Gluconeogenesis

Gluconeogenesis is the biosynthesis of glucose from non-carbohydrate precursors, primarily occurring in the liver (~80-90% of total gluconeogenic output) and kidney cortex (~10-20%). It is the principal mechanism by which mammals maintain blood glucose during interprandial periods, prolonged fasting, low-carbohydrate diets, and sustained exercise, when exogenous glucose is unavailable and hepatic glycogen stores have been depleted.

Substrates

Three principal substrate classes feed gluconeogenesis. Lactate from anaerobic glycolysis in muscle and erythrocytes is transported to the liver via the Cori cycle and converted back to glucose. Glycerol released from adipose triglyceride hydrolysis during lipolysis contributes increasingly during prolonged fasting. Glucogenic amino acids — principally alanine (from muscle via the glucose-alanine cycle) and glutamine — provide carbon skeletons via transamination to pyruvate and TCA cycle intermediates. In prolonged fasting, muscle protein breakdown contributes substantial alanine and glutamine to support gluconeogenic flux.

Pathway enzymes

Gluconeogenesis is not simply the reversal of glycolysis; three glycolytic steps are thermodynamically irreversible and require distinct gluconeogenic enzymes to bypass them. Pyruvate carboxylase (mitochondrial) carboxylates pyruvate to oxaloacetate. Phosphoenolpyruvate carboxykinase (PEPCK; cytosolic PCK1 and mitochondrial PCK2) converts oxaloacetate to phosphoenolpyruvate. Fructose-1,6-bisphosphatase hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate. Glucose-6-phosphatase (endoplasmic reticulum) dephosphorylates glucose-6-phosphate to free glucose for release into circulation. Expression of PEPCK and G6Pase is under direct transcriptional control of FOXO1, CREB, and glucagon-cAMP signaling.

Hormonal regulation

Gluconeogenesis is stimulated by glucagon (via cAMP-PKA-CREB and FOXO1 transcriptional activation), cortisol (via glucocorticoid response elements on PEPCK and G6Pase promoters), and catecholamines. It is suppressed by insulin, which activates AKT to phosphorylate and exclude FOXO1 from the nucleus. In type 2 diabetes, hepatic insulin resistance produces inappropriately elevated gluconeogenesis, contributing to fasting hyperglycemia — a target of metformin, which partially inhibits hepatic gluconeogenesis through mitochondrial glycerol-3-phosphate dehydrogenase inhibition and AMPK activation.

Quantitative contribution

After an overnight fast (12-14 hours), gluconeogenesis contributes approximately 50-70% of endogenous glucose production, with glycogenolysis contributing the remainder. At 24 hours of fasting, hepatic glycogen is largely depleted and gluconeogenesis approaches 100% of endogenous glucose production. On low-carbohydrate or ketogenic diets, gluconeogenesis provides most of the glucose that obligate glucose-consuming tissues (brain, erythrocytes, renal medulla) continue to require.

Low-carbohydrate diets

A common misconception is that ketogenic diets "run on ketones" exclusively; in reality, gluconeogenesis produces 80-150 g/day of glucose on very-low-carbohydrate intake, enough to meet obligate brain and red blood cell demands that cannot be replaced by ketones or fatty acids. Brain glucose utilization is reduced by 40-60% as beta-hydroxybutyrate displaces glucose as primary fuel, but not eliminated. This sets a floor on protein intake for ketogenic dieters, since excess protein (beyond satisfying amino acid requirements) can be gluconeogenically converted to glucose, partially attenuating ketosis.