The liver is the body’s dominant glucose-manufacturing organ, responsible for the large majority of new glucose that enters your bloodstream between meals and overnight. But it does not work alone. The kidneys, the small intestine, and even certain brain cells can also build glucose from scratch, each stepping in under particular circumstances. How much each organ contributes depends on whether you have just eaten, how long you have been fasting, whether you are exercising, and whether conditions like diabetes have shifted the balance.
The Liver Does Most of the Heavy Lifting
Your liver produces glucose in two distinct ways. The first is by breaking down glycogen, a stored form of glucose that the liver packs away after meals. The second is gluconeogenesis, which literally means “new glucose creation,” the assembly of glucose molecules from non-sugar raw materials like lactate, amino acids, and glycerol. Together, these two pathways let the liver regulate how much glucose it releases into the blood, keeping levels stable even when you are not eating.1PubMed Central. Molecular pathophysiology of hepatic glucose production
Right after an overnight fast, most of the glucose your liver puts out actually comes from glycogen breakdown rather than from building new glucose. In one study that tracked these pathways in fasting humans using isotope-labeled tracers, about 36% of liver glucose output came from gluconeogenesis after an overnight fast, with glycogen breakdown covering the rest.2JCI Insight. Hepatic gluconeogenic fluxes and glycogen turnover during fasting in humans. A stable isotope study That ratio flips dramatically if the fast continues. After a prolonged fast, gluconeogenesis accounted for roughly 78% of liver glucose output in the same study, because glycogen stores had been substantially depleted. The liver essentially switches gears: early in a fast it draws down its glycogen reserves, but as those run low, it increasingly builds glucose from whatever substrates are available.3PubMed Central. Energy metabolism in the liver
The Kidneys as a Second Glucose Factory
For decades, the textbook answer to “where is glucose made?” was simply “the liver.” The kidneys were acknowledged to contribute a little glucose during extreme conditions like prolonged starvation or metabolic acidosis, but they were treated as a footnote.4PubMed Central. Bench-to-bedside review: glucose production from the kidney That picture has changed. Physiological and molecular work over the past few decades has established the kidney as a genuine second site of glucose production, not just a backup for emergencies.5PubMed. Renal gluconeogenesis: a key metabolic hub in health and kidney disease
The kidney contributes an estimated 20% of all glucose production during the post-absorptive phase, the stretch between meals when your body relies on internally made glucose.6PubMed Central. Renal gluconeogenesis in insulin resistance: A culprit for hyperglycemia in diabetes Unlike the liver, however, the kidneys cannot store glycogen in meaningful quantities and release it. Their contribution comes entirely from gluconeogenesis.
Within the kidney, this glucose-making machinery is concentrated in a very specific location: the proximal tubule, which is the first stretch of the tiny tubes that process urine. The enzymes needed for gluconeogenesis are found almost exclusively there, with the highest levels in the earliest, convoluted portion of the tubule.7PubMed. Distribution along the rat nephron of three enzymes of gluconeogenesis in acidosis and starvation During acidosis, when the blood becomes too acidic, one key enzyme in the proximal convoluted tubule can increase roughly fourfold, ramping up glucose production. Starvation triggers its own distinct pattern of enzyme changes along the tubule. The kidney adapts its gluconeogenic output to nutritional state, acid-base conditions, and hormonal signals, integrating all of these inputs to decide how much glucose to release.5PubMed. Renal gluconeogenesis: a key metabolic hub in health and kidney disease
The Small Intestine and an Unexpected Link to Appetite
The intestine is not a place most people would expect to find glucose being made. Its primary job, after all, is to absorb dietary glucose from food. Yet the small intestine expresses the enzymes for gluconeogenesis and can produce glucose on its own, particularly after eating a protein-rich meal.
Research in rodents showed that when animals eat a high-protein diet, the intestine ramps up its own glucose production during the period after food has been digested. This newly made glucose is released into the portal vein, the blood vessel that carries blood from the gut to the liver.8Cell Metabolism. Intestinal Gluconeogenesis Yields a Novel Mechanism for Protein-Induced Anorexia The portal vein wall contains glucose sensors, and when they detect this trickle of new glucose, they send signals to hypothalamic brain regions involved in regulating hunger. The result is a reduction in food intake. When the portal vein nerve was severed in experiments, the satiety effect disappeared, confirming that the signal travels along that nerve pathway.
To test whether intestinal glucose production was truly responsible, researchers engineered mice whose intestinal cells lacked glucose-6-phosphatase, the enzyme required for the final step of gluconeogenesis. These mice were completely insensitive to the appetite-suppressing effect of a high-protein diet, while normal mice ate less on the same diet.9PubMed. Protein-induced satiety is abolished in the absence of intestinal gluconeogenesis This provided strong evidence that intestinal gluconeogenesis is not a metabolic curiosity but an active participant in how your body regulates hunger in response to what you eat.10PubMed. Intestinal gluconeogenesis and protein diet: future directions
The intestine’s overall contribution to whole-body glucose supply is much smaller than the liver’s or kidneys’. Its significance lies less in volume and more in the signaling it triggers. The glucose it makes acts as a messenger that influences brain circuits controlling appetite, not as a major fuel source.
Glucose Synthesis in the Brain
The brain is famously a glucose consumer, burning through a large share of the body’s daily glucose supply. The idea that it could also produce glucose locally seems counterintuitive, but there is evidence that a specific type of brain cell can do exactly that. Astrocytes, the star-shaped support cells that surround neurons, have been shown to possess gluconeogenic activity.11PubMed Central. Cerebral Gluconeogenesis and Diseases
Astrocytes are already known for using glycolysis to produce lactate, which they shuttle over to neurons as fuel. The gluconeogenic pathway in astrocytes may function as an alternative local glucose source, and interest in it has grown particularly in the context of ischemic stroke and brain tumors, conditions where normal glucose delivery from the blood can be disrupted. No convincing evidence has been found for gluconeogenesis in neurons themselves. The quantities involved are small compared to what the liver sends to the brain through the bloodstream, but the ability to generate glucose locally could matter when blood flow is compromised.
Hormonal Signals That Turn Glucose Production Up or Down
Glucose synthesis is not left to run on autopilot. A network of hormones tells these organs when to ramp production up and when to shut it down. The two most important players are insulin and glucagon, both made by the pancreas.
After you eat, rising blood sugar prompts your pancreas to release insulin. Insulin signals the liver to stop producing glucose, partly by suppressing the activity of key gluconeogenic enzymes at the gene level.12PubMed Central. Insulin regulation of gluconeogenesis When blood sugar drops during fasting, glucagon and cortisol take over. These fasting hormones activate the genes that encode the enzymes driving gluconeogenesis, particularly two critical ones often referred to by their abbreviations PEPCK and G6Pase. The result is increased glucose output from the liver to keep blood sugar from falling too low.13PubMed Central. CREB and FoxO1: two transcription factors for the regulation of hepatic gluconeogenesis
Insulin does not just flip a single switch. It works through several molecular pathways that either modify the proteins responsible for gluconeogenesis or activate proteins that suppress them. This redundancy helps explain why the system is so finely tuned under normal conditions and why things go wrong so thoroughly in insulin-resistant states.
What Happens During Exercise
Physical activity creates a spike in glucose demand from working muscles. Your body meets this demand through a combination of glycogen breakdown and gluconeogenesis, both ramped up in the liver. Research comparing trained and untrained men during exercise found that gluconeogenesis plays an essential role in maintaining glucose production regardless of fitness level.14PubMed Central. Gluconeogenesis and hepatic glycogenolysis during exercise at the lactate threshold
The lactate your muscles produce during intense exercise is not simply a waste product. It circulates back to the liver, where it serves as a raw material for gluconeogenesis. In untrained subjects exercising near their lactate threshold, experimentally raising lactate levels roughly doubled gluconeogenic output. Trained individuals showed lower gluconeogenesis at the same workloads, likely because their bodies are more efficient at managing fuel. But in all groups, gluconeogenesis contributed meaningfully to keeping blood glucose stable during sustained effort.
When Glucose Production Goes Wrong in Diabetes
In type 2 diabetes, the tightly controlled system of glucose production becomes dysregulated. The liver, which should shut down glucose output after a meal in response to insulin, keeps producing glucose because the insulin signal is not getting through effectively. This is one of the major reasons fasting blood sugar runs high in people with type 2 diabetes.
The kidneys add to the problem. In people with diabetes, the rate of renal glucose release increases beyond normal levels, contributing to both fasting hyperglycemia and elevated blood sugar after meals.6PubMed Central. Renal gluconeogenesis in insulin resistance: A culprit for hyperglycemia in diabetes Studies in both diabetic rodents and humans have found increased expression of gluconeogenic enzymes in the kidneys, suggesting that the kidney actively ramps up glucose production in an insulin-resistant state rather than just passively overproducing it. This dual source of excess glucose, from both the liver and the kidneys, helps explain why blood sugar can be so stubbornly difficult to control in diabetes.
Metformin, the most widely prescribed drug for type 2 diabetes, works in large part by targeting this problem. Rather than increasing insulin levels or sensitivity directly, metformin inhibits the liver’s gluconeogenic output. Studies in rats showed that at concentrations matching what patients achieve with standard doses, metformin blocks gluconeogenesis from certain substrates by altering the cell’s internal chemical balance, specifically by shifting the ratio of oxidized and reduced molecules in the cell’s cytoplasm.15PubMed Central. Metformin inhibits gluconeogenesis by a redox-dependent mechanism in vivo When researchers corrected this chemical shift by infusing a compound that reverses it, metformin’s glucose-lowering effect vanished, confirming the mechanism.16Nature Medicine. Metformin inhibits gluconeogenesis via a redox-dependent mechanism in vivo
How Newborns Start Making Their Own Glucose
Before birth, the fetus relies on its mother’s blood supply for glucose and does not need to produce its own. The enzymatic machinery for gluconeogenesis exists in fetal tissue, but one rate-limiting enzyme, PEPCK, does not appear until after birth. This means that while the fetus has the potential for gluconeogenesis, it does not actually produce glucose from non-sugar sources until delivery triggers the process.17PubMed. Gluconeogenesis in the fetus and neonate
Once a newborn is born, gluconeogenesis kicks in quickly and becomes a major contributor to the baby’s glucose supply. Available data suggest that gluconeogenesis accounts for somewhere between 30% and 70% of glucose produced in the human newborn. That is a remarkably wide range, which reflects both the difficulty of measuring this in tiny patients and the variability that comes with different feeding situations and birth circumstances. The rapid switch-on of this pathway is critical, because the sudden loss of maternal glucose supply at birth could otherwise cause dangerously low blood sugar.
Cancer Cells and Hijacked Glucose Pathways
Cancer cells are famous for their enormous appetite for glucose, a trait known as the Warburg effect. They burn through glucose at high rates even when oxygen is plentiful, using the energy and building blocks for rapid growth. What has surprised researchers more recently is that some cancers also express gluconeogenic enzymes, running what appear to be truncated or partial versions of gluconeogenesis.
Rather than making glucose to release into the bloodstream the way the liver does, these cancer cells seem to use pieces of the gluconeogenic pathway in reverse to feed biosynthetic processes. When glucose is scarce in the tumor environment, cancer cells expressing the enzyme PEPCK can synthesize crucial metabolites from lactate or amino acids, funneling them into pathways that branch off the normal route between glucose and pyruvate.18PubMed Central. Gluconeogenesis in cancer cells – Repurposing of a starvation-induced metabolic pathway? Some types of cancer appear to use this truncated gluconeogenesis to gain metabolic flexibility, redistributing glucose-derived carbon for antioxidant production and other survival needs.19Trends in Cancer. Gluconeogenesis and cancer: from mechanisms to therapeutic opportunities
The gluconeogenic enzymes PEPCK, FBPase, and G6Pase have also been found to play roles beyond metabolism in cancer cells, influencing signaling, proliferation, and the behavior of cancer stem cells.20PubMed. Gluconeogenesis in Cancer: Function and Regulation of PEPCK, FBPase, and G6Pase This has opened up interest in whether targeting these enzymes could offer new therapeutic strategies, though that work remains early-stage.
How Researchers Actually Measure Glucose Production
Measuring where glucose comes from inside a living person is not straightforward. You cannot simply take a blood sample and tell whether a glucose molecule was made by the liver, the kidneys, or the intestine. Researchers have developed several clever approaches over the years.
One pioneering method used carbon-13 nuclear magnetic resonance spectroscopy to directly measure glycogen concentrations in the liver of living, conscious humans at intervals during a 68-hour fast. By watching how quickly glycogen levels dropped and comparing that rate to total glucose production measured with a different tracer, the researchers could calculate how much glucose was coming from glycogen breakdown versus how much was being newly synthesized.21PubMed. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR Other approaches use isotope-labeled substrates like deuterated water or carbon-13-labeled lactate, following how these labels get incorporated into glucose molecules to determine which pathway produced them.
These techniques are part of why our understanding of glucose production has shifted over recent decades. Older “net organ balance” methods, which measured glucose going into and out of an organ by sampling blood on either side, tended to underestimate the kidney’s contribution because the kidney simultaneously uses and produces glucose. More sophisticated tracer methods revealed that the kidney’s total glucose output is larger than the net balance suggests, which is how the kidney went from a footnote in glucose metabolism to a recognized contributor worth studying in its own right.4PubMed Central. Bench-to-bedside review: glucose production from the kidney