How GLP-1 and GIP Medicines Work, and Which Trial Number Measures Which Effect

The short version#

Incretin medicines such as semaglutide and tirzepatide are laboratory-built copies of two gut hormones, and they do three things at once: they nudge the pancreas to release more insulin but only while blood glucose is elevated, they slow how fast the stomach hands food to the intestine, and they calm the brain circuits that drive hunger. The reason this matters for anyone reading a study is that each of those three actions is measured by a different trial number. Glucose control appears as HbA1c, the metabolic effect appears as percent body-weight change, and the effect that patients care about most appears as counts of real cardiovascular events. Line up the biology with the measurement and a busy headline becomes much easier to judge.

Key points#

• These drugs mimic gut hormones (GLP-1 and GIP) that the intestine releases at every meal.
• Their insulin effect is glucose-dependent, so the pushing stops when blood sugar returns to normal, which keeps low-sugar episodes uncommon.
• The same molecule also slows gastric emptying and reduces appetite, which is where the weight change comes from.
• HbA1c, percent weight change, and cardiovascular event counts each measure a different one of these mechanisms.
• A change in a lab value is not the same as a change in what happens to people.

The hormones these drugs are copying#

Within minutes of a meal, the lining of the intestine releases two signalling molecules: glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Both belong to a family called incretins, a name that simply means they warn the pancreas that food is on the way so the insulin response can be sized to the meal rather than to the bloodstream alone. The catch is that the body's own versions are short-lived. An enzyme called DPP-4 clears them in a couple of minutes.

The medicines solve that problem by redesign. They are shaped to resist DPP-4, so instead of a two-minute signal you get one that a single weekly injection can sustain. That durability is the practical difference between a natural hormone and a drug built from it.

Three doors the same molecule opens#

The most important word in the whole mechanism is glucose-dependent. A mechanistic review in the endocrine literature describes GLP-1 as driving insulin release in a glucose-dependent way, together with slowed gastric emptying and reduced appetite. In everyday language, the drug only leans on the pancreas while blood sugar is high; once glucose settles back toward normal, that push eases off on its own. This built-in brake is why the agents can lower glucose strongly while keeping episodes of low blood sugar uncommon, an advantage the SURPASS program documented for the dual agonist tirzepatide.

These receptors are not confined to one organ. The review notes GLP-1 receptors across the pancreas, brain, gut, cardiovascular system, kidney, and fat tissue. Three of those sites produce the effects a patient actually feels, and it helps to picture them as three doors the same molecule opens:

• The pancreas. The drug amplifies insulin release and restrains glucagon, the hormone that raises blood sugar. This is the glucose-lowering door.
• The stomach. By slowing gastric emptying, the drug lets glucose enter the blood more gradually and lets a meal feel filling for longer.
• The brain. Acting on appetite-regulating circuits, the drug lowers hunger and reduces how much a person eats.

GIP opens a second pathway alongside GLP-1. Tirzepatide is a dual agonist, meaning it activates both the GIP and the GLP-1 receptor, and in head-to-head testing that pairing produced larger effects on glucose and weight than agents that hit a single receptor. The newest compounds described in the Endocrine Reviews overview go further still, combining GLP-1 with GIP, glucagon, or amylin signalling, which is why triple agonists and amylin combinations now appear in the pipeline. The through-line is straightforward: recruit more of the gut-hormone system at once.

Matching each mechanism to its trial number#

Here is the translation that makes headlines readable. Every mechanism corresponds to a specific measurement, and the three measurements answer different questions.

HbA1c: the pancreatic door, measured#

HbA1c reflects average blood glucose over roughly three months, so it captures the insulin and glucagon effects. In the SURPASS phase 3 program for tirzepatide in type 2 diabetes, HbA1c fell by close to 2 percentage points versus placebo, with the precise figure shifting by dose and by comparator. An HbA1c drop, in other words, is the pancreatic door shown as a number.

Percent body-weight change: the stomach and brain doors, measured#

Weight loss reported as a percentage of starting body weight is mostly the readout of reduced appetite and slowed emptying. It is the figure that has driven the obesity trials. The STEP program for semaglutide reported sustained weight reduction out to 104 weeks, and the SURMOUNT program for tirzepatide reported larger mean reductions again. The direction is consistent and the effect is genuine, yet the exact percentage depends heavily on dose, duration, and who was enrolled, which is why two trials of the same molecule can print different numbers.

Cardiovascular events: the outcome that counts#

The endpoint that is hardest to overstate, and the one that matters most to patients, is a tally of events that actually happened. The SELECT trial, published in the New England Journal of Medicine in 2023, followed more than 17,000 adults with overweight or obesity and established cardiovascular disease but without diabetes. Its primary endpoint combined cardiovascular death, non-fatal heart attack, and non-fatal stroke, and semaglutide 2.4 mg lowered that composite by about 20 percent relative to placebo. A surrogate such as HbA1c tells you the drug moves a lab value; an outcome trial such as SELECT tells you it changes what happens to people.

Four habits for reading the claims#

Once you know which number measures which door, a short checklist keeps a headline in proportion.

1. Ask what the endpoint really was. A change in a surrogate marker is not a change in heart attacks, and a claim resting on a lab value should not be dressed up as if an outcome trial had proven it.
2. Look at the comparator. Beating placebo answers a different question than beating an active treatment. SURPASS-2 tested tirzepatide directly against semaglutide, and a head-to-head design carries more information than two separate placebo comparisons.
3. Note the population and the duration. Both weight change and HbA1c depend on who was studied and for how long, so a figure from a two-year obesity trial and one from a 40-week diabetes trial are not interchangeable even for the same molecule.
4. Separate relative from absolute. A 20 percent relative risk reduction sounds larger than the absolute difference underneath it, and both are true at the same time. Reading them together is the honest way to size an effect.

None of this passes judgment on any specific product, and none of it is a recommendation. It is a way to set the biology beside the measurement so a claim can be weighed on what it genuinely showed.