After a meal the body stores fuel, and between meals it releases it. Insulin and glucagon from the pancreas control this switch and keep blood glucose steady.
Fed state
In the hours after eating, insulin rises. It moves glucose into muscle and fat cells, the liver and muscles store it as glycogen, and extra fuel is stored as fat.
Early fasting
A few hours after a meal, insulin falls and glucagon rises. The liver breaks down glycogen and releases glucose to keep the brain supplied.
Longer fasting
Liver glycogen runs low within about a day. The liver then makes new glucose from amino acids, lactate and glycerol (gluconeogenesis), and fat breakdown supplies fatty acids to muscles.
Starvation and ketones
After several days the liver turns fatty acids into ketone bodies, which the brain can also use. This spares muscle protein.
| State | Main hormone | Main fuel source |
|---|---|---|
| Fed (0 to 4 h) | Insulin | Glucose from the meal |
| Early fasting (4 to 24 h) | Glucagon | Liver glycogen |
| Longer fasting (over 24 h) | Glucagon, low insulin | Gluconeogenesis and fat |
| Starvation (days) | Glucagon, low insulin | Ketone bodies and fat |
At the bedside
Without insulin, as in type 1 diabetes, the body acts as if it is starving despite high glucose and makes ketones fast, leading to diabetic ketoacidosis. Check blood glucose and ketones in any unwell patient with diabetes.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).