GLP-1 drugs get most of the headlines, but GLP-1 is just one node in a much larger network of peptide and protein hormones that regulate hunger and fullness. Understanding where it fits among leptin, ghrelin, PYY, and CCK helps explain both why these drugs work the way they do and why researchers keep looking at the rest of this hormone network for the next generation of obesity and metabolic treatments.

Leptin: The Long-Term Fat-Storage Signal

Leptin is produced by fat tissue itself and signals the brain about the body's long-term energy reserves, rather than responding to any single meal. In most common obesity, leptin levels are actually high, not low, because fat tissue produces more of it; the problem is leptin resistance, where the brain stops responding normally to that signal. Leptin isn't just a research concept: metreleptin (brand name Myalept) is an FDA-approved leptin analog, approved specifically for generalized lipodystrophy, a rare condition marked by an abnormal lack of body fat and correspondingly low natural leptin levels. It carries an FDA boxed warning for anti-metreleptin antibody formation, which has been associated with serious infections and loss of the drug's effectiveness, along with a risk of T-cell lymphoma and autoimmune disorders, and it's approved under a Risk Evaluation and Mitigation Strategy (REMS) that requires certified prescribers and pharmacies, a topic covered in more depth in this site's guide to REMS programs. It is not approved or appropriate for common obesity, where leptin levels are typically already elevated rather than deficient.

Ghrelin: The Hunger Signal

Ghrelin works in something close to the opposite direction from GLP-1: it's released mainly by the stomach, rises before meals, and is widely described as the body's main "hunger hormone." Bariatric surgery procedures that remove or bypass part of the stomach tend to lower circulating ghrelin, which researchers believe contributes to their appetite-suppressing effect independent of the mechanical restriction on food intake. GLP-1 drugs don't directly block ghrelin production, but some research suggests they may blunt its appetite-stimulating signal downstream; this remains an active area of study rather than a settled mechanism.

PYY and CCK: Gut-Released Fullness Signals

Peptide YY (PYY) and cholecystokinin (CCK) are both released from the gut after eating and contribute to short-term feelings of fullness, working on a faster timescale than leptin's long-term signal. PYY is released from the small intestine and colon in response to food and has been studied, alongside GLP-1, as part of why bariatric surgery produces such durable appetite changes, since surgery increases post-meal release of both hormones. CCK triggers gallbladder contraction and slows stomach emptying in addition to its satiety signaling, a mechanism that overlaps conceptually with how GLP-1 drugs slow gastric emptying, discussed in this site's coverage of GLP-1 medications and gastroparesis.

Why Combination Approaches Are a Growing Research Focus

The existence of this whole network, rather than a single hunger switch, is a big part of why newer obesity drug research keeps moving toward multi-hormone combinations instead of single-target drugs. Amylin, covered in this site's dedicated amylin agonists guide, is already a validated combination target alongside GLP-1. GDF15, covered separately on this site, represents yet another distinct appetite-regulating pathway under active investigation. Direct drug targeting of ghrelin, PYY, or CCK receptors for obesity remains largely in earlier-stage or preclinical research rather than approved therapy, but the broader scientific logic, that appetite is controlled by a redundant network of signals rather than one single pathway, is the throughline connecting metreleptin's narrow approved use, amylin-GLP-1 combinations, and the multi-agonist peptides now in late-stage trials.

The Bottom Line

GLP-1 drugs work within a much larger, overlapping hormonal system, not instead of it. That's useful context for understanding both why isolated GLP-1 agonism has real limits and why so much current drug development is aimed at combining GLP-1 with other hormones in this same network rather than replacing it with something unrelated.