw-.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Fuel Homeostasis and the Fed to Fasted Switch After a carbohydrate-containing meal, insulin promotes fuel use and storage. During fasting, lower insulin and relatively greater glucagon action favor liver glucose production and fuel mobilization. Fuel pathways overlap, and greater brain ketone use during prolonged fasting can reduce protein demand. Subject: Science · Area: The Human Body · Ages 21 to 22 Page: https://lightmysky.com/learn/science/fuel-homeostasis-and-the-fed-to-fasted-switch-mt_zZrXwbWqw- ## Ready when they can - Names how liver, muscle and fat tissue handle fuel in fed and fasted states. - Explains how changing insulin and glucagon signals coordinate liver storage and glucose production. - Explains overlapping fuel use during fasting and how greater brain ketone use can reduce protein demand. ## Lesson: Fed or fasted: how your body switches fuels 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. *(drawing: Read left to right for the response to a carbohydrate-containing meal, not every possible meal.)* 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. *(drawing: This route explains why muscle glycogen does not directly supply free glucose to blood.)* 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. **Recap.** Insulin favors fuel use and storage after meals. Fasting favors glucose production and fat use. Pathways overlap, and greater brain ketone use can reduce protein demand. *This lesson, question bank and flashcards were written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Optional connections - [Gluconeogenesis and the Reciprocal Regulation of Opposing Pathways](https://lightmysky.com/learn/science/gluconeogenesis-and-the-reciprocal-regulation-of-opposing-pathways-mt_D1MbKcb6Le) - [Blood Glucose Control and Diabetes](https://lightmysky.com/learn/science/blood-glucose-control-and-diabetes-mt_eOzNtiBvX2) - [Digestion and Absorption in the Mammalian Gut](https://lightmysky.com/learn/science/digestion-and-absorption-in-the-mammalian-gut-mt_xZ2-3ZClDa) ## Opens up - [The Stress Response: Fast Signals, Slower Cortisol Effects and Chronic Costs](https://lightmysky.com/learn/science/the-stress-response-fast-signals-slower-cortisol-effects-and-chronic-costs-mt_4ZxCU21ziH)