GLP-1 Agonists Guide

GLP-1 (glucagon-like peptide-1) agonists mimic a gut hormone that regulates appetite and blood sugar. They slow gastric emptying, reduce hunger signals to the brain, and improve insulin sensitivity. This leads to reduced calorie intake and weight loss of 15-25% of body weight in clinical trials.

Semaglutide (Ozempic/Wegovy) is a pure GLP-1 agonist — it suppresses appetite but cannot directly signal fat cells (GLP-1 receptors aren't expressed in adipose tissue), producing ~15% weight loss and a 60:40 fat-to-lean ratio. Its key advantage is the strongest cardiovascular outcome data (SELECT trial). Tirzepatide (Mounjaro/Zepbound) adds GIP receptor agonism, which directly engages fat cells via thermogenesis — producing 20-22% weight loss and a better 75:25 fat-to-lean ratio in non-diabetic obesity. Retatrutide is a triple agonist with redesigned receptor potencies (2× GIP, 0.5× GLP-1, 0.2× glucagon) — the glucagon component directly mobilizes liver fat (86% reduction), and the stronger GIP was engineered to push through the impaired GIP signaling seen in type 2 diabetes. Phase 3 trials are ongoing.

Retatrutide showed the highest weight loss at 24.2% at 48 weeks (phase 2, non-diabetic obesity, no plateau reached), followed by tirzepatide at 22.5% over 72 weeks (SURMOUNT-1) and semaglutide at 14.9% over 68 weeks (STEP-1). In the head-to-head SURMOUNT-5 trial, tirzepatide produced 47% more weight loss than semaglutide (20.2% vs 13.7%). Important: these numbers are from non-diabetic obesity trials — in type 2 diabetes, all incretins show substantially less weight loss due to impaired GIP signaling. Retatrutide is still investigational; semaglutide and tirzepatide are FDA-approved.

Most people notice reduced appetite within the first week. Weight loss typically begins in weeks 2-4. By week 8-12, users commonly see 5-10% weight loss. Maximum effects usually occur at 16-20 weeks on maintenance dose, with continued gradual loss through 68-72 weeks.

The most common side effects are gastrointestinal: nausea (affects 20-40%), vomiting, diarrhea, and constipation. These typically decrease after the first few weeks as the body adjusts. Slow dose escalation reduces side effect severity. Serious but rare risks include pancreatitis and gallbladder issues.

Yes — GLP-1 agonists were originally developed for type 2 diabetes. Semaglutide (Ozempic) and tirzepatide (Mounjaro) are FDA-approved for diabetes management, improving HbA1c by 1.5-2.3%. However, weight loss results differ between diabetic and non-diabetic populations. In T2D, GIP signaling is impaired (the "incretin defect"), which blunts tirzepatide's body-composition advantage — head-to-head data shows similar fat-to-lean ratios for tirzepatide and semaglutide in diabetic patients. Retatrutide was specifically designed with stronger GIP (2× native) and added glucagon to partially overcome this limitation.

Resistance training 2-4x weekly and high protein intake (1.6-2.2g per kg body weight) are essential. Without exercise, lean mass losses vary by drug: semaglutide shows a 60:40 fat-to-lean ratio (40% of weight lost is lean mass), while tirzepatide achieves 75:25 in non-diabetic populations thanks to GIP receptor activation in fat cells. This body-composition advantage narrows in type 2 diabetes where GIP is impaired. Regardless of drug, training and protein are the biggest levers for preserving muscle.