Gluconeogenesis is the process by which the body generates glucose from certain non-carbohydrate substrates including glycogenic (glucose producing) amino acids from the breakdown of proteins and from glycerol which is one of the breakdown products of triglycerides. Fatty acids themselves do not enter the gluconeogenic pathway but instead enter the Krebs cycle as an energy substrate in their own right.
Gluconeogenesis is one of several mechanisms within the human body that is designed to maintain blood glucose levels and therefore avoid low levels otherwise known as hypoglycaemia. The process takes place mainly in the liver and to a lesser extent in the kidney. It typically occurs during periods of fasting, starvation, and low carbohydrate diets or during intense exercise.
The main gluconeogenic precursors are lactate, glycerol, alamine and glutamine the last two being amino acids. Lactate is transported back to the liver where it is converted to pyruvate and from there, ???
There are two alternatives firstly to either enter the Krebs cycle to generate energy or alternatively to be converted to phosphoenolpyruvate and then to glucose 6-phosphate which eventually produces glucose. This glucose can then travel via the blood stream to cells that have an obligatory requirement for glucose namely the central nervous system.
The glycogenic amino acids can enter the Krebs cycle via conversion to the various elements of the cycle which eventually comes back to oxaloacetate which itself can be converted to phosphoenolpyruvate and thus to glucose. Additionally, some amino acids can be converted to pyruvate directly and then converted to glucose in this fashion.
Gluconeogenesis is under the control of various metabolic enzymes in particular insulin which inhibits it and adrenaline and glucagon both of which promote it. As previously mentioned the whole concept of gluconeogenesis is to maintain blood glucose at or above its baseline level thereby making it available immediately as a primary energy source.