Much of metabolic research centers on the incretin system — hormones released after eating that signal the pancreas to release insulin. Two receptors, GLP-1R and GIPR, are the main focus here, and many of today’s most-studied peptides are designed to activate one or both at once. A separate hormone, ghrelin, works in the opposite direction: it signals hunger and triggers growth hormone release through its own receptor, GHSR-1a.

In the brain, a set of receptors called the melanocortin system (especially MC4R) helps regulate appetite and how much energy the body burns at rest. At the cellular level, two “energy sensor” pathways — AMPK and mTOR — read the body’s fuel status and adjust processes like fat storage and breakdown accordingly.

Researchers commonly study these pathways in diet-induced obesity mouse models, or in genetic models bred to lack a specific receptor, which helps isolate exactly what that pathway does. At the cell level, researchers use fat cells (to study fat storage and insulin sensitivity), pancreatic beta-cells (to study insulin release), and liver or muscle cells (to study how glucose is used and produced).

The peptides used in this field generally fall into a few groups: incretin mimetics (single, dual, or triple receptor agonists), ghrelin-pathway modulators, and compounds that act on satiety hormones like amylin. A growing area of interest is “biased agonism” — the idea that different peptides binding the same receptor can trigger different downstream effects, not just a stronger or weaker version of the same one.