What Is the GLP Peptide Family?
The glucagon-like peptides (GLPs) are a family of hormones derived from the proglucagon gene. This single gene produces different hormones depending on which tissue is processing it. In the pancreatic alpha cells, proglucagon is cleaved to produce glucagon — the hormone that raises blood sugar. In the intestinal L-cells and certain brain neurons, the same proglucagon protein is processed differently. It produces GLP-1 and GLP-2 — hormones with entirely distinct receptor targets and physiological roles.
This shared genetic origin is why the family is called "glucagon-like." GLP-1 and GLP-2 share structural similarity with glucagon but act through different receptors. The GLP-1 receptor (GLP-1R) is expressed on pancreatic beta cells, the hypothalamus, the heart, and the kidneys. The GLP-2 receptor (GLP-2R) is expressed almost exclusively on the gastrointestinal tract. The glucagon receptor (GCGR) is expressed primarily in the liver.
The pharmaceutical development of GLP-1 receptor agonists is one of the most significant advances in metabolic medicine in decades. These GLP-1 medications and GLP-1 drugs — including GLP-1 research peptides (also called GLP 1 peptides) — slow stomach emptying, suppress appetite, and lower blood sugar. They are used to treat diabetes and obesity under health care supervision. Robust long term safety data comes from the SUSTAIN and STEP trial programs. Each generation has expanded receptor coverage. The progression runs from GLP-1 monoagonism (semaglutide) to dual GLP-1/GIP agonism (tirzepatide vs semaglutide) to triple GLP-1/GIP/glucagon agonism (retatrutide). Each step produces greater weight loss in clinical trials. Rybelsus is the only approved oral medication in this class; all others are injectable.
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All three are encoded by the same proglucagon gene — tissue-specific post-translational processing determines which hormone is produced.
The Evolution of GLP Therapeutics
Mojsov et al. identify GLP-1 as a potent insulin secretagogue. The 1–2 minute half-life makes it impractical as a therapeutic — but establishes the receptor biology that drives all subsequent drug development.
Exenatide (Byetta) — derived from Gila monster saliva peptide exendin-4 — becomes the first GLP-1 receptor agonist approved by the FDA for type 2 diabetes. Twice-daily injection required.
Fatty acid modifications enable albumin binding, extending half-life to 13 hours (liraglutide, daily) and 7 days (semaglutide, weekly). Semaglutide's STEP 1 trial shows 14.9% weight loss — unprecedented for a drug.
Tirzepatide (Mounjaro/Zepbound) adds GIP receptor agonism to GLP-1R targeting. SURMOUNT-1 shows 22.5% weight loss at 72 weeks — surpassing all prior GLP-1 monoagonists.
Retatrutide adds glucagon receptor agonism to GLP-1R + GIP-R. Phase II NEJM data: 24.2% weight loss at 48 weeks — the highest ever documented for a pharmacological agent. Phase III ongoing.
Weight Loss by Generation
Mean body weight reduction from pivotal clinical trials. Each generation of receptor coverage has produced meaningfully greater weight loss.
Compound Profiles
Native GLP-1
NativeGlucose-dependent insulin secretion, glucagon suppression, appetite reduction via hypothalamic GLP-1R, gastric emptying delay
Parent molecule of the entire GLP-1 agonist drug class. Foundational research tool for GLP-1R biology. Rapidly degraded by DPP-IV at the His⁷-Ala⁸ bond.
Liraglutide
1st Gen AnalogueDPP-IV-resistant GLP-1 analogue with C16 fatty acid modification for albumin binding; daily subcutaneous injection
First GLP-1 agonist approved for obesity (Saxenda®, 2014). LEADER trial demonstrated cardiovascular risk reduction in T2D patients.
Semaglutide
1st Gen AnalogueDPP-IV-resistant GLP-1 analogue with Aib8 substitution and C18 fatty diacid modification; once-weekly dosing via albumin binding
STEP 1: 14.9% mean body weight reduction. SELECT trial: 20% reduction in major cardiovascular events. Currently the most prescribed GLP-1 agonist globally.
Tirzepatide
2nd Gen DualFirst-in-class dual agonist; GIP-R agonism enhances insulin secretion and may improve GLP-1R sensitivity; once-weekly injection
SURMOUNT-1: 22.5% mean body weight reduction — surpassing all GLP-1 monoagonists. First dual incretin receptor agonist approved for obesity.
Retatrutide (GLP-3 R)
3rd Gen Triple/BlendTriple agonism adds glucagon receptor activation → increased energy expenditure via brown adipose thermogenesis and hepatic fatty acid oxidation; once-weekly injection
Phase II NEJM 2023: 24.2% mean body weight reduction — highest ever documented for a pharmacological agent in a clinical trial. Phase III ongoing.
Full Comparison Table
| Compound | Type | Receptors | Half-Life | Weight Loss | Status |
|---|---|---|---|---|---|
| Native GLP-1 | Endogenous | GLP-1R | 1–2 min | N/A | Research only |
| Liraglutide | GLP-1 Analogue | GLP-1R | 13 hrs | ~5–8% | FDA approved |
| Semaglutide (Purgo "GLP-1") | GLP-1 Analogue | GLP-1R | 7 days | ~14.9% | FDA approved |
| Tirzepatide (Purgo "GLP-2 T") | Dual Agonist | GLP-1R + GIP-R | 5 days | ~22.5% | FDA approved |
| Retatrutide (Purgo "GLP-3 R") | Triple Agonist | GLP-1R + GIP-R + GCGR | ~6 days | ~24.2% | Phase III |
| Teduglutide (not the vendor's "GLP-2 T") | GLP-2 Analogue | GLP-2R | 1.3 hrs | N/A | FDA approved |
GLP-1 vs GLP-2: Same Gene, Different Organs
Frequently Asked Questions
What is the difference between GLP-1, semaglutide, and tirzepatide?
GLP-1 (glucagon-like peptide-1) is the native hormone your gut produces after eating — it stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite. Its half-life is only 1–2 minutes in vivo because it is rapidly degraded by the enzyme DPP-IV. Semaglutide is a synthetic GLP-1 analogue engineered to resist DPP-IV degradation, with a half-life of approximately 7 days — enabling once-weekly dosing. It targets only the GLP-1 receptor. Tirzepatide is a dual agonist that targets both the GLP-1 receptor and the GIP receptor (glucose-dependent insulinotropic polypeptide receptor), producing superior weight loss compared to GLP-1 monoagonists in head-to-head trials.
What is retatrutide and how does it differ from semaglutide?
Retatrutide (LY3437943) is a triple receptor agonist developed by Eli Lilly that simultaneously activates the GLP-1 receptor, GIP receptor, and glucagon receptor. The addition of glucagon receptor agonism is the key distinction from semaglutide (GLP-1 only) and tirzepatide (GLP-1 + GIP). Glucagon receptor activation increases energy expenditure through thermogenic effects in brown adipose tissue and hepatic fatty acid oxidation — a mechanism not present in semaglutide or tirzepatide. Phase II trials published in the New England Journal of Medicine (2023) showed retatrutide produced up to 24.2% mean body weight reduction at 48 weeks, the highest ever reported for a pharmacological agent in a clinical trial.
What is GLP-2 and how is it different from GLP-1?
GLP-1 and GLP-2 are both derived from the same proglucagon gene, produced by the same L-cells in the gut, and released simultaneously after eating — but they have entirely different receptor targets and physiological roles. GLP-1 acts systemically on the pancreas, brain, and cardiovascular system to regulate glucose and appetite. GLP-2 acts almost exclusively on the gastrointestinal tract, where it promotes intestinal mucosal growth, enhances nutrient absorption, and maintains gut barrier integrity. Teduglutide (GLP-2 T) is a stabilized GLP-2 analogue that has been FDA-approved since 2012 for short bowel syndrome under the brand name Gattex®.
What is the proglucagon gene and why does it matter?
The proglucagon gene encodes a large precursor protein (proglucagon) that is processed differently in different tissues. In the pancreatic alpha cells, proglucagon is cleaved to produce glucagon — the hormone that raises blood sugar. In the intestinal L-cells and certain brain neurons, the same proglucagon protein is cleaved differently to produce GLP-1 and GLP-2. This tissue-specific processing means the same gene produces hormones with opposite effects on blood sugar: glucagon raises it, GLP-1 lowers it. Understanding this shared origin is key to understanding why retatrutide's glucagon receptor agonism is counterbalanced by its GLP-1 receptor agonism.
Is KLOW a GLP blend?
No. An earlier version of this guide described KLOW as a GLP-1/GLP-2 T/GLP-3 R blend; that was wrong. Purgo Labs' KLOW is an 80mg four-peptide vial of GHK-Cu (50mg), BPC-157 (10mg), TB-500 (10mg) and KPV (10mg) — a copper-peptide and tissue-repair stack with no incretin component. It does not belong in a GLP comparison and is covered on its own profile page.
What does 'GLP-1 receptor agonist' mean?
A GLP-1 receptor agonist is any compound that binds to and activates the GLP-1 receptor (GLP-1R), mimicking the effects of native GLP-1. The GLP-1R is a class B G-protein coupled receptor expressed on pancreatic beta cells, the hypothalamus, the gastrointestinal tract, the heart, and the kidneys. Activation stimulates adenylyl cyclase, increasing intracellular cAMP, which potentiates glucose-stimulated insulin secretion. GLP-1 receptor agonists include native GLP-1 (very short half-life), semaglutide (weekly), liraglutide (daily), and the GLP-1R component of tirzepatide and retatrutide.
Is native GLP-1 the same as Ozempic?
No. Native GLP-1 is the endogenous hormone your body produces naturally — it has a half-life of approximately 1–2 minutes and is rapidly degraded by DPP-IV. Ozempic (semaglutide) is a synthetic GLP-1 analogue engineered to resist DPP-IV degradation through amino acid substitutions and a fatty acid modification that enables albumin binding, extending the half-life to approximately 7 days. Ozempic is an FDA-approved pharmaceutical drug. Research-grade native GLP-1 is an investigational compound used to study GLP-1 receptor biology in laboratory settings — it is not a pharmaceutical drug and is not approved for human use.
What is the clinical evidence for GLP-1 receptor agonists for weight loss?
The clinical evidence base for GLP-1 receptor agonists in weight management is among the strongest in modern pharmacology. Key trials: STEP 1 trial (semaglutide 2.4mg weekly) showed 14.9% mean body weight reduction at 68 weeks vs 2.4% placebo. SURMOUNT-1 trial (tirzepatide 15mg weekly) showed 22.5% mean body weight reduction at 72 weeks. Phase II retatrutide trial (12mg weekly) showed 24.2% mean body weight reduction at 48 weeks. These represent the largest pharmacologically-induced weight reductions ever documented in clinical trials, with the trajectory appearing to improve with each generation of receptor coverage.
Related Guides & Compound Profiles
Key Published Research
Peer-reviewed studies from verified investigators — linked directly to PubMed
GLP-1 receptor agonist research has been led by multiple groups at Novo Nordisk (semaglutide) and Eli Lilly (tirzepatide, retatrutide). The foundational GLP-1 biology was established by Jens Juul Holst, MD, DMSc, at the University of Copenhagen, whose work on incretin hormones spans 40+ years and 1,000+ publications.
The Physiology of Glucagon-like Peptide 1
Holst JJ.
Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1)
Jastreboff AM, Aronne LJ, Ahmad NN, et al.
Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial
Jastreboff AM, Kaplan LM, Frias JP, et al.
Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1)
Wilding JPH, Batterham RL, Calanna S, et al.
All citations link to verified PubMed records. This site does not fabricate or assign authorship — only real published investigators are listed.
Research GLP Peptides at Purgo Labs
Third-party tested, ≥99% purity, COA with every batch.