What Is GLP-1 and How Do GLP-1 Receptor Agonists Work?

When food reaches the digestive tract, the body receives more than nutrients. The gut also releases signals that tell the pancreas, stomach, and brain that a meal has arrived. One of them is the hormone GLP-1.

Ozempic mimics the same signal but keeps GLP-1R activated for much longer. To understand why people taking it may feel less hungry, feel full sooner, and achieve better blood glucose control, it helps to look at what natural GLP-1 does to the pancreas, liver, stomach, and brain after a meal.

What GLP-1 is

GLP-1 stands for glucagon-like peptide-1. It is a natural hormone released by cells in the intestinal wall when food arrives.¹

GLP-1 can be understood as a brief message from the gut: “Food is here; prepare to process it.” Only cells with a GLP-1 receptor, known as GLP-1R, on their surface can receive that message. The receptor acts like a receiver: when GLP-1 binds to it, the receptor passes the signal into the cell and changes what the cell does.²

The same signal then has different effects depending on the target tissue:

• The pancreas releases more insulin when blood glucose rises after a meal and reduces glucagon release at the same time.
• The stomach may empty more slowly, delaying the movement of food into the intestine.
• Brain circuits involved in regulating food intake receive a signal that supports satiety.

Together, these processes help the body respond to a meal. Their strength and duration differ, however, depending on whether the signal comes from the body's own short-acting GLP-1, administered GLP-1, or a long-acting receptor agonist.

What GLP-1 does to insulin and blood glucose

After a meal, glucose from digested carbohydrate enters the bloodstream. The pancreas responds by releasing insulin, which helps move glucose from the blood into cells and store the excess.

GLP-1 strengthens this insulin response mainly when blood glucose is elevated. It does not trigger insulin release regardless of the situation; it helps the pancreas respond to the meal that has just arrived.^1,3

In practice, this means that glucose is processed more efficiently after a meal and blood levels usually do not rise as sharply. This effect is one reason GLP-1 receptor agonists are used to treat type 2 diabetes.⁴

Glucagon is insulin's counterweight

Insulin and glucagon regulate blood glucose in opposite directions. After a meal, insulin helps the body use and store glucose. Between meals, glucagon tells the liver to release stored glucose or make more of it.⁵

Both hormones are produced in the pancreas. Insulin lowers blood glucose, while glucagon raises it or helps prevent it from falling too far. Glucagon is therefore not a harmful hormone; between meals, it helps maintain an adequate supply of energy in the circulation.

After a meal, GLP-1 temporarily shifts this balance: it supports insulin and suppresses glucagon. The practical result is twofold. The body processes the glucose that came from the meal more effectively, while the liver adds less extra glucose from its stores.^3,5

Does GLP-1 reduce hunger, or does it only slow the stomach?

Reduced hunger cannot be explained by a slower stomach alone. GLP-1 signaling also affects brain circuits that regulate hunger and satiety.¹ In a controlled experiment, administered GLP-1 increased satiety and reduced subsequent energy intake. In adults with obesity, semaglutide also reduced hunger and food cravings after 20 weeks, when the study found no evidence of delayed gastric emptying.^6,7

For the body's own GLP-1, slowing the stomach is not always the main mechanism. In a study in which participants drank glucose, naturally released GLP-1 mainly affected insulin and glucagon, while its effect on gastric emptying was small.³ In another study, administered GLP-1 did slow the emptying of a liquid meal.⁸ The importance of this effect therefore depends on the form of the signal, the type of meal, and the duration of exposure.

What a GLP-1 receptor agonist is

An agonist is a substance that activates a particular receptor. Natural GLP-1 activates GLP-1R, but the body breaks the hormone down within minutes.⁹ The gut can continue releasing more GLP-1 while digestion is under way, but each individual molecule acts only briefly.

Ozempic contains semaglutide, a modified molecule that activates the same receptor. The body removes it much more slowly, and its biological half-life is approximately one week.^10,4 Ozempic is therefore not an injection of natural GLP-1. It is a longer-acting mimic of the same signal.

The practical difference is duration. Natural GLP-1 helps process a particular meal, while semaglutide provides prolonged receptor stimulation over several days. Its effects on blood glucose, hunger, and satiety are therefore not limited to a brief period after eating.

Mounjaro and the investigational drug retatrutide share part of Ozempic's mechanism: they also activate GLP-1R. However, they add other mechanisms. Mounjaro contains tirzepatide, which also activates the GIP receptor, while retatrutide activates the GIP and glucagon receptors as well. They are therefore described as dual and triple agonists.^11,12

What to take away

GLP-1 is a brief message sent by the gut after a meal. Ozempic contains semaglutide, which mimics that message and keeps the receptor activated for much longer.

In practice, four main effects work together:

• the pancreas releases more insulin when blood glucose is elevated,
• glucagon is suppressed after a meal, so the liver adds less extra glucose,
• the stomach may move food into the intestine more slowly,
• the brain receives a stronger signal of satiety and a weaker signal of hunger.

None of these processes explains the effect of Ozempic on its own. Together, they show why the same medicine can affect blood glucose, portion size, and how often a person feels hungry during the day.