Gluconeogenesis is the process by which your body builds new glucose molecules from raw materials that are not themselves carbohydrates. The liver handles most of this work, stitching together glucose from leftovers like lactate, certain amino acids, and glycerol released from fat stores. This pathway kicks in whenever dietary glucose runs low, whether you have gone overnight without eating, are deep into a multi-day fast, or are burning through fuel during prolonged exercise. What makes it interesting is not just that the body can manufacture its own sugar, but the elaborate system of hormonal signals, organ-to-organ recycling loops, and molecular switches that keep the process tightly controlled.
What the Body Uses as Raw Material
Gluconeogenesis does not create something from nothing. It reassembles glucose from three main categories of non-sugar building blocks, each arriving in the liver from a different part of the body.
- Lactate: Produced by muscles during intense activity and by red blood cells (which lack mitochondria and always ferment glucose), lactate is the single largest contributor to new glucose production. The liver takes it up, converts it back to pyruvate, and runs the gluconeogenic pathway forward to glucose.
- Amino acids: When protein is broken down, several amino acids can feed into gluconeogenesis. Alanine is the most important one quantitatively. Others, like glutamine, are particularly relevant in the kidneys.
- Glycerol: When fat cells break down stored triglycerides for energy, each triglyceride molecule releases one glycerol backbone. That glycerol travels to the liver and enters the gluconeogenic pathway partway through.
Which of these substrates dominates depends on your metabolic state. After an ordinary overnight fast, lactate recycling accounts for a large share. During prolonged starvation lasting days, glycerol from accelerated fat breakdown becomes increasingly important, contributing roughly a fifth of total glucose production after two to three days without food, up from under five percent after a normal overnight fast.1PubMed. Glycerol gluconeogenesis in fasting humans Amino acid contributions also rise during extended fasts as the body starts breaking down muscle protein to keep the glucose supply going.
The Recycling Loops Between Organs
A striking feature of gluconeogenesis is that much of it is not truly “new” glucose at all but recycled glucose. Two named cycles describe how this works.
The Cori cycle is the loop between muscles and the liver. Muscles burn glucose, producing lactate as a byproduct. That lactate circulates to the liver, which spends energy converting it back into glucose, then ships the glucose back out for muscles to use again. Researchers have tracked this recycling process across different fasting durations and confirmed it accounts for a meaningful fraction of the glucose appearing in the blood after an overnight fast.2PubMed. Gluconeogenesis and the Cori cycle in 12-, 20-, and 40-h-fasted humans The cycle is metabolically expensive for the liver but allows muscles to keep working under conditions where oxygen is limited.
The Cahill cycle, also called the glucose-alanine cycle, works similarly but uses the amino acid alanine instead of lactate. Muscles break down amino acids and transfer their nitrogen onto pyruvate, forming alanine. The liver takes up that alanine, strips off the nitrogen (which eventually becomes urea), and converts the remaining carbon skeleton into glucose.3JCI Insight. Hungry for your alanine: when liver depends on muscle proteolysis This cycle becomes more prominent during fasting, when muscle protein is being mobilized to feed gluconeogenesis.
How Hormones Run the Show
Your body does not leave gluconeogenesis running at full blast all the time. A handful of hormones act as the main on-off switches.
Glucagon, released by the pancreas when blood sugar drops, is the primary signal that turns gluconeogenesis on. It activates gene expression in liver cells that ramp up the enzymes needed to produce glucose. Cortisol, the stress hormone released by the adrenal glands, amplifies glucagon’s effects. Research in liver cells shows that glucagon and cortisol together drive amino acid breakdown and glucose production more powerfully than either hormone alone, a synergistic effect that depends on two specific gene-activating proteins working in concert.4PubMed Central. Fasting Hormones Synergistically Induce Amino Acid Catabolism Genes to Promote Gluconeogenesis Older work on isolated liver cells confirmed this pattern: glucocorticoids amplify glucagon’s long-term stimulation of gluconeogenic enzyme activity.5PubMed. Coordinate regulation of gluconeogenesis by the glucocorticoids and glucagon: evidence for acute and chronic regulation by glucagon
Insulin is the counterweight. After a meal, rising insulin suppresses gluconeogenesis so the liver stops pumping out glucose while you are absorbing it from food. This suppression works through a signaling chain involving a protein called FOXO1, which normally helps activate gluconeogenic genes. Insulin triggers FOXO1 to be tagged and moved out of the nucleus, effectively silencing those genes.6Nature. Insulin-regulated hepatic gluconeogenesis through FOXO1–PGC-1α interaction Mouse studies have shown that deleting FOXO1 from liver cells restores normal glucose control even when insulin signaling itself is broken, underscoring how central this single protein is to the whole regulatory circuit.7PubMed Central. Hepatic Insulin Signaling is Dispensable for Suppression of Glucose Output by Insulin in Vivo
The Molecular Toggle That Prevents Traffic Jams
Gluconeogenesis and glycolysis (the pathway that breaks glucose down) share many of the same enzymes, just running in opposite directions. The body needs a way to prevent both pathways from running at the same time, which would waste energy in a futile loop. A small signaling molecule called fructose-2,6-bisphosphate acts as the main traffic controller. When its levels are high, it activates a key glycolysis enzyme and simultaneously inhibits a key gluconeogenesis enzyme, pushing the system toward glucose breakdown.8PubMed. Roles for fructose-2,6-bisphosphate in the control of fuel metabolism: beyond its allosteric effects on glycolytic and gluconeogenic enzymes When levels drop, as they do in response to glucagon, the inhibition lifts and gluconeogenesis can proceed. This regulatory molecule has been demonstrated to work the same way across species, from chicken liver to human liver.9PubMed. Role of fructose 2,6-bisphosphate in the regulation of glycolysis and gluconeogenesis in chicken liver
There is also a compartmentalization puzzle the cell has to solve. One critical intermediate in the pathway, oxaloacetate, is produced inside the mitochondria but needs to reach the cytoplasm where most gluconeogenic enzymes operate. The problem is that oxaloacetate itself cannot cross the mitochondrial membrane. Instead, it is temporarily converted to either malate or aspartate, shuttled across the membrane, and then converted back.10PubMed. Roles of malate and aspartate in gluconeogenesis in various physiological and pathological states The transporter responsible for the aspartate route is a protein called citrin, which swaps aspartate for glutamate across the mitochondrial membrane and also participates in the broader shuttle system that moves electrons between cellular compartments.11PubMed Central. Aspartate-glutamate carrier 2 (citrin): a role in glucose and amino acid metabolism in the liver
Not Just the Liver
The liver handles the lion’s share of gluconeogenesis, but it is not the only organ that can make glucose from scratch. The kidneys and the small intestine also contribute, and each does so for somewhat different reasons.
The kidneys ramp up gluconeogenesis during metabolic acidosis, when the blood becomes too acidic. The preferred fuel for this renal glucose production is glutamine, an amino acid. When kidney cells break down glutamine, the process generates ammonium ions, which are excreted in the urine to help clear excess acid, and bicarbonate, which buffers the blood.12PubMed. Mechanism of increased renal gene expression during metabolic acidosis The glucose produced along the way is released into the bloodstream. So in the kidneys, gluconeogenesis serves double duty: it both supplies glucose and helps regulate the body’s acid-base balance. When the acidosis resolves, kidney glutamine metabolism and glucose production return to baseline.13PubMed Central. Glutamine metabolism in the kidney during induction of, and recovery from, metabolic acidosis in the rat
The intestine’s role is more recently discovered and has surprising implications for appetite. After digesting a protein-rich meal, cells in the small intestine activate gluconeogenic enzymes and release newly made glucose into the portal vein, the vessel that carries blood from the gut to the liver. Nerves surrounding the portal vein detect this glucose, sending signals to the brain that reduce hunger and improve insulin sensitivity.14Cell Metabolism. Intestinal Gluconeogenesis Is Key to Sensing Protein in Satiety and Portal Glucose When the portal vein’s nerve connections were severed in rats, both the appetite-suppressing effect and the brain activation pattern disappeared. This has led researchers to propose intestinal gluconeogenesis as a mechanism explaining why high-protein diets tend to be more satiating.15PubMed. Hypothalamic integration of portal glucose signals and control of food intake and insulin sensitivity More broadly, this intestinal glucose signaling appears to influence whole-body energy management in ways researchers are still mapping out.16PubMed. Gut-Brain Glucose Signaling in Energy Homeostasis
Gluconeogenesis During Exercise and Recovery
During prolonged or intense exercise, your muscles devour glucose far faster than normal. The liver responds by increasing both glycogen breakdown and gluconeogenesis to keep blood sugar from dropping too far. Lactate flowing from working muscles to the liver fuels a large share of this exercise-driven glucose production through the Cori cycle.
What happens after you stop is equally interesting. As muscle oxygen consumption drops during recovery, less lactate gets burned directly for energy. Instead, the liver shifts toward using accumulated lactate as a raw material for gluconeogenesis, producing glucose that helps replenish muscle glycogen stores.17PubMed. The lactate shuttle during exercise and recovery This helps explain the blood sugar rebound some people notice after a hard workout. Studies have found that after short, high-intensity exercise, blood glucose rises during recovery, and the pattern correlates with lactate availability, consistent with lactate feeding post-exercise gluconeogenesis.18Frontiers in Nutrition. Delayed Rebound of Glycemia During Recovery Following Short-Duration High-Intensity Exercise: Are There Lactate and Glucose Metabolism Interactions?
When Gluconeogenesis Drives High Blood Sugar
In type 2 diabetes, the normal regulation breaks down. Insulin resistance in the liver means that insulin’s suppressive signal no longer shuts gluconeogenesis down effectively, so the liver keeps producing glucose even when blood sugar is already elevated. This over-active hepatic glucose production is considered a major contributor to the high fasting blood sugar characteristic of the disease.19PubMed. Increased hepatic gluconeogenesis and type 2 diabetes mellitus
Studies measuring actual gluconeogenic rates in people with type 2 diabetes have confirmed this. After an overnight fast, patients with the disease showed higher absolute rates of gluconeogenesis and a larger share of total liver glucose output coming from gluconeogenesis compared with matched controls.20PubMed. Gluconeogenesis in moderately and severely hyperglycemic patients with type 2 diabetes mellitus The same research suggested that this increase becomes pronounced under conditions of significant hepatic insulin deficiency, pointing to it as a feature that worsens as the disease progresses.
Fat metabolism compounds the problem. People with type 2 diabetes show higher rates of lipolysis, meaning more glycerol is being released from fat stores. That extra glycerol feeds directly into gluconeogenesis, adding fuel to the fire. Glycerol-derived glucose production was roughly 70 percent higher in patients with type 2 diabetes than in matched controls in one study.21The Journal of Clinical Endocrinology & Metabolism. Lipolysis and gluconeogenesis from glycerol are increased in patients with noninsulin-dependent diabetes mellitus Certain newer diabetes medications, such as SGLT2 inhibitors like empagliflozin, can actually stimulate gluconeogenesis by augmenting lipolysis and increasing glycerol availability, which is something clinicians consider when evaluating these drugs’ metabolic effects.22Diabetes. 1274-OR: Effects of Empagliflozin on Hepatic Glucose Production, Gluconeogenesis, and Lipolysis in Type 2 Diabetes
How Metformin Puts the Brakes On
Metformin, the most widely prescribed drug for type 2 diabetes, works in large part by suppressing hepatic gluconeogenesis. For decades, researchers debated exactly how it does this, and the picture that has emerged involves multiple mechanisms rather than a single clean target.
One line of evidence shows that metformin inhibits a mitochondrial enzyme called glycerophosphate dehydrogenase, which alters the balance of molecules the liver uses to convert lactate and glycerol into glucose. By disrupting this enzyme, metformin shifts the cell’s internal chemistry in a way that slows glucose production.23Nature. Metformin suppresses gluconeogenesis by inhibiting mitochondrial glycerophosphate dehydrogenase A separate investigation demonstrated that metformin, at concentrations matching what patients actually achieve in their blood, inhibits gluconeogenesis through a mechanism dependent on changing the cell’s internal redox balance, independent of effects on several other pathways previously suspected.24PubMed Central. Metformin inhibits gluconeogenesis by a redox-dependent mechanism in vivo
A third study identified the gluconeogenic enzyme fructose-1,6-bisphosphatase as another contributor to metformin’s effect. When researchers engineered mice carrying a mutant version of this enzyme that could not be inhibited by AMP (the molecule metformin raises), the mice responded significantly less to metformin treatment.25PubMed Central. Metformin reduces liver glucose production by inhibition of fructose-1-6-bisphosphatase The emerging view is that metformin does not have one magic bullet target but instead hits gluconeogenesis from several angles, which may help explain why it is so effective clinically.
Low-Carb Diets and Gluconeogenic Rates
A common assumption is that people eating very few carbohydrates must be churning out much more glucose through gluconeogenesis to compensate. The actual data from endurance athletes tells a more nuanced story. Cyclists habituated to a long-term low-carbohydrate, high-fat diet had lower overall glucose production during exercise compared to those on a mixed diet, primarily because they drew less on glycogen. But their gluconeogenic rates were essentially the same as the mixed-diet group, both at rest and during exercise.26PubMed Central. Gluconeogenesis during endurance exercise in cyclists habituated to a long-term low carbohydrate high-fat diet Rather than ramping up glucose synthesis to replace missing dietary carbohydrate, these athletes adapted by shifting their muscles to burn more fat directly, leaving gluconeogenesis largely unchanged. The body’s adaptation to carbohydrate restriction, in other words, leans more on altering which fuels tissues prefer than on forcing the liver to produce extra glucose.
How Quickly Newborns Start Making Glucose
Before birth, the fetus receives a steady glucose supply through the placenta. At delivery, that supply cuts off abruptly, and the newborn must begin producing its own glucose within hours. Research measuring gluconeogenic rates in newborns has shown that the pathway is active remarkably soon. In healthy full-term babies, gluconeogenesis from pyruvate contributed about 31 percent of total glucose production after just a five-hour fast. In preterm infants, the contribution was more variable, ranging from about 6 to 60 percent depending on how much intravenous glucose the baby was receiving.27PubMed. Estimation of gluconeogenesis in newborn infants The wide range in preterm babies reflects both the immaturity of their metabolic systems and the influence of clinical nutrition support. Either way, the ability to synthesize glucose is present from the first hours of life, not something that develops gradually over weeks.
Why the Process Costs So Much Energy
Gluconeogenesis is metabolically expensive. Building one molecule of glucose from two molecules of pyruvate costs the cell the equivalent of six high-energy phosphate bonds, drawn from ATP and GTP. In practice, the real energy cost tends to run even higher than the theoretical minimum, because the pathway is not perfectly efficient and some of the supporting reactions generate heat rather than useful work.28Metabolism. Energy cost of gluconeogenesis in rat liver This high cost is not a design flaw; it is what makes gluconeogenesis thermodynamically favorable in the forward direction and allows it to run even when conditions would stall a cheaper pathway. The energy is supplied primarily by fat oxidation in the liver, which is why prolonged fasting drives both fat burning and glucose synthesis simultaneously. The fat does not turn into glucose directly (a common misconception), but the energy from burning fat is what powers the liver’s glucose factory.
That energetic link between fat oxidation and gluconeogenesis helps explain a pattern that confuses people: during fasting, your body breaks down fat and makes glucose at the same time. These are not competing processes. Fat provides the fuel that the liver needs to run gluconeogenesis, while the raw materials for the glucose itself come from lactate, amino acids, and glycerol. The carbon skeletons of most fatty acids cannot be converted to glucose in humans because they are broken down into acetyl-CoA, which enters a cycle that fully oxidizes it rather than feeding into the gluconeogenic pathway. The glycerol backbone is the exception, and as noted earlier, its contribution grows during extended fasts.