Gluconeogenesis
Although fat and protein can be used as an energy source, glucose must be available at all times because the nervous tissue (particularly the brain) requires it. Glycogen reserves can supply it for a period of time but when glycogen reserves are depleted glucose must be synthesized from fat and protein by a process called gluconeogenesis.
By gluconeogenesis glucose can come from three sources:
1. Glycerol - A component of triglycerides. Glycerol can be converted into glycerol phosphate and enter the glycolytic pathway in reverse.
2. Lactate - Lactate can be converted into pyruvate and then enter the glycolytic pathway in reverse.
3. Amino Acids - Some amino acids can be converted after conversion to pyruvate. Others can be converted to oxaloacetic acid which can then be converted to phosphoenolpyruvate which can enter the glycolytic pathway in reverse.