The liver manufactures glucose from non-sugar building blocks through a process called gluconeogenesis, and it does so primarily to keep the brain and red blood cells fueled when dietary carbohydrates run out. After roughly 12 hours without food, glycogen stores in the liver are largely depleted, and gluconeogenesis takes over as the dominant source of blood glucose. The process is ancient, tightly regulated by hormones, and central to survival during fasting, sleep, and prolonged exercise. It also sits at the heart of what goes wrong in type 2 diabetes, where the liver overproduces glucose even when blood sugar is already elevated.
The Raw Materials
Gluconeogenesis literally means “new glucose creation,” and the liver builds that new glucose from three main categories of precursor: lactate, amino acids, and glycerol. Lactate is the single largest contributor. It arrives at the liver through the bloodstream after being released by working muscles, red blood cells, and other tissues that break glucose down only partway. The liver then reassembles those carbon skeletons back into glucose and ships it out again. This continuous loop between tissues that produce lactate and the liver that recycles it is called the Cori cycle, and it accounts for a substantial fraction of glucose turnover even at rest.
Among amino acids, alanine and glutamine are the principal glucogenic contributors.1PubMed Central. Origin and Roles of Alanine and Glutamine in Gluconeogenesis in the Liver, Kidneys, and Small Intestine under Physiological and Pathological Conditions Alanine participates in its own recycling loop, the glucose-alanine cycle: muscles break down glucose to pyruvate, transfer a nitrogen group onto it to form alanine, and release it into the blood. The liver strips the nitrogen off and converts the remaining carbon backbone back into glucose.2Metabolism. The glucose-alanine cycle During starvation, muscles release increasing amounts of alanine, lactate, and glutamine, and the liver extracts these substrates more aggressively to sustain glucose output.3PubMed. Alanine metabolism and gluconeogenesis in the rat
Glycerol, the third major precursor, comes from the breakdown of stored fat. When fat cells release fatty acids for energy, glycerol is freed as a byproduct and travels to the liver, where it enters the gluconeogenic pathway partway through. During prolonged fasting or vigorous exercise, this glycerol contribution grows as fat breakdown accelerates.
How the Pathway Actually Works
Gluconeogenesis is often described as “glycolysis in reverse,” and while the two pathways do share most of their enzymatic steps, they are not simply mirror images of each other. Three reactions in glycolysis release so much energy that they are effectively irreversible, so the liver uses different enzymes to bypass each of those bottlenecks.
The three bypass enzymes are phosphoenolpyruvate carboxykinase (PEPCK), fructose-1,6-bisphosphatase (FBPase), and glucose-6-phosphatase (G6Pase).4Trends in Cancer. Gluconeogenesis and cancer: from mechanisms to therapeutic opportunities PEPCK catalyzes the first committed step, converting oxaloacetate to phosphoenolpyruvate. FBPase removes a phosphate group to convert fructose-1,6-bisphosphate into fructose-6-phosphate. And G6Pase strips the final phosphate from glucose-6-phosphate to produce free glucose that the liver can export into the bloodstream. These enzymes are reciprocally regulated with their glycolytic counterparts: when conditions favor glucose production, these bypass enzymes are turned up while the corresponding glycolytic enzymes are turned down, and vice versa.5PubMed Central. Regulation of glycolysis and gluconeogenesis by acetylation of PKM and PEPCK
One aspect of the pathway that trips people up is the fact that it spans two cellular compartments. The first step, where pyruvate is converted to oxaloacetate, happens inside the mitochondria. But most of the remaining enzymes sit in the cytoplasm. Oxaloacetate cannot cross the mitochondrial membrane directly, so it is converted to malate, shuttled out, and then converted back to oxaloacetate in the cytoplasm before the pathway continues.6PubMed. Roles of malate and aspartate in gluconeogenesis in various physiological and pathological states This shuttle system adds complexity but also adds a layer of control: the cell can regulate traffic between compartments to fine-tune glucose output.
Hormones Running the Show
The on-off switch for hepatic gluconeogenesis is primarily hormonal. Glucagon, released by the pancreas when blood sugar drops, is the main activator. It triggers a signaling cascade inside liver cells that increases expression of the bypass enzymes, especially PEPCK and G6Pase. Glucocorticoids like cortisol act synergistically with glucagon, amplifying the signal during stress or prolonged fasting. Research has shown that this activation works through the transcription factor CREB, which induces the coactivator PGC-1, a master regulator of gluconeogenic gene expression. Mice lacking functional CREB develop low blood sugar during fasting because their livers cannot properly turn on the gluconeogenic program.7PubMed. CREB regulates hepatic gluconeogenesis through the coactivator PGC-1
Insulin is the primary brake. After you eat, rising insulin levels suppress gluconeogenesis by activating signaling pathways that phosphorylate and inactivate FoxO1, a transcription factor that otherwise keeps gluconeogenic genes switched on. When FoxO1 is phosphorylated, it gets shuttled out of the nucleus and can no longer drive PEPCK and FBPase expression.8PubMed Central. Insulin regulation of gluconeogenesis This is why a healthy liver stops making glucose shortly after a meal, when insulin is high and there is plenty of glucose arriving from digestion.
Beyond hormones, the pathway is also regulated at the molecular level by metabolites themselves. Fructose-2,6-bisphosphate is a powerful allosteric signal: when its levels are high (in the fed state), it activates glycolysis and inhibits FBPase, effectively shutting down gluconeogenesis. When its levels fall (during fasting), the brake comes off and gluconeogenesis ramps up.9PubMed. Roles for fructose-2,6-bisphosphate in the control of fuel metabolism: beyond its allosteric effects on glycolytic and gluconeogenic enzymes This kind of instant, enzyme-level control complements the slower, gene-expression-level control exerted by hormones.
What Happens as a Fast Gets Longer
The liver has two ways to release glucose: breaking down stored glycogen (glycogenolysis) and building new glucose from scratch (gluconeogenesis). In the first several hours after a meal, glycogenolysis does most of the work. But glycogen stores are finite, typically enough to cover roughly 12 to 24 hours of normal metabolic demand, and as they deplete, gluconeogenesis picks up the slack.10PubMed Central. Energy metabolism in the liver
Detailed measurements in fasting volunteers illustrate this shift vividly. After a 12-hour fast, gluconeogenesis contributed about 41% of glucose production, with the rest coming from glycogen breakdown and other sources. By 20 hours, that fraction had risen to roughly 71%. By 40 hours, gluconeogenesis accounted for about 92% of glucose output.11PubMed. Gluconeogenesis and the Cori cycle in 12-, 20-, and 40-h-fasted humans Blood glucose dropped from about 93 mg/dL at 12 hours to about 71 mg/dL at 40 hours, but it did not collapse, because gluconeogenesis ramped up aggressively enough to maintain a functional level. The Cori cycle also grew in importance, with recycling of glucose molecules roughly doubling from 18% at 12 hours to 36% at 40 hours, showing how the body gets increasingly thrifty with its carbon resources as the fast extends.11PubMed. Gluconeogenesis and the Cori cycle in 12-, 20-, and 40-h-fasted humans
The Kidney Contributes More Than People Think
The liver gets nearly all the attention as the body’s glucose factory, and for good reason: it is the primary bulk contributor to gluconeogenesis under most conditions, partly because it is a large organ (over a kilogram in an average adult). But the kidneys are underappreciated players. Each kidney weighs only about 100 grams, yet on a per-gram basis, kidney cells may be just as efficient at making glucose as liver cells.12PubMed Central. Liver or kidney: Who has the oar in the gluconeogenesis boat and when? Renal gluconeogenesis occurs exclusively in the proximal tubule cells of the kidney cortex, and unlike the liver, the kidney releases glucose almost entirely through gluconeogenesis rather than glycogen breakdown.
The two organs also show a preference for different raw materials. The liver is the primary site for converting alanine to glucose, while the kidney preferentially uses glutamine.13PubMed. Human kidney and liver gluconeogenesis: evidence for organ substrate selectivity During epinephrine-driven stress responses, both organs increase gluconeogenesis substantially, with lactate serving as the dominant precursor for both liver and kidney under those conditions.14PubMed. Relative importance of liver, kidney, and substrates in epinephrine-induced increased gluconeogenesis in humans The kidney’s contribution becomes particularly relevant in conditions like liver disease, where the remaining liver tissue may not be able to meet the body’s glucose needs alone.
Gluconeogenesis and Ketogenesis Are Linked
During fasting, the liver is doing two big jobs simultaneously: making glucose through gluconeogenesis and making ketone bodies through ketogenesis. These two processes are tightly connected because they depend on the same initial fuel source: fatty acids. When fat is broken down in the liver’s mitochondria, it produces acetyl-CoA. Some of that acetyl-CoA gets channeled toward ketone body production, providing an alternative fuel for the brain and other tissues. The rest feeds into the TCA cycle, generating the ATP and reducing equivalents that power gluconeogenesis.15Journal of Biological Chemistry. Pyruvate carboxylase is required for hepatic gluconeogenesis but preserves metabolic health during fasting
In healthy fasting individuals, ketogenesis accounts for the large majority of acetyl-CoA disposal, roughly 86%, with the remainder going to TCA cycle oxidation. In people with non-alcoholic fatty liver disease, this balance shifts: ketogenesis drops to about 77% of acetyl-CoA disposal, and more acetyl-CoA is diverted to TCA cycle oxidation.16The Journal of Clinical Investigation. Impaired ketogenesis and increased acetyl-CoA oxidation promote hyperglycemia in human fatty liver This shift helps explain why fatty liver is associated with excessive glucose production: more fuel gets funneled toward processes that support gluconeogenesis at the expense of ketone body production.
There may even be cross-talk at the level of mitochondrial transport. Experiments on isolated liver cells found that the ketone body 3-hydroxybutyrate stimulated gluconeogenesis from lactate by about 41%, while acetoacetate (another ketone body) had the opposite effect and could abolish the stimulation caused by other substrates. Researchers proposed that pyruvate and acetoacetate may compete for the same mitochondrial carrier, creating a direct link between how fast the liver makes ketones and how fast it makes glucose.17PubMed. Possible interrelationship between gluconeogenesis and ketogenesis in the liver
When Gluconeogenesis Goes Wrong in Diabetes and Fatty Liver
In type 2 diabetes, one of the central problems is that the liver keeps making glucose even when blood sugar is already high. Normally, rising insulin after a meal shuts gluconeogenesis down. But in insulin-resistant individuals, the liver fails to respond to that stop signal. Gluconeogenesis stays inappropriately active in the fed state, pumping glucose into the bloodstream on top of whatever glucose is arriving from digested food.8PubMed Central. Insulin regulation of gluconeogenesis
Non-alcoholic fatty liver disease makes this worse. Both NAFLD and type 2 diabetes are characterized by high circulating insulin, hepatic insulin resistance, and elevated hepatic glucose production driven disproportionately by gluconeogenesis. The fasting hyperglycemia typical of type 2 diabetes tracks closely with how much gluconeogenesis is increased, while postprandial (after-meal) hyperglycemia reflects the liver’s failure to suppress glucose output when insulin levels rise.18PubMed. The role of the liver in the modulation of glucose and insulin in non alcoholic fatty liver disease and type 2 diabetes This is why controlling hepatic glucose output is a primary therapeutic target in managing type 2 diabetes.
How Metformin Tames the Liver
Metformin, the most widely prescribed drug for type 2 diabetes, works in large part by suppressing hepatic gluconeogenesis. For years, the standard explanation was that metformin inhibits mitochondrial complex I, reducing the cell’s energy charge and activating AMP-activated protein kinase, which in turn dials down glucose production. But the picture has gotten more complicated. Studies in mice and humans show that at clinically relevant doses, metformin inhibits gluconeogenesis through a redox-dependent mechanism that does not depend on reductions in overall energy charge or complex I inhibition in the way originally proposed.19PubMed Central. Metformin Inhibits Gluconeogenesis by a Redox-Dependent Mechanism In Vivo
A separate line of research has identified a more direct target: metformin inhibits fructose-1,6-bisphosphatase (FBP1), one of the three bypass enzymes that makes gluconeogenesis possible. Rising AMP levels in the liver cell, triggered by metformin, directly inhibit FBP1, blocking a rate-controlling step in the pathway.20PubMed Central. Metformin reduces liver glucose production by inhibition of fructose-1-6-bisphosphatase The truth is probably that metformin works through multiple mechanisms simultaneously, which may explain why it has been so clinically effective for decades even as scientists continue to debate exactly how it works.
Why Alcohol Drops Your Blood Sugar
There is a well-known clinical phenomenon where heavy drinking on an empty stomach can cause dangerously low blood sugar. The explanation lies directly in gluconeogenesis. When the liver metabolizes alcohol, the enzyme that breaks down ethanol (alcohol dehydrogenase) consumes a coenzyme called NAD+ and generates NADH. This dramatically shifts the ratio of NAD+ to NADH inside the liver cell. That shift lowers the concentration of pyruvate, because the reaction that converts lactate to pyruvate (which requires NAD+) slows down. Since pyruvate is the entry point for gluconeogenesis from lactate, alanine, and serine, the entire pathway gets choked at its first step.21PubMed Central. Inhibition of hepatic gluconeogenesis by ethanol
This matters most when someone drinks without eating, because glycogen stores are already depleted and the body is depending heavily on gluconeogenesis to maintain blood sugar. It also has implications for people with diabetes who take glucose-lowering medication: combining those drugs with alcohol can compound the drop in blood sugar and lead to hypoglycemia.
Gluconeogenesis in Newborns
One of the more remarkable aspects of gluconeogenesis is how abruptly it switches on at birth. In the fetus, the pathway is essentially silent. The developing baby receives a continuous glucose supply from the mother across the placenta, and there is no need for independent glucose production. The rate-limiting enzyme PEPCK is absent from the fetal liver under normal conditions.22PubMed. Hormonal control of the development of hepatic gluconeogenesis in the neonate
At birth, everything changes within hours. The umbilical cord is cut, the glucose supply from the mother stops, and the newborn’s blood sugar drops. The hormonal shift is dramatic: glucagon surges, insulin falls, and liver cyclic AMP rises. These signals switch on PEPCK gene transcription, and the enzyme appears rapidly in the newborn liver. Gluconeogenesis and ketogenesis both emerge after birth and typically reach adult-level capacity within about 24 hours.23PubMed. Metabolic adaptations to change of nutrition at birth In human newborns, gluconeogenesis appears soon after birth and contributes an estimated 30% to 70% of the glucose produced in the immediate postnatal period.24PubMed. Gluconeogenesis in the fetus and neonate That wide range reflects how variable the transition can be from infant to infant, depending on birth weight, gestational age, and feeding timing. For premature babies or those who are small for gestational age, the failure to activate gluconeogenesis efficiently is one reason neonatal hypoglycemia is so common and closely monitored.
How Diet Influences Hepatic Glucose Output
Dietary composition can alter gluconeogenesis in ways that are not always intuitive. Fructose, for instance, is metabolized almost entirely by the liver, and unlike glucose it enters the metabolic pathway below the main regulatory checkpoints. The liver converts fructose to intermediates that feed directly into gluconeogenesis. High fructose intake has been shown to increase hepatic gluconeogenesis and raise fasting blood glucose and triglyceride levels.25PubMed Central. Fructose Consumption, Lipogenesis, and Non-Alcoholic Fatty Liver Disease This is one of the mechanisms by which excessive fructose consumption, particularly from sweetened beverages and processed foods, is thought to contribute to fatty liver and metabolic dysfunction.
Very low-carbohydrate and ketogenic diets produce their own interesting gluconeogenic dynamics. When carbohydrate intake is extremely low, the body relies heavily on gluconeogenesis to supply the tissues that absolutely require glucose, such as red blood cells. In animal studies, ketogenic feeding reduced hepatic glycogen, lowered insulin levels, and suppressed PEPCK activity, while increasing lactate and ketone body concentrations.26PubMed Central. The Biochemical Role of Bitter Melon (Momordica charantia L.) Aqueous Extract in Regulating Hepatic Gluconeogenesis in Rats Fed Ketogenic Diet The relationship between ketogenic diets and gluconeogenesis is more nuanced than the popular narrative that “the liver just makes all the glucose you need.” In practice, gluconeogenesis operates under tight hormonal control and does not simply ramp up to replace whatever carbohydrate you have removed from your diet. Protein intake, cortisol levels, and the availability of gluconeogenic substrates all influence how much glucose the liver produces.
Splanchnic glucose output measurements after an overnight fast illustrate the liver’s overall throughput: roughly 123 grams of glucose released per 24 hours, with an estimated 99 grams of that coming from gluconeogenesis alone.27PubMed Central. Hepatic ketogenesis and gluconeogenesis in humans That is a substantial amount of glucose, roughly equivalent to the carbohydrate content of two medium potatoes, produced entirely from non-carbohydrate sources while you sleep. It underscores why hepatic gluconeogenesis is not just a backup system for emergencies. It is an active, continuous process that plays a central role in everyday metabolism.