Mounjaro outperforms Ozempic for a specific hormonal reason.
| Stat | Value |
|---|---|
| Post-meal insulin driven by incretins in healthy people | 50–70% |
| Average body weight loss with semaglutide (GLP-1 only) | ~15% |
| Average body weight loss with tirzepatide (GLP-1 + GIP) | ~22.5% |
| Main incretin hormones: GLP-1 and GIP | 2 |
Key Takeaways
- What incretins are: Gut hormones released when food hits the small intestine that amplify insulin release before blood sugar rises — a pre-emptive metabolic response to eating.
- Two main players: GLP-1 (glucagon-like peptide-1) slows digestion, suppresses appetite, and stimulates insulin. GIP (glucose-dependent insulinotropic polypeptide) primarily stimulates insulin and enhances GLP-1 receptor sensitivity.
- Why dual beats single: Tirzepatide (GIP+GLP-1) produced 22.5% average weight loss vs. roughly 15% for semaglutide (GLP-1 only) — a 50% larger effect driven by targeting an additional hormonal lever.
- The incretin effect: In healthy metabolism, 50–70% of post-meal insulin is incretin-driven. In type 2 diabetes, this effect is severely blunted — one reason blood sugar control is difficult.
- Triple agonists: Adding a glucagon receptor to the GIP+GLP-1 combination introduces a fat-burning pathway that pushes efficacy further still.
- Real limitation: Greater hormonal coverage comes with greater complexity — dual and triple agonists have more potential interactions and side effect variability than single-target drugs.
That is not a marketing claim — it is a direct consequence of incretin biology. Once you understand what incretins are and how they work together, the progression from semaglutide to tirzepatide to retatrutide follows a logical arc. Each step is not a new invention so much as a deeper reach into the same underlying hormonal system. And understanding that system tells you exactly why the drugs that come after today's leaders will almost certainly outperform them.
What incretins actually are
The word "incretin" has an elegant definition.
Incretins are hormones produced in the gut in response to food that amplify the insulin response from the pancreas — specifically, they do this in a glucose-dependent way, meaning the insulin release they trigger occurs only when blood sugar is elevated. The effect is anticipatory: even before digested food hits the bloodstream as glucose, the intestine senses that food is coming and sends a signal ahead that prepares the pancreas to release insulin.
In a healthy person eating a normal meal, roughly 50–70% of the total post-meal insulin release is triggered not by the rise in blood glucose itself but by these incretin hormones. The pancreas is being signaled by the gut before the sugar arrives. This pre-emptive insulin response is part of why blood sugar after eating stays relatively controlled — the insulin is already mobilizing glucose uptake before the glucose peak hits.
In type 2 diabetes, this incretin effect is severely blunted. The pancreatic beta cells' response to GLP-1 and GIP is diminished, meaning most post-meal insulin has to be triggered by blood glucose rising rather than by anticipatory incretin signals. The result is a slower, smaller insulin response to eating — and higher post-meal blood sugar spikes.
GLP-1: the incretin most people have heard of
GLP-1 does several things simultaneously.
Produced by L-cells in the small intestine and colon, GLP-1 secretion begins within minutes of food entering the gut. It stimulates pancreatic beta cells to release insulin in a glucose-dependent manner. It suppresses glucagon release from alpha cells, reducing the liver's glucose output. It slows gastric emptying, causing food to move from the stomach into the intestine more gradually — blunting the speed and height of post-meal blood sugar spikes. And it acts on the hypothalamus and brainstem to reduce appetite and increase the feeling of satiety.
All of these effects work toward the same outcome: preventing blood sugar from rising too high, too fast, after a meal. By the time natural GLP-1 does its job, it has been inactivated by DPP-4 within about two minutes. But in that brief window, it has initiated a cascade of metabolic responses that continue to play out over the next 1–2 hours.
GLP-1 agonist medications reproduce this signal continuously — at pharmacological levels far above anything natural GLP-1 achieves — producing sustained appetite suppression, slower gastric emptying, and continuous insulin-sensitizing effects rather than the brief post-meal pulse the body produces naturally.
GIP: the incretin that got overlooked
GIP was actually discovered before GLP-1.
Glucose-dependent insulinotropic polypeptide is produced by K-cells in the upper small intestine (duodenum and jejunum) — closer to the top of the GI tract than GLP-1-producing L-cells, which are more abundant further down. GIP is released quickly after eating, primarily in response to fat and carbohydrates, and its primary role in the incretin response is stimulating insulin release from beta cells.
For a long time, GIP was considered a less interesting target for metabolic drugs. There was a puzzle: in people with type 2 diabetes, GIP's insulinotropic effect is blunted — the pancreas loses its responsiveness to GIP signaling. This led many researchers to assume that targeting GIP would not add much to GLP-1 agonism. They were wrong, and the reason they were wrong is what makes tirzepatide work.
GIP appears to enhance GLP-1 receptor sensitivity. When both GIP and GLP-1 receptors are activated simultaneously, the response from the GLP-1 receptor is amplified beyond what GLP-1 agonism alone produces. The two incretin signals interact synergistically, not just additively. GIP also has direct effects in the brain and in fat tissue that contribute to the metabolic response — effects that GLP-1 does not replicate.
The reason dual GIP+GLP-1 agonism produces 50% more weight loss than GLP-1 alone is not simply that two signals are better than one. It is that GIP and GLP-1 potentiate each other — each signal makes the other more effective. Tirzepatide was not designed just to add GIP on top of GLP-1; it was designed to activate both receptors simultaneously with the right balance to achieve synergistic rather than additive effects.
The incretin effect — and why it breaks down in type 2 diabetes
Understanding this explains why these drugs work differently in different people.
In a healthy person without metabolic dysfunction, the incretin effect is powerful and intact. Between 50% and 70% of post-meal insulin is incretin-driven. The anticipatory response is fast and efficient. Blood sugar rarely spikes dramatically after a normal meal because insulin is already in motion before the glucose arrives.
In someone with type 2 diabetes or advanced insulin resistance, the incretin effect is severely reduced — sometimes to near zero. The beta cells that should respond to GLP-1 and GIP have become less sensitive to these signals, partly because chronic high blood sugar and metabolic stress impair beta cell function. The result is that most post-meal insulin has to come from the direct glucose stimulus on beta cells, which is slower and produces a lower insulin peak — contributing to elevated post-meal blood sugar.
GLP-1 agonist medications partially restore this response. By delivering pharmacological levels of GLP-1 receptor activation, they bypass some of the desensitization and force a larger insulin response than the blunted natural incretin effect could achieve. This is why GLP-1 drugs lower HbA1c even in people with longstanding type 2 diabetes whose beta cells have reduced incretin responsiveness.
| Feature | GLP-1 | GIP |
|---|---|---|
| Source | L-cells (lower small intestine, colon) | K-cells (upper small intestine) |
| Stimulated by | Fat, protein, fiber, carbohydrates | Fat and carbohydrates (early meal) |
| Insulin stimulation | Yes (glucose-dependent) | Yes (glucose-dependent) |
| Appetite suppression | Strong (brain, gut) | Modest (some central effects) |
| Gastric emptying | Slows significantly | Minimal effect |
| Effect in T2D | Blunted but partially restored by agonists | Blunted — but GIP agonism restores GLP-1 sensitivity |
How dual agonists outperformed single agonists
The trial data tells a clear story.
In the SURMOUNT-1 trial, tirzepatide at the highest dose (15mg weekly) produced 22.5% average body weight loss over 72 weeks. In the STEP-1 trial, semaglutide at 2.4mg weekly produced approximately 14.9% average body weight loss over 68 weeks. Both were placebo-controlled, double-blind trials in people with obesity without type 2 diabetes — directly comparable populations.
A 50% difference in weight loss between the best GLP-1-only drug and the best dual agonist is not a minor refinement. It represents a meaningfully different clinical outcome for many patients — the difference between reducing body weight from 250 pounds to 213 pounds vs. 194 pounds. That is a real difference in health outcomes, joint loading, cardiovascular risk, and quality of life.
The addition of GIP did not reduce the tolerability of the drug — gastrointestinal side effects were similar in profile to semaglutide, though tirzepatide had somewhat lower rates of certain GI adverse events in comparative analyses. This suggests that the GIP component is adding efficacy without meaningfully adding to the burden of side effects.
Triple agonists: adding glucagon to the mix
The next step was adding a third hormonal lever.
Retatrutide targets GLP-1, GIP, and glucagon receptors simultaneously. The glucagon component adds a fat-burning (lipolytic) pathway that is distinct from the appetite suppression and insulin-stimulating mechanisms of GLP-1 and GIP. Phase 2 data showed 24.2% average body weight loss over 48 weeks at the 12mg dose — and the curve had not plateaued, suggesting continued weight loss beyond that timepoint.
The incretin system — originally understood as just GLP-1 and GIP — turns out to be connected to glucagon biology in ways that are pharmacologically exploitable. All three of these hormones ultimately come from the same proglucagon gene, expressed in different tissues. The newest drugs are reaching back into that shared biology and activating it more completely than any previous pharmacological approach.
Frequently Asked Questions
What is the incretin effect, simply explained?
When you eat, your gut sends hormonal signals to your pancreas telling it to prepare insulin before blood sugar actually rises. These hormone signals are incretins. In healthy people, they drive 50–70% of post-meal insulin release. In type 2 diabetes, this anticipatory system is impaired, and most insulin release has to happen reactively after blood sugar rises — which is slower and less effective.
Is tirzepatide (Mounjaro/Zepbound) better than semaglutide (Ozempic/Wegovy)?
In clinical trials, tirzepatide produces larger average weight loss and similar or better blood sugar control compared to semaglutide. Whether it is "better" for any individual depends on their response, tolerance, insurance coverage, dosing preferences, and prescriber's assessment. Both are effective and approved. The decision is a clinical one, not a general ranking.
Why doesn't everyone produce enough GLP-1 naturally?
Several factors impair natural GLP-1 secretion: obesity itself, type 2 diabetes, certain dietary patterns (high chronic saturated fat), sleep deprivation, and sedentary lifestyle all reduce L-cell output. The body's natural GLP-1 system is also designed for brief post-meal signaling, not the sustained pharmacological activation that weight loss drugs produce — so even a perfectly healthy GLP-1 system produces much lower levels than what drugs deliver.
Are there natural ways to boost incretin activity?
Yes, with caveats. High-fiber diets, fermented foods, foods rich in prebiotics, and regular aerobic exercise have documented GLP-1-stimulating effects. Protein-rich meals also produce meaningful incretin responses. These lifestyle factors genuinely improve incretin function — but the magnitudes involved are a fraction of what pharmacological agonists achieve. Natural approaches support metabolic health; they do not replicate drug-level effects.
How does tirzepatide compare to insulin for type 2 diabetes?
Tirzepatide and insulin work through very different mechanisms. Insulin directly lowers blood sugar regardless of metabolic context. Tirzepatide works through incretin pathways — glucose-dependent insulin stimulation, appetite suppression, and weight loss — and does not cause hypoglycemia on its own. Head-to-head trials have shown tirzepatide achieving comparable or superior HbA1c reduction to basal insulin, with significant weight loss rather than weight gain (a common side effect of insulin). Your prescriber can advise on which approach fits your specific situation.
This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting any medication or treatment.
