Gluconeogenesis

When all the glycogen stores are depleted, the body will switch to an other way to produce energy.
(In practice the body won't wait untill all glycogen is gone to start with this process. As mentioned before there is always an overlap between the processes. Goal is to keep the blood sugar level steady.)

The brain is not capable of burning fats. It can only get energy from glucose (and from ketones, but that is described later) and the glucose has to be supplied continuously because the brain also can't create its own supply.

Because all glucose and glycogen is (almost) used up at this moment, the body will have to find a way to create glucose from totally different substances. The liver is able to do this in a process called gluconeogenesis ("gluco" = glucose; "neo" = new and "genese" = to create).

In this process the liver will make glucose of endproducts of other metabolic processes:

• amino acids from breaking down protein in the muscles,
• lactate from anaerobic break down of glucose in the muscles,
• glycerol from breaking down body fat into one glycerol and 3 fatty acids. The fat metabolism is accelerated at this moment.

Besides this some other substances are necessary for this process to take place. One of these substances ("oxaloacetate") is made from acetyl-CoA.
As described in the page on fat metabolism, this acetyl-CoA is produced from the beta-oxidation.

This is where it starts to go wrong in patients with MCAD deficiency. Because the beta-oxidation is not properly working, not enough of this acetyl-CoA is created (and as a consequence also not enough oxaloacetate). That is why the process of gluconeogenesis can not properly take place in these patients.
Additionally, gluconeogenesis is a process that creates something and thus requires energy. Energy is also not plenty in a patient with MCAD deficiency at a moment when the body has to get energy from the fat metabolism.

Without gluconeogenesis no glucose is made and as a consequence the blood sugar level may become very low.
This is why some patients with MCAD deficiency who are going through a metabolic crisis can have a dangerously low blood sugar level.

Read further: ketogenesis >