How Zepbound Works: Tirzepatide's Two Receptors, Appetite and Fat Loss

Zepbound (tirzepatide) works by switching on two hormone receptors at once: the receptor for GLP-1 and the receptor for GIP, two gut hormones released after a meal. Acting on both, it turns down appetite, so people eat less, and it slows how fast the stomach empties, mostly in the first weeks.^[1] The weight that comes off is mostly fat. In a body-scan substudy of the main Zepbound trial, about three-quarters of the weight lost was fat and one-quarter was lean mass.^[4] Zepbound does not burn fat directly. The fat is lost because the body runs on less food than it needs.

One molecule, two receptors

Tirzepatide is a single peptide that fits two locks. Its FDA label says it binds and switches on two receptors, one for GIP and one for GLP-1, which are the same receptors the body’s own hormones use.^[1] A fatty-acid chain attached to the molecule makes it stick to albumin in the blood, which is why one injection lasts a week.^[1]

The two receptors are not hit equally. In laboratory work by Lilly scientists, doses used in patients engaged the GIP receptor to a greater degree than the GLP-1 receptor, and at the GLP-1 receptor tirzepatide signaled in a different pattern from natural GLP-1.^[2] That is why tirzepatide is described as a dual agonist and not as a stronger version of semaglutide, which acts on the GLP-1 receptor alone. What GIP does when it is given by itself, without the GLP-1 half, is covered in what GIP does on its own.

Appetite: the main way Zepbound causes weight loss

The label is direct about where the weight loss comes from: tirzepatide lowers calorie intake, and it probably does so by acting on appetite.^[1] Both receptors are found in brain areas that regulate appetite, and in animals the drug reaches and activates neurons there.^[1]

A six-week trial in 114 adults without diabetes measured this directly. At week three, people on tirzepatide ate about 525 fewer calories at a buffet lunch than people on placebo. They also reported less hunger, fewer food cravings and less of a tendency to overeat. Their ability to hold back on purpose did not change.^[5] In other words, the drug lowered the pull toward food rather than adding willpower.

Stomach emptying slows, then mostly recovers

Tirzepatide slows the rate at which food leaves the stomach. The label says the effect is biggest after the first dose and fades with time.^[1] That early slowing helps explain the fullness and nausea many people notice early on and again whenever the dose goes up. Because it wears off, slow stomach emptying cannot account for weight loss that continues for more than a year. Appetite can.

Insulin, glucagon and blood sugar

When blood sugar rises after a meal, tirzepatide makes the pancreas release more insulin; when sugar is normal, the push stops. That is what the label means by glucose-dependent.^[1] It also lowers glucagon, the hormone that tells the liver to release sugar. In the diabetes label, the 15 mg dose cut fasting glucagon by 28% and post-meal glucagon by 43% after 28 weeks, with no change on placebo.^[10]

The same 28-week study measured insulin sensitivity with a clamp test, the reference method. Tirzepatide improved it more than semaglutide 1 mg did.^[7] A later analysis of that trial found that tirzepatide gave more improvement in insulin sensitivity per kilogram lost, and that the gap was not explained by weight or fat loss alone.^[8] These studies were in people with type 2 diabetes, so they show what the drug does to insulin, not how it causes weight loss in people without diabetes.

Does tirzepatide burn fat?

Not in the sense of a fat burner that speeds up metabolism. In a placebo-controlled trial that measured energy use in a sealed metabolic chamber, tirzepatide did not change how much the body’s calorie burn slowed during weight loss, but it did shift the fuel mix toward fat.^[11] The main effect is still lower intake: when the body takes in less energy than it uses, it draws on stored fat. The label puts the result plainly: weight comes down with more fat lost than lean mass.^[1]

The best measurement comes from the SURMOUNT-1 trial, the 72-week study that led to Zepbound’s approval. In the full trial of 2,539 adults without diabetes, people lost an average of 15.0% (5 mg dose), 19.5% (10 mg) or 20.9% (15 mg) of their starting weight; the placebo group lost 3.1%.^[3] A subgroup of 160 participants had DXA body scans at the start and at week 72.^[4] The full trial results are at SURMOUNT-1.

Two things stand out. People on tirzepatide lost about a third of their body fat. And the split between fat and lean mass, roughly 75 to 25, was the same on the drug as on placebo, which means it follows the amount of weight lost rather than anything special about the drug.^[4] Lean mass includes muscle but also water, organs and other non-fat tissue, so a 10.9% drop in lean mass is not the same as losing 10.9% of muscle. How that compares with dieting is covered in lean mass on a drug versus dieting, and the muscle question in hair loss and muscle loss on GLP-1 drugs.

Why Zepbound differs from semaglutide

Semaglutide, sold as Wegovy for weight loss, acts on the GLP-1 receptor only. Zepbound adds the GIP receptor. In SURMOUNT-5, the one head-to-head trial in adults with obesity and without diabetes, people on tirzepatide lost 20.2% of their body weight at 72 weeks and people on semaglutide lost 13.7%. Waists shrank by 18.4 cm and 13.0 cm. That trial was open label, so participants knew which drug they were taking.^[9] The two brands are set side by side at Wegovy vs Zepbound.

Why the gap exists is not settled. In a 28-week study in type 2 diabetes, tirzepatide 15 mg and semaglutide 1 mg lowered appetite scores and lunch intake by similar amounts, yet tirzepatide produced more weight and fat loss. The authors wrote that the lunch measurements were not enough to explain the difference.^[6] The GIP half may add to appetite control, as the label suggests from animal work, and the better insulin response may play a part.^[1][8] Neither has been proved as the reason.

What this means if you are starting Zepbound

• Eating less is how it works. The drug lowers hunger; the weight loss depends on that lower intake continuing.
• The early fullness and nausea partly come from slower stomach emptying, an effect that peaks with the first injection and then weakens.^[1]
• Roughly one pound in four that comes off is lean tissue, on the drug or on placebo.^[4] Eating enough protein and doing strength training are the usual ways to protect muscle; the drug does not do it for you.
• Mounjaro contains the identical molecule; Lilly sells it under that name for type 2 diabetes. Same doses, same mechanism, different approved uses.

How fast the weight comes off is covered in the tirzepatide results timeline, and what happens after stopping in stopping tirzepatide and weight regain.