Glucagon is a hormone that raises your blood sugar by signaling your liver to release stored glucose into the bloodstream. It’s produced by alpha cells in the pancreas and acts as the counterbalance to insulin. While insulin lowers blood sugar after you eat, glucagon kicks in when blood sugar drops too low, during fasting, or during exercise to keep your brain and muscles fueled.
What Triggers Glucagon Release
The primary trigger is falling blood sugar. Alpha cells in the pancreas constantly monitor glucose levels through a surprisingly direct mechanism: glucose flows into the cell, gets converted to energy (ATP), and the amount of energy produced tells the cell how much sugar is in the blood. When blood sugar drops, energy production inside the alpha cell drops too. This sets off a chain reaction involving ion channels in the cell membrane that ultimately lets calcium flood into the cell, and that calcium surge is what causes the cell to release its stored glucagon granules into the bloodstream.
Beyond low blood sugar, glucagon release is also triggered by prolonged fasting, exercise, and protein-rich meals. That last one surprises many people: eating a steak with no carbohydrates causes both insulin and glucagon to rise. Insulin handles the incoming amino acids, while glucagon prevents that insulin from crashing your blood sugar.
Fasting glucagon levels in healthy adults typically fall between 25 and 50 pg/mL. Those levels climb quickly when your body needs more glucose.
What Happens When Glucagon Reaches the Liver
The liver is glucagon’s primary target. When glucagon binds to receptors on liver cells, it triggers a signaling cascade that raises blood sugar through two distinct processes.
The first and fastest response is glycogenolysis: breaking down glycogen, a starch-like molecule the liver uses to stockpile glucose. Your liver can store enough glycogen to maintain blood sugar for roughly 12 to 24 hours of fasting, and glucagon is the main signal to start tapping into that reserve.
The second process is gluconeogenesis, literally “making new glucose.” When glycogen stores run low, glucagon ramps up production of enzymes that build fresh glucose molecules from raw materials like lactate (a byproduct of muscle activity), amino acids from protein breakdown, and glycerol released from fat. Glycerol is the dominant building block, but glucagon preferentially increases the use of the amino acid glutamine as fasting extends. This flexibility means your body can manufacture glucose from multiple sources depending on what’s available.
Inside the liver cell, the signaling works like a relay. Glucagon activates an enzyme that produces a messenger molecule called cAMP, which in turn activates a protein called PKA. PKA then switches on the genes for key glucose-producing enzymes. The whole sequence from glucagon binding to measurable increases in blood glucose takes only minutes.
The Insulin-Glucagon Balance
Your metabolic state at any given moment is largely determined not by insulin or glucagon alone, but by the ratio between them. A high insulin-to-glucagon ratio, like after a carb-heavy meal, tells your body to store energy: build proteins, pack away glucose as glycogen, and stop burning fat. A low ratio, like during an overnight fast, flips the switch to energy release: break down glycogen, produce new glucose, and liberate fatty acids from fat tissue.
This ratio acts as a metabolic fulcrum. It’s lowest during starvation, when the body needs maximum glucose production and fat burning, and highest after a carbohydrate load, when storage is the priority. The system is remarkably responsive, shifting within minutes of eating or fasting.
What Shuts Glucagon Off
When blood sugar rises after a meal, the pancreas needs to quickly suppress glucagon so it doesn’t keep pushing glucose into the blood on top of what you just ate. Two hormones handle this job through complementary mechanisms.
Insulin, released from neighboring beta cells, acts directly on alpha cells. It activates an enzyme that breaks down cAMP inside the alpha cell, essentially dismantling the same signaling molecule that glucagon uses in the liver. Without cAMP, the alpha cell’s internal “release” signal weakens.
Somatostatin, produced by delta cells also located in the pancreas, takes a different approach. It blocks the production of new cAMP in the first place by inhibiting the enzyme that makes it. Together, insulin reduces cAMP by speeding up its destruction while somatostatin reduces it by slowing down its creation. The combined effect efficiently shuts down glucagon secretion when blood sugar is high.
Effects Beyond the Liver
Glucagon receptors exist in several tissues outside the liver, including the kidneys, digestive tract, brain, and fat tissue. The extent to which glucagon acts on these tissues in humans, however, is still debated.
In rodents, glucagon clearly stimulates fat breakdown in fat cells. In humans, the picture is murkier. At the concentrations glucagon normally reaches in human blood (1 to 40 picomoles per liter), researchers have struggled to demonstrate a direct fat-burning effect on fat cells. Clinical studies using glucagon levels within the physiological range found no significant increase in fat breakdown. The exception may be situations where insulin is very low, such as prolonged fasting or uncontrolled diabetes, where glucagon could contribute to the release of fatty acids. For most practical purposes, glucagon’s dominant action in humans is on the liver.
Glucagon also influences how the liver processes amino acids. It increases activity of the urea cycle, the pathway that converts nitrogen waste from protein breakdown into urea for excretion by the kidneys. This means glucagon doesn’t just mobilize glucose; it also helps the body handle the byproducts of using amino acids as fuel.
Glucagon as an Emergency Treatment
For people with diabetes who experience severe low blood sugar (hypoglycemia), glucagon is available as an emergency medication. It comes in two main forms: injectable kits that require mixing a powder with a liquid before injection, and a nasal powder that’s sprayed into the nose.
The standard adult dose is 1 mg injected into the upper arm, thigh, or buttocks. Children weighing under 25 kg (about 55 pounds) receive 0.5 mg. Blood sugar typically begins rising within 10 minutes, with peak levels reached around 30 minutes after injection. In one study of healthy subjects, a 1 mg injection raised blood sugar by about 79 mg/dL on average, with the peak occurring around 50 minutes.
Nasal glucagon is simpler to administer since it requires no mixing or needles, but it delivers a lower effective dose to the bloodstream. Its bioavailability is roughly 16% compared to injectable glucagon, meaning much less of the drug actually reaches circulation. Despite this, the nasal form absorbs faster and has been shown effective enough for emergency use.
Nausea and vomiting are the most common side effects of glucagon administration. This happens because glucagon receptors in the gut and brain affect gastric motility and autonomic signaling, not just glucose metabolism. The nausea is typically short-lived but can be significant enough that the person should be positioned on their side to prevent choking if they vomit while unconscious.