Your liver acts as a glucose factory, continuously manufacturing and releasing sugar into the bloodstream to keep your brain, muscles, and other organs fueled between meals. It does this through two distinct processes: breaking down its stored glycogen (a starch-like reserve) and building brand-new glucose molecules from scratch using raw materials like amino acids and lactate. The balance between these two routes shifts depending on how long it has been since you last ate, and the whole operation is tightly regulated by hormones. When that regulation breaks down, the consequences ripple across your metabolism in ways that are central to type 2 diabetes, fatty liver disease, and several rarer genetic conditions.
Two Pathways, One Goal
The liver stores glucose in a compact form called glycogen, essentially long chains of glucose molecules packed together. When blood sugar starts to dip, the liver breaks these chains apart and releases free glucose into the bloodstream. This process, glycogenolysis, is fast. It can respond within minutes and is the body’s first line of defense against dropping blood sugar after a meal is digested.
The second pathway is gluconeogenesis, which literally means “making new glucose.” Instead of unpacking a stored reserve, the liver assembles glucose from non-sugar building blocks. The main raw materials are lactate (a byproduct of muscle activity), certain amino acids like alanine (released from muscle protein), and glycerol (freed when fat is broken down). Both pathways funnel through a final step that requires a specific enzyme, glucose-6-phosphatase, found mainly in the liver and kidneys. This enzyme strips a phosphate group off the glucose molecule so it can exit the liver cell and enter the blood.1PubMed Central. The glucose-6-phosphatase system Without it, glucose stays trapped inside the cell, unable to serve the rest of the body.2PubMed Central. The absence of hepatic glucose-6 phosphatase/ChREBP couple is incompatible with survival in mice
How the Balance Shifts During a Fast
Right after a meal, glycogen stores are full and the liver leans heavily on glycogenolysis to keep blood sugar steady. But glycogen reserves are limited. A classic study using carbon-13 magnetic resonance spectroscopy in fasting humans found that during the first 22 hours without food, gluconeogenesis already accounted for roughly 64% of total glucose production. By 36 hours of fasting, that figure climbed to about 82%, and after 40 hours it was essentially 96%.3PubMed. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR In other words, once glycogen runs low, gluconeogenesis takes over almost entirely.
This shift explains why prolonged fasting eventually becomes a whole-body affair. When the liver needs raw materials for gluconeogenesis, it pulls alanine from skeletal muscle. Research into what is sometimes called the Cahill cycle (or glucose-alanine cycle) has shown that muscle-derived alanine becomes the rate-limiting factor for the liver’s ability to keep churning out glucose during extended fasts.4PubMed Central. Hungry for your alanine: when liver depends on muscle proteolysis This is one reason people lose muscle mass during starvation; the body is literally cannibalizing protein to maintain blood sugar.
The Hormones Controlling the Switch
The liver does not decide on its own when to ramp up or shut down glucose production. It takes instructions from a hormonal relay system, and the two main players sit on opposite sides of a seesaw.
Glucagon, released by the pancreas when blood sugar drops, is the primary “go” signal. It activates a chain of molecular events inside liver cells that switch on gluconeogenic genes and trigger glycogen breakdown.5PubMed. Hepatic glucagon action: beyond glucose mobilization Recent research has mapped out how glucagon’s signal gets relayed: a scaffolding protein called RACK1 physically connects the glucagon receptor at the cell surface to the enzymes that carry its message into the nucleus, where gluconeogenic genes get turned on. When researchers knocked out RACK1 in mouse livers, the animals developed low blood sugar during fasting because their livers could no longer respond properly to glucagon.6PubMed Central. A Dual-compartment Scaffolding Role for Receptor for Activate C Kinase 1 in Hepatic Glucagon Signaling and Gluconeogenesis
Insulin is the “stop” signal. After a meal, rising insulin tells the liver to quit releasing glucose and start storing it instead. This suppression is partly direct, acting through insulin receptors on the liver itself, and partly indirect, working through signals from fat and muscle tissue. A key mediator is the transcription factor FoxO1, which normally promotes gluconeogenic gene expression.7PubMed Central. Dual role of transcription factor FoxO1 in controlling hepatic insulin sensitivity and lipid metabolism Insulin signaling normally shuts FoxO1 down. In mouse studies, when both the insulin receptor and FoxO1 were knocked out in the liver simultaneously, insulin could still fully suppress glucose production through those indirect, non-hepatic pathways.8PubMed Central. Hepatic Insulin Signaling is Dispensable for Suppression of Glucose Output by Insulin in Vivo The implication is that insulin restrains the liver from multiple angles at once, a redundancy that underscores how important it is to keep glucose output in check.
Beyond glucagon and insulin, stress hormones pile on during illness, injury, or intense physical strain. Cortisol sustains gluconeogenesis by ensuring the liver has a steady supply of precursors and by maintaining glycogen availability. In dog studies, removing cortisol from an otherwise full cocktail of stress hormones cut the rise in blood sugar roughly in half.9PubMed. Role of cortisol in the metabolic response to stress hormone infusion in the conscious dog Epinephrine (adrenaline) acts even more quickly, primarily by revving up glycogen breakdown. When epinephrine was removed from the same stress-hormone mixture, the glucose surge was markedly blunted, whereas norepinephrine had a comparatively minor effect.10PubMed. Role of epinephrine and norepinephrine in the metabolic response to stress hormone infusion in the conscious dog This is why people in intensive care units or experiencing severe trauma often develop high blood sugar even without diabetes: their stress hormones are driving the liver to flood the bloodstream with glucose.
When the Liver Overproduces Glucose
In type 2 diabetes, the liver’s glucose factory runs too hot. Insulin resistance means the “stop” signal is muffled: the liver keeps pumping out glucose even when blood sugar is already elevated. A landmark study found that in people with type 2 diabetes whose fasting blood sugar exceeded 140 mg/dL, basal hepatic glucose production was significantly elevated and correlated closely with rising fasting glucose levels.11Metabolism. Fasting hyperglycemia in non-insulin-dependent diabetes mellitus: Contributions of excessive hepatic glucose production and impaired tissue glucose uptake As diabetes progresses, this excessive hepatic output becomes the main driver of worsening fasting hyperglycemia.12PubMed Central. Molecular pathophysiology of hepatic glucose production
An especially frustrating wrinkle of this insulin resistance is that it is selective. In obese or insulin-resistant livers, insulin fails to suppress glucose production through the FoxO1 pathway. But the arm of insulin signaling that drives fat synthesis, through a factor called SREBP-1c, remains active. The result is a paradox sometimes called “mixed insulin resistance”: the liver keeps making glucose it should not, while simultaneously ramping up fat production it also should not.13Cell Press (Cell Metabolism). Hepatic lipid metabolism and insulin resistance: A current overview This helps explain why type 2 diabetes and fatty liver disease so often travel together. Dysregulated glucose metabolism and selective insulin resistance promote both hepatic gluconeogenesis and fat accumulation at the same time.14PubMed Central. Role of amino acids in the regulation of hepatic gluconeogenesis and lipogenesis in metabolic dysfunction-associated steatotic liver disease
How Metformin Puts the Brakes On
Metformin, still the first-line drug for most people newly diagnosed with type 2 diabetes, works largely by dialing down the liver’s glucose output. But exactly how it achieves this has been debated for decades. Early theories focused on an energy-sensing enzyme called AMPK, which seemed like a natural candidate. Research has complicated that picture. Studies in mouse hepatocytes lacking both functional copies of the AMPK catalytic subunit showed that metformin still suppressed glucose production just fine.15JCI Insight. Metformin inhibits hepatic gluconeogenesis in mice independently of the LKB1/AMPK pathway via a decrease in hepatic energy state The current understanding is that metformin works through multiple overlapping mechanisms, including altering the cell’s energy balance and changing redox chemistry inside liver cells, rather than flipping a single molecular switch.16PubMed Central. Metformin Inhibits Gluconeogenesis by a Redox-Dependent Mechanism In Vivo There is also growing evidence that some of metformin’s benefits come from its actions in the gut, not just the liver.17PubMed Central. The mechanisms of action of metformin
Newer therapeutic strategies aim directly at glucagon signaling. Experimental glucagon receptor antagonists, antibodies that block glucagon from reaching its receptor on liver cells, have shown promise in animal models. One such antibody suppressed hepatic glucose production and improved insulin sensitivity in obese mice, with the ability of insulin to suppress liver glucose output jumping from a negligible 2% in controls to about 33%.18PubMed Central. Human monoclonal antibodies against glucagon receptor improve glucose homeostasis by suppression of hepatic glucose output in diet-induced obese mice Another glucagon receptor antagonist also improved blood sugar and, surprisingly, enhanced insulin action in both the liver and skeletal muscle.19Cell Reports. Glucagon Receptor Antagonism Improves Glycemic Control and Renal Function in Type 2 Diabetes These approaches are still being studied, but they highlight how central the glucagon-liver axis is to blood sugar control.
Alcohol and Liver Glucose Production
Drinking alcohol can temporarily shut down gluconeogenesis, which is one reason heavy drinking on an empty stomach can lead to dangerously low blood sugar. The mechanism is surprisingly direct. When the liver metabolizes ethanol, the chemical reaction shifts the balance of a key cellular molecule, NAD+, toward its reduced form, NADH. That shift starves the gluconeogenic pathway of a molecule it needs to convert lactate into pyruvate, one of the early steps in building new glucose.20PubMed Central. Inhibition of hepatic gluconeogenesis by ethanol
In human studies, alcohol ingestion reduced gluconeogenesis by about 45% over five hours compared to a placebo, and the availability of gluconeogenic precursors inside the liver dropped by roughly 61%.21PubMed. The inhibition of gluconeogenesis following alcohol in humans The participants’ blood sugar did not actually crash, though, because the body compensated by increasing glycogen breakdown and reducing how much glucose other tissues burned. Still, people whose glycogen stores are already low, such as someone who has been fasting, exercising heavily, or eating poorly, lose that safety net. This is why clinicians worry about hypoglycemia when someone with diabetes drinks without eating, and why binge drinking on an empty stomach can land otherwise healthy people in the emergency room.
Your Liver Runs on a Clock
If you have ever noticed that your blood sugar is highest in the early morning, before you have eaten anything, you have experienced the dawn phenomenon. The liver’s glucose output follows a circadian rhythm, and research in mice has pinpointed the liver’s own internal clock as a major driver. Mice engineered to lack Bmal1, a core clock gene, specifically in the liver lost the normal daily rhythm of fasting blood glucose and showed reduced expression of a glucose transporter that helps the liver export glucose.22PubMed. Daily Fasting Blood Glucose Rhythm in Male Mice: A Role of the Circadian Clock in the Liver Interestingly, suppressing growth hormone in these mice did not eliminate the dawn rise, suggesting the liver clock itself, rather than circulating hormones, is the primary timekeeper.
For people with diabetes, this circadian surge can be especially troublesome. The liver ramps up glucose output in the hours before waking, at exactly the time when many people’s insulin sensitivity is at its lowest. Understanding that the dawn phenomenon is driven by the liver’s internal clock rather than by what you ate the night before can affect treatment decisions: long-acting insulin or medication timing, rather than dietary changes alone, is often the appropriate response.
Glucose Production in Newborns
Newborns face an immediate metabolic challenge: the continuous glucose supply from the placenta is suddenly cut off at birth. In the first hours of life, the baby’s liver relies on rapid glycogen breakdown to keep blood sugar stable until feeding begins. Mouse studies have illustrated just how critical this is. When researchers inactivated the gene for liver glycogen phosphorylase (the enzyme that starts glycogen breakdown), the pups developed low blood sugar after birth that persisted until they began nursing.23Journal of Lipid Research. Metabolic switch from glycogen to lipid in the liver maintains glucose homeostasis in neonatal mice Once milk suckling provided an alternative energy source, their blood sugar recovered, but the window of vulnerability was real. This research helps explain why premature babies, who may have smaller glycogen reserves, are at heightened risk for neonatal hypoglycemia.
Glycogen Storage Diseases
A handful of rare inherited disorders directly sabotage the liver’s glucose-producing machinery. Glycogen storage diseases are caused by mutations that impair enzymes involved in either building or breaking down glycogen.24PubMed Central. Hepatic glycogen storage diseases: pathogenesis, clinical symptoms and therapeutic management The most clinically significant liver form is GSD type I (also called von Gierke disease), caused by deficient activity of glucose-6-phosphatase, the very last enzyme in the glucose release pathway. Without it, glycogen and fat accumulate excessively in the liver, and patients develop severe hypoglycemia between meals along with an enlarged liver, elevated blood fats, and growth delays.25PubMed. Diagnosis and management of glycogen storage disease type I: a practice guideline of the American College of Medical Genetics and Genomics
The metabolic consequences go beyond just low blood sugar. Because glucose-6-phosphate is trapped inside the cell, it gets diverted into alternative metabolic routes. Research has shown that the resulting buildup enhances glycolysis and other sugar-processing pathways in the liver, creating a metabolic environment that can promote liver tumors over time.26PubMed Central. Hepatic glucose-6-phosphatase-α deficiency leads to metabolic reprogramming in glycogen storage disease type Ia Management of GSD type I revolves around frequent feedings and uncooked cornstarch (which digests slowly) to prevent fasting hypoglycemia, and gene therapy approaches are under investigation.
How Fasting and Diet Affect the System
Intermittent fasting and ketogenic diets have attracted enormous interest for metabolic health, and both interact directly with the liver’s glucose and energy pathways. Prolonged fasting beyond about 12 hours depletes hepatic glycogen stores and pushes the liver toward increased fat breakdown, which can reduce fat accumulation in the liver.27PubMed Central. Current Evidence Concerning Effects of Ketogenic Diet and Intermittent Fasting in Patients with Nonalcoholic Fatty Liver Very-low-carbohydrate ketogenic diets accomplish something similar by keeping insulin levels persistently low, which promotes fatty acid oxidation and reduces the liver’s drive to make new fat.
The benefits are not universal, though. A recent study in mice on a high-fat diet found that intermittent fasting substantially reduced liver fat and fibrosis in males, but female mice saw limited improvement. The liver-protective effects in males depended in part on ketogenesis, the process of converting fats into ketone bodies. When the enzyme responsible for ketogenesis was knocked down, even male mice lost much of the fasting benefit, while female mice became more vulnerable to liver fibrosis regardless of fasting status.28bioRxiv. Sex- and ketogenesis-dependent effects of intermittent fasting against diet-induced obesity and fatty liver disease This is a single mouse study and should be interpreted cautiously, but it points toward an emerging theme: the liver’s response to fasting is modulated by sex and by the capacity for ketone production, which means the same dietary strategy might not work identically for everyone.
Sepsis and the Liver Under Siege
During severe infection, the liver’s glucose-producing machinery can be disrupted in unusual ways. In a rat model of sepsis, gluconeogenesis from the usual substrates, alanine, lactate, and pyruvate, was significantly impaired in both early and late stages of the infection. Yet gluconeogenesis from glycerol, which enters the pathway at a later step, was maintained throughout.29PubMed. Gluconeogenesis is reduced from alanine, lactate and pyruvate, but maintained from glycerol, in liver perfusion of rats with early and late sepsis The liver, in other words, does not simply shut down glucose production during sepsis. It reroutes it, leaning on whichever raw materials can still be processed. This finding matters clinically because critically ill patients often develop unpredictable swings between high and low blood sugar, and understanding which parts of the gluconeogenic machinery are still working helps explain why.
Fatty liver disease introduces its own complications during metabolic stress. Under normal conditions, fasting prompts fat tissue to release fatty acids, which the liver takes up and uses partly to fuel gluconeogenesis. In people with metabolic dysfunction-associated steatotic liver disease (the condition formerly known as NAFLD), this process is impaired, contributing to a broader breakdown in the coordination between liver and fat tissue.30Clinical Science. The physiology of MASLD: molecular pathways between liver and adipose tissues The liver is already overloaded with fat and cannot process the incoming fatty acids properly, creating a vicious cycle of metabolic dysfunction.