[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Fed, fasting and starvation states [Biochemistry](https://durgical.com/en/biochemistry/) · Topic # Fed, fasting and starvation states The body switches fuel with the time since the last meal: it stores in the fed state and draws on reserves while fasting. Updated 2026-11-01 ## Key facts - The insulin/glucagon ratio rules: high after a meal (store), low in fasting (mobilise). - Liver glycogen keeps blood glucose up for the first hours and runs out around 24 hours of fasting; after that glucose comes from gluconeogenesis. - In prolonged starvation the brain draws much of its energy from ketone bodies, which lowers the glucose need and spares muscle protein. - The red cell (no mitochondria) always depends on glucose; muscle and heart prefer fatty acids in fasting. - The switches work at three levels: allosteric (seconds), enzyme phosphorylation (minutes) and gene expression (hours to days). ## In detail - Fed state (0–4 h): insulin rises. The liver takes up glucose through GLUT2 and glucokinase, fills glycogen and turns the surplus into fatty acids exported in VLDL; muscle and fat take up glucose through GLUT4, and adipose lipoprotein lipase stores the triglycerides. - Post-absorptive (4–24 h): insulin falls and glucagon rises. The liver breaks down glycogen (glycogen phosphorylase) and starts gluconeogenesis; adipose tissue releases fatty acids through hormone-sensitive lipase. - Early fasting (1–3 days): with no liver glycogen, glucose comes from lactate (Cori cycle), glycerol and muscle alanine (glucose–alanine cycle); the liver oxidises fatty acids and makes ketone bodies. - Starvation (over 3 days): ketones rise sharply and the brain uses them; muscle proteolysis slows and the kidney adds glucose made from glutamine. T3 and leptin fall and energy expenditure drops. - Key fasting switches: PKA phosphorylates and inactivates glycogen synthase, liver pyruvate kinase and acetyl-CoA carboxylase, and fructose 2,6-bisphosphate falls; cortisol and glucagon induce PEPCK and glucose 6-phosphatase. - Survival in starvation depends mostly on the fat reserve; death comes when protein loss hits the breathing muscles, heart and immunity. **Also called** Metabolic integration, Fed–fast cycle ## Clinical notes - Defects of gluconeogenesis, glycogen or beta-oxidation show up as fasting hypoglycaemia (von Gierke disease, MCAD deficiency). - Refeeding a malnourished patient with a lot of carbohydrate triggers insulin and drives phosphate, potassium and magnesium into cells: refeeding syndrome. - Kwashiorkor and marasmus are the severe forms of childhood malnutrition; diabetic ketoacidosis is a metabolic fast with high glucose because insulin is missing. ## Pathways - [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/) - [Gluconeogenesis](https://durgical.com/en/biochemistry/pathways/gluconeogenesis/) - [Glycogen metabolism](https://durgical.com/en/biochemistry/pathways/glycogen-metabolism/) - [Fatty acid synthesis (lipogenesis)](https://durgical.com/en/biochemistry/pathways/fatty-acid-synthesis/) - [Fatty acid β-oxidation](https://durgical.com/en/biochemistry/pathways/fatty-acid-oxidation/) - [Ketogenesis (ketone body synthesis)](https://durgical.com/en/biochemistry/pathways/ketogenesis/) ## Related conditions - [Kwashiorkor](https://durgical.com/en/conditions/kwashiorkor/) - [Marasmus](https://durgical.com/en/conditions/marasmus/) - [Von Gierke disease (glycogen storage disease type I)](https://durgical.com/en/conditions/von-gierke-disease/) - [Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency](https://durgical.com/en/conditions/medium-chain-acyl-coa-dehydrogenase-deficiency/) - [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/) ## Related - [Insulin signalling](https://durgical.com/en/biochemistry/topics/insulin-signalling/) - [Glucagon signalling](https://durgical.com/en/biochemistry/topics/glucagon-signalling/) - [Cori cycle](https://durgical.com/en/biochemistry/topics/cori-cycle/) - [Glucose–alanine cycle](https://durgical.com/en/biochemistry/topics/glucose-alanine-cycle/) - [Hormone-sensitive lipase (HSL)](https://durgical.com/en/biochemistry/enzymes/hormone-sensitive-lipase/) - [Lipoprotein lipase (LPL)](https://durgical.com/en/biochemistry/enzymes/lipoprotein-lipase/) - [β-Hydroxybutyrate](https://durgical.com/en/biochemistry/molecules/hydroxybutyrate/) - [Glycogen](https://durgical.com/en/biochemistry/molecules/glycogen/) - [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) - [Protein kinase A (PKA)](https://durgical.com/en/biochemistry/enzymes/protein-kinase-a/) - [Exercise metabolism](https://durgical.com/en/biochemistry/topics/exercise-metabolism/) - [Leptin, appetite and obesity](https://durgical.com/en/biochemistry/topics/leptin-appetite-and-obesity/) ## Test yourself Think of the answer, then tap the question to see it. ## What keeps blood glucose up during the first hours of fasting? Liver glycogen, which runs out around 24 hours; after that glucose comes from gluconeogenesis. ## Why does prolonged starvation use less muscle protein than a 2-day fast? Because the brain switches to ketone bodies and needs less glucose, so less gluconeogenesis from amino acids is required. ## Which cell always depends on glucose, even in starvation? The red blood cell, because it has no mitochondria and makes ATP only by glycolysis. ## Which hormone dominates the fed state, and what does it do to adipose lipoprotein lipase? Insulin; it raises adipose lipoprotein lipase so the triglycerides of chylomicrons and VLDL are stored. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/fed-fasting-and-starvation-states/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Hypoglycemia: MedlinePlus](https://medlineplus.gov/hypoglycemia.html) (MedlinePlus, US National Library of Medicine, 2026); [Malnutrition: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000404.htm) (MedlinePlus, US National Library of Medicine, 2026); [INS: Insulin (human)](https://www.uniprot.org/uniprotkb/P01308/entry) (UniProt, 2026); [GCG: Pro-glucagon (human)](https://www.uniprot.org/uniprotkb/P01275/entry) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/biochemistry/topics/fed-fasting-and-starvation-states/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/fed-fasting-and-starvation-states/