GLP-1 Receptor Agonists: Semaglutide, Tirzepatide, and Retatrutide

The Incretin System: Why These Compounds Work

Incretin hormones are released from the gut after eating. Their primary role is to amplify the pancreatic insulin response to food - the so-called incretin effect - and to signal satiety to the brain. The two main incretins are GLP-1 (glucagon-like peptide-1) and GIP (gastric inhibitory polypeptide, also called glucose-dependent insulinotropic polypeptide).

GLP-1 in particular produces several effects relevant to fat loss and metabolic health:

• Stimulates insulin secretion from the pancreas in a glucose-dependent manner (so hypoglycaemia risk is low in non-diabetic users)
• Suppresses glucagon secretion, reducing hepatic glucose output
• Slows gastric emptying, which blunts post-meal glucose spikes and prolongs satiety
• Acts centrally in the hypothalamus to reduce appetite via GLP-1 receptors in appetite-regulating circuits
• Reduces food reward signalling, making highly palatable food less compelling

The result is a compound that makes it substantially easier to maintain a calorie deficit by removing most of the hunger and food-craving signal that makes calorie restriction difficult. Users do not need willpower to avoid overeating because the biological signal driving eating is substantially reduced.

Natural GLP-1 has a half-life of under 2 minutes in the bloodstream, it is rapidly degraded by the enzyme DPP-4. The pharmaceutical approach was to develop GLP-1 analogues resistant to DPP-4 degradation, allowing once-weekly dosing at plasma concentrations far higher than any endogenous GLP-1 pulse.

Semaglutide: The First-Generation Standard

Semaglutide (Ozempic for type 2 diabetes, Wegovy for weight management) is a GLP-1 receptor agonist with an approximately 7-day half-life, enabling once-weekly subcutaneous injection. In the STEP trial program, semaglutide at 2.4 mg/week produced average weight loss of approximately 15% of body weight over 68 weeks in obese non-diabetic adults.

Mechanism: Selective GLP-1 receptor agonism. It produces all the GLP-1 effects described above at sustained supraphysiological concentrations.

Clinical and performance context:

The typical performance use of semaglutide is as a fat-loss adjunct during a cut, particularly for users who find extreme calorie restriction difficult to maintain due to hunger. The compound does not directly build muscle or improve training performance - it creates conditions where a large calorie deficit is tolerable.

The critical problem with any aggressive fat-loss compound (pharmacological or dietary) is lean mass loss. Rapid weight loss without adequate protein intake and resistance training leads to significant muscle loss alongside fat. Semaglutide accelerates this risk by dramatically suppressing appetite, which can make it difficult to hit protein targets.

A practical rule: if appetite suppression prevents you from consuming adequate protein (1.6–2.2 g/kg bodyweight), the dose is too high for the context. Dial it back until protein intake is manageable.

Protocol design:

Titration is mandatory. Starting at a high dose produces severe nausea, vomiting, and often discontinuation. A functional titration approach:

• Week 1–4: 0.25 mg/week subcutaneous
• Week 5–8: 0.5 mg/week
• Week 9–12: 1.0 mg/week
• Week 13+: 1.7 mg/week or 2.4 mg/week if tolerated

Many performance users find effective appetite suppression at 0.5–1.0 mg/week without the gastrointestinal discomfort that higher doses produce. There is no requirement to reach maximum dose; stop titrating when appetite suppression is achieving the intended result.

Tirzepatide: Dual Agonism Improves Results

Tirzepatide (Mounjaro for diabetes, Zepbound for weight management) adds GIP receptor agonism to GLP-1 receptor agonism. The SURMOUNT-1 trial showed approximately 20.9% body weight loss at 72 weeks at the 15 mg/week dose - substantially more than semaglutide.

Why dual agonism is more effective: GIP receptor activation has a complementary mechanism. In the brain, GIP receptor agonism appears to amplify the GLP-1-mediated satiety signal. In peripheral tissue, GIP receptor agonism has distinct metabolic effects. The combination produces greater appetite suppression, faster gastric emptying normalisation, and better metabolic outcomes than GLP-1 agonism alone.

Practical differences from semaglutide:

• Typically greater appetite suppression at equivalent tolerance - some users find tirzepatide more effective at lower doses for the same outcome
• Gastrointestinal side effect profile is similar during titration but often perceived as more manageable once at steady state
• More expensive than semaglutide in most markets
• Available in pre-filled pens at fixed doses; research peptide versions require reconstitution

Protocol:

• Week 1–4: 2.5 mg/week
• Week 5–8: 5 mg/week
• Week 9–12: 7.5 mg/week (often sufficient for significant appetite suppression)
• Week 13+: 10 mg, 12.5 mg, or 15 mg if needed

Many performance users plateau at 7.5 mg/week and find further titration adds GI distress without meaningfully greater appetite suppression. The principle is the same as with semaglutide: stop at the dose that achieves the goal.

Retatrutide: Triple Agonism and Current Evidence

Retatrutide (LY3437943) extends the dual-agonist model by adding glucagon receptor agonism to GLP-1 and GIP agonism. The glucagon receptor component increases hepatic fat oxidation and resting energy expenditure, providing an additional fat-loss mechanism beyond appetite suppression.

Phase 2 trial results: In a 48-week trial in obese adults, retatrutide at 12 mg/week produced approximately 24% weight loss - the largest pharmacological weight loss ever observed in a clinical trial at the time of publication. At lower doses (8 mg/week), weight loss was approximately 17%.

What the glucagon component adds:

Glucagon receptor agonism increases hepatic glucose output and fatty acid oxidation. In isolation, this is undesirable (it raises blood glucose). In combination with GLP-1 and GIP agonism, the insulin-stimulating effects counteract the glucose-raising effects, while the lipolysis and energy expenditure effects remain. The net result is more aggressive fat oxidation without net hyperglycaemia in non-diabetic users.

Current limitations:

Retatrutide is not yet commercially available as a pharmaceutical product. It is available from grey-market research peptide vendors, with all the purity and dosing accuracy caveats that entails. The 48-week phase 2 trial establishes efficacy and a safety profile for that period; long-term data (beyond approximately 12 months) does not exist. Safety and efficacy in the absence of pharmaceutical-grade formulation is unknown.

Resting heart rate increase has been observed in retatrutide users in clinical data, the glucagon receptor component has sympathomimetic properties. This is worth monitoring and is more significant than with semaglutide or tirzepatide.

Practical positioning: Retatrutide is appropriate only for users who understand they are using a research compound with less established safety data than semaglutide or tirzepatide. For most performance fat-loss applications, tirzepatide produces results sufficient to make retatrutide an unnecessary escalation.

Common Considerations Across All GLP-1 Class Compounds

Lean mass preservation:

All GLP-1 agonists carry the same lean mass risk - calorie deficit severity combined with appetite suppression that makes eating feel aversive. Strategies:

• Prioritise protein at every meal even when appetite is poor; protein shakes are useful when solid food is undesirable
• Resistance training must continue throughout the cut - loss of training stimulus accelerates muscle loss under calorie restriction
• Track bodyweight and waist measurements separately; if waist is not decreasing proportionally to bodyweight, muscle loss may be disproportionate
• Consider rate of loss: more than 0.5–1% of bodyweight per week is high and increases lean mass loss risk

Injection sites and rotation:

All three compounds are subcutaneous. Rotate between abdomen, outer thigh, and upper arm. Repeated injection into the same site produces local lipodystrophy (fat pad formation or loss) over time.

Gastrointestinal management:

Nausea is the primary side effect, worst in the first 4 weeks of any titration step. Strategies: eat smaller meals, avoid high-fat meals (which interact with slowed gastric emptying), stay hydrated, and reduce dose back one step if symptoms are severe. Most nausea resolves within 1–2 weeks at a stable dose.

Discontinuation:

Appetite suppression reverses after stopping. Users who stop without a maintenance plan frequently regain weight rapidly. Tapering dose or transitioning to a lower maintenance dose is preferable to abrupt cessation for multi-month protocols.