Insulin actions in the fed state
After a meal, insulin, released when blood glucose is high, makes the body store fuel and stops it releasing any more.
Remember
After a meal, insulin is the storage hormone. It moves GLUT4 to the surface of muscle and fat cells, so glucose can enter (GLUT4 is a glucose transporter, a protein that carries glucose across the cell membrane), and it switches off the release of fuel.
Memory aid
Insulin opens the door to glucose for muscle and fat: GLUT4 is the door.
GLUT4 acts as a door that insulin opens, so glucose can enter muscle and fat cells.
The fed state is the time just after a meal, when blood glucose rises and plenty of glucose is arriving from the gut. The pancreas releases insulin, the main anabolic hormone (anabolic means building up and storing fuel). Insulin is there to store fuel, not to break it down.
Insulin starts storage. In muscle and fat (adipose tissue), it moves GLUT4 to the cell surface, so glucose can enter the cell and leave the blood. Inside the cell, glucose can be stored as glycogen (in muscle) or as fat (in adipose tissue). Insulin also promotes glycogen synthesis (storing glucose as glycogen), protein synthesis and lipogenesis (making and storing fat), and it increases amino acid uptake.
At the same time, insulin stops the pathways that release fuel: gluconeogenesis (making new glucose, mainly in the liver), glycogenolysis (breaking down glycogen), lipolysis (breaking down fat), proteolysis (breaking down protein) and ketogenesis (making ketone bodies). New glucose is not needed when plenty is already arriving, so insulin switches gluconeogenesis off.
Ask whether a process stores fuel or releases fuel. Insulin stimulates the storing processes and inhibits the releasing ones. In fasting, glucagon rises and does the reverse, so glycogenolysis, lipolysis and gluconeogenesis increase.
Brain and red blood cells take up glucose through other transporters that do not depend on insulin, so insulin does not control their glucose uptake.
Key facts
- Fed-state hormone
- insulin is released after a meal, when there is plenty of glucose available in the blood.
- Insulin starts
- GLUT4-mediated glucose uptake into muscle and fat, glycogen synthesis and protein synthesis.
- Insulin stops
- gluconeogenesis, glycogenolysis, lipolysis, proteolysis and ketogenesis.
- Storage rule
- insulin stimulates a process that stores fuel and inhibits one that releases fuel.
- Fasting
- glucagon is released and prompts the release of molecules that can be used as a fuel source.
- Brain and red blood cells
- they take up glucose through other transporters that do not depend on insulin.
Exam traps
- Gluconeogenesis (making new glucose, mainly in the liver) does not belong to the fed state, because insulin switches it off when plenty of glucose is already arriving from the gut.
- Lipolysis (breaking down fat to release fatty acids) is not promoted after a meal, because insulin inhibits it while the body is storing fuel; it fits fasting, when insulin is low and glucagon is high.
- Ketogenesis, the making of ketone bodies in the liver, is stopped by insulin; it occurs in prolonged fasting, when insulin is low.
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