How do weight loss peptides work?

Weight loss peptides with proven human efficacy work primarily by mimicking incretin hormones — gut-derived peptides released after eating that signal fullness to the brain. They do not "burn fat" directly; people eat less, and the energy deficit does the rest.

• GLP-1 receptor agonism. Semaglutide is a modified version of glucagon-like peptide-1. Activating GLP-1 receptors slows gastric emptying, enhances glucose-dependent insulin secretion, and acts on appetite centers in the hypothalamus and brainstem to reduce hunger. In STEP 1 (n=1,961), that translated to a mean 14.9% weight loss versus 2.4% for placebo over 68 weeks (NEJM 2021).
• Dual agonism (GLP-1 + GIP). Tirzepatide adds activity at the glucose-dependent insulinotropic polypeptide (GIP) receptor, which appears to further influence appetite and insulin sensitivity. In SURMOUNT-1 (n=2,539), the 15 mg dose produced a mean 20.9% weight loss versus 3.1% for placebo at 72 weeks (NEJM 2022).
• Triple agonism (GLP-1 + GIP + glucagon). Retatrutide adds glucagon-receptor activity, hypothesized to raise energy expenditure on top of appetite suppression. Its phase 2 trial (n=338) reported a mean 24.2% weight reduction at the 12 mg dose versus 2.1% for placebo at 48 weeks (NEJM 2023). It remains investigational.

A pattern worth noticing: each added receptor has, so far, increased average weight loss in trials — evidence that appetite and energy balance are governed by multiple overlapping hormone signals rather than a single switch.

Compounds marketed as "fat-burning" peptides claim to work downstream, at the fat cell or its metabolism, rather than on appetite. The evidence here is thinner and mostly preclinical:

• AOD-9604 is a synthetic fragment of human growth hormone's C-terminus, designed to keep hGH's fat-mobilizing (lipolytic) activity without its effects on glucose. In obese mice, two weeks of treatment reduced body weight and fat, apparently through the beta-3 adrenergic receptor — the effect largely disappeared in mice lacking that receptor (Endocrinology 2001). A phase 2 human obesity program existed in the early 2000s, but no peer-reviewed efficacy results were published and the compound was never approved.
• 5-Amino-1MQ is not a peptide at all — it is a small-molecule inhibitor of the enzyme nicotinamide N-methyltransferase (NNMT), which is overexpressed in the fat tissue of obese animals. In diet-induced obese mice it reduced body weight, white adipose mass, and adipocyte size (Biochem Pharmacol 2018). No human trials have been published.

Mechanistic plausibility in a mouse is the starting line of drug development, not the finish; most compounds with promising rodent data fail to replicate in humans.

Body weight is regulated by a feedback network spanning the brain (hunger and satiety circuits), the gut (incretin hormones, gastric emptying), the pancreas (insulin, glucagon), the liver, and adipose tissue itself (leptin and other adipokines). Any single-target intervention is pushing on one node of that network while the rest push back — which is why appetite typically returns and weight is regained when incretin drugs are stopped, and why trials run for a year or more rather than weeks.

This systems view also explains the evidence hierarchy. Drugs acting on brain–gut appetite circuits have produced double-digit percentage weight loss in randomized trials because reduced intake reliably produces an energy deficit. Compounds targeting fat-cell metabolism directly have so far shown effects only in animal models, where energy balance is far easier to manipulate than in free-living humans.

For more context:

• What are weight loss peptides?
• What peptide kills fat cells?
• Semaglutide vs tirzepatide comparison
• Catalog entries: semaglutide, tirzepatide, retatrutide