Fed vs Fasted State: Insulin and Glucagon

Fed vs Fasted State: Insulin, Glucagon and Metabolic Switches

By StudyStore · Published 2026-11-04 · 1 min read · 262 words

After a meal, insulin dominates and the body stores fuel. During fasting, glucagon (with epinephrine and cortisol) dominates and the body releases it.

Hormone comparison

	Insulin (fed)	Glucagon (fasting)
Source	Pancreatic beta cells	Pancreatic alpha cells
Blood glucose	Lowers	Raises
Liver glycogen	Synthesis (glycogen synthase)	Breakdown (glycogen phosphorylase)
Liver glucose	Glycolysis, fatty acid synthesis	Gluconeogenesis
Adipose	Stores triglycerides (lipoprotein lipase)	Lipolysis (hormone-sensitive lipase)
Signaling	Receptor tyrosine kinase	G protein, cAMP, PKA

Insulin promotes dephosphorylation of key enzymes (active glycogen synthase). Glucagon promotes phosphorylation (active glycogen phosphorylase).

Time course of fasting

Stage	Glucose source	Main fuel for other tissues
Fed (0 to 4 h)	Diet	Glucose
Early fasting (4 to 16 h)	Liver glycogenolysis	Glucose; fatty acids in muscle
Fasting (about 1 to 3 days)	Gluconeogenesis (lactate, glycerol, amino acids)	Fatty acids; brain still uses glucose
Prolonged starvation	Gluconeogenesis, less protein breakdown	Ketone bodies used by brain

Organ roles

Liver: the glucose supplier; the only organ that releases free glucose (it has glucose-6-phosphatase) besides kidney.
Muscle: keeps its own glycogen; lacks glucose-6-phosphatase, so it cannot export glucose. Releases lactate and alanine.
Adipose: releases fatty acids and glycerol.
Brain: glucose first, ketones in long fasts. It cannot use fatty acids directly.
Red blood cells: always glucose (no mitochondria).

Common traps

Letting muscle glycogen raise blood glucose. It cannot.
Saying the brain burns fatty acids. It uses ketones.
Mixing up the cell types. Beta makes insulin; alpha makes glucagon.

One-minute recap

Fed: insulin, store. Fasted: glucagon, release.
Glycogen first, then gluconeogenesis, then ketones.
Only liver (and kidney) export glucose.

Check yourself: 5-question mini-quiz

Try each question first, then click it to reveal the answer.

1. Which hormone activates glycogen phosphorylase in the liver?

Glucagon, through cAMP and PKA.

2. Which tissue cannot release glucose from glycogen into blood?

Muscle, which lacks glucose-6-phosphatase.

3. What is the main glucose source in the first hours of fasting?

Liver glycogenolysis.

4. What fuel does the brain use in prolonged starvation?

Ketone bodies, along with some glucose.

5. Which cells secrete insulin?

Pancreatic beta cells.

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Written by StudyStore from standard scientific facts. Study aid only. Always check values against the reference your exam provides.