Fed-fast cycle

A human typically eats a meal followed by a period of fasting. The amount of energy consumed during a meal is often 100 times greater than the energy needed for the duration of the meal. This way humans can live without having to nibble continuously.
The human body knows different ways to make sure that the supply of energy and the blood sugar level remains steady. Influenced by different hormones the body will switch from one way of producing energy to the other.

The body knows of 4 different ways to produce energy. The 4 ways are used one after the other but not in a strictly sequential way. In practice some of these processes occur simultaneously.

Here below the time after the consumption of a meal is divided in 4 periods, and for each period the processes are described that occur at that time.
A process that ends in "-lysis" is a process that breaks something down. These processes yield energy.
A process that ends in "-genesis" is a process that creates something. These processes cost energy. The end product of such a process is then used in an other process to break it down for energy.

In the next pages each of these processes is explained. It is also indicated what goes wrong in patients with MCAD deficiency.

Here is an overview:

• Just after eating – 0 to 3 hours after the meal:
  Glycolysis: breaking down sugars (glucose) from the meal for energy.
  Glycogenesis: if the supply of glucose exceeds the demand, glycogen is created and stored for later use.
  Read more about glycolysis and glycogenesis >

• Postabsorptive state – 3 to 12 hours after the meal:
  Glycogenolysis: the breakdown of stored glycogen yields energy.
  Read more about glycogenolysis >

• Fasting state – 12 hours to 2 days after the meal:
  Gluconeogenesis: the creation of glucose from other substances. This glucose can then be broken down for energy using glycolysis.
  Read more about gluconeogenesis >

• Starvation state – a few days to a few weeks after the meal:
  Ketogenesis: the creation of ketones that can be broken down for energy.
  Read more about ketogenesis >

The times indicated are only approximately. A lot depends on the amount of calories in the meal, the level of activity of the person and the speed of the metabolism of the person.

Only processes for supplying energy are discussed. All the other biochemical processes (build up tissue, muscle movements, maintenance etc.) that are continuously going on in the human body are left aside.