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[Physiology](https://durgical.com/en/physiology/) · [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative)

# Fed and fasting states: the hormonal switch

- Regulation
- Exam priority: high

After a meal insulin stores energy; when fasting, glucagon pulls it from stores so the brain never runs short of glucose.

Updated 2026-11-01

Also called: Absorptive and post-absorptive states, Insulin/glucagon ratio

**See it in 3D:** [Liver](https://durgical.com/en/anatomy/liver/) · [Pancreas](https://durgical.com/en/anatomy/pancreas/)

## Key points

- High insulin/low glucagon: glycogen, fat and protein synthesis; GLUT4 brings glucose into muscle and fat.
- High glucagon/low insulin: hepatic glycogenolysis and gluconeogenesis, lipolysis and ketogenesis.
- Liver glycogen holds glucose up for ≈12–24 h; after that gluconeogenesis takes over (lactate, alanine, glycerol).
- Muscle cannot release glucose (no glucose-6-phosphatase): its glycogen is for its own use.
- Insulin suppresses ketogenesis; without insulin (type 1 diabetes) unchecked lipolysis leads to ketoacidosis.

## What it is

The body alternates between storing (fed or absorptive state) and mobilizing (post-absorptive and fasting). The switch is the insulin/glucagon ratio in portal blood, acting mainly on liver, muscle and fat. The fixed goal is a blood glucose of ≈70–100 mg/dL for the brain and red cells.

## How it works, step by step

- A meal raises portal glucose and amino acids; incretins (GLP-1, GIP) amplify insulin secretion.
- Insulin activates glycogen synthase and lipogenesis in the liver and inserts GLUT4 in muscle and fat cells.
- In fat it activates lipoprotein lipase and brakes hormone-sensitive lipase: fat is stored.
- Hours later insulin falls and glucagon rises; via cAMP/PKA the liver breaks down glycogen and releases glucose.
- Low insulin releases lipolysis: fatty acids fuel muscle and, by β-oxidation, the liver.
- Excess hepatic acetyl-CoA forms ketone bodies, which the brain begins to use after several days of fasting.

## Who does what in each state

| Tissue | Fed (insulin) | Fasting (glucagon, catecholamines) |
| --- | --- | --- |
| Liver | Stores glycogen, makes triglycerides | Releases glucose, gluconeogenesis, ketones |
| Muscle | Takes up glucose (GLUT4), builds protein | Burns fatty acids and ketones; exports alanine |
| Adipose tissue | Stores triglycerides | Lipolysis: fatty acids and glycerol |
| Brain | Glucose | Glucose; after days, ketones |
| Red blood cells | Glucose (glycolysis) | Always glucose (no mitochondria) |

## Clinical notes

- Diabetic ketoacidosis: absent insulin with high glucagon: hyperglycemia, ketones and high anion gap acidosis.
- Fasting hypoglycemia in children or people who drink heavily: ethanol blocks gluconeogenesis once glycogen is gone.
- Cortisol, epinephrine and growth hormone also oppose insulin (counter-regulatory hormones).

## Memory aid

«Insulin puts IN, glucaGONE takes out»: one stores energy, the other releases it.

## Test yourself

Think of the answer, then tap the question to see it.

## Why doesn't muscle glycogen help correct fasting hypoglycemia?

Muscle lacks glucose-6-phosphatase, so glucose-6-phosphate cannot leave as free glucose; it is used in the muscle's own glycolysis.

## Which substrates does the liver use for gluconeogenesis after 24 h of fasting?

Lactate (Cori cycle), alanine and other amino acids from muscle, and glycerol from lipolysis. Even-chain fatty acids give no net glucose.

## Why does lack of insulin, not just hyperglycemia, cause ketoacidosis?

Without insulin lipolysis is unchecked and glucagon routes fatty acids into hepatic β-oxidation; surplus acetyl-CoA forms keto-acids.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/fed-and-fasting-states-the-hormonal-switch/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Liver](https://durgical.com/en/anatomy/liver/)
- [Pancreas](https://durgical.com/en/anatomy/pancreas/)

## Related topics

- [Prolonged fasting and starvation](https://durgical.com/en/physiology/prolonged-fasting-and-starvation/)
- [Exercise and training physiology](https://durgical.com/en/physiology/exercise-and-training-physiology/)
- [Energy balance, food intake and obesity](https://durgical.com/en/physiology/energy-balance-food-intake-and-obesity/)

## Related conditions

- [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/)
- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)

## In Biochemistry

- [Fed, fasting and starvation states](https://durgical.com/en/biochemistry/topics/fed-fasting-and-starvation-states/)
- [Insulin signalling](https://durgical.com/en/biochemistry/topics/insulin-signalling/)
- [Glucagon signalling](https://durgical.com/en/biochemistry/topics/glucagon-signalling/)
- [Ketogenesis (ketone body synthesis)](https://durgical.com/en/biochemistry/pathways/ketogenesis/)
- [Gluconeogenesis](https://durgical.com/en/biochemistry/pathways/gluconeogenesis/)
- [Glycogen metabolism](https://durgical.com/en/biochemistry/pathways/glycogen-metabolism/)

## In Pharmacology

- [Insulins](https://durgical.com/en/pharmacology/classes/insulins/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [24.5 Metabolic States of the Body](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-5-metabolic-states-of-the-body) (OpenStax (Rice University), 2022); [24.3 Lipid Metabolism](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-3-lipid-metabolism) (OpenStax (Rice University), 2022); [Diabetic ketoacidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000320.htm) (MedlinePlus, US National Library of Medicine, 2026); [Low blood sugar: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000386.htm) (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 79). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/fed-and-fasting-states-the-hormonal-switch/