Gluconeogenesis is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. Gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the cortex of the kidneys in vertebrates. It is one of two primary mechanisms used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia), the other being degradation of glycogen (glycogenolysis).

The main gluconeogenic precursors in humans are lactate, glycerol, alanine, and glutamine, which account for over 90% of the overall gluconeogenesis. Other glucogenic amino acids and all citric acid cycle intermediates can also function as substrates for gluconeogenesis. Generally, human consumption of gluconeogenic substrates in food does not result in increased gluconeogenesis.

Gluconeogenesis is essentially the reversal of glycolysis, but to bypass the three highly exergonic (and essentially irreversible) steps of glycolysis, gluconeogenesis utilizes four unique enzymes. The enzymes unique to gluconeogenesis are pyruvate carboxylase, PEP carboxykinase, fructose 1,6-bisphosphatase, and glucose 6-phosphatase. Because these enzymes are not present in all cell types, gluconeogenesis can only occur in specific tissues. The purpose of gluconeogenesis is to maintain blood glucose levels during a fast.

Multiple factors contribute to the regulation of substrates, enzymes, and reactions involved in gluconeogenesis, including changes in allosteric regulators, covalent modification of enzyme activity, and induction of enzymes gene expression. Glucagon, insulin, and cortisol are the hormones that regulate the rate of gluconeogenesis. In a fed, high-energy state, gluconeogenesis decreases by inhibiting PEPCK and fructose 1,6-bisphosphatase.

In summary, gluconeogenesis is a metabolic pathway that generates glucose from non-carbohydrate carbon substrates and occurs mainly in the liver and kidneys of vertebrates. It is one of the two primary mechanisms used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia).