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THEPeptide Academy
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LESSON 1 OF 2

Metabolism and the growth hormone axis

By the end you can

  • Describe the gut hormones behind GLP-1 research
  • Trace the growth hormone axis from brain to liver
  • Match products to each system

Gut hormones and metabolism

After a meal, the gut and pancreas release peptide hormones that tell the body food has arrived. GLP-1 and GIP signal the pancreas to release insulin, and GLP-1 also acts on appetite centers in the brain. Amylin, released with insulin, adds a fullness signal. Glucagon, also from the pancreas, works the other way: it is released between meals to free up stored energy.

This system is the most active area of peptide research today. Semaglutide targets the GLP-1 receptor, tirzepatide targets GLP-1 and GIP, and retatrutide adds the glucagon receptor. Cagrilintide is a long-acting amylin analog.

The growth hormone axis

Growth hormone (GH) is controlled by a chain of signals. The hypothalamus in the brain releases GHRH, which tells the pituitary gland to release GH. GH travels to the liver, which makes IGF-1, the hormone behind many of GH's effects. A brake called somatostatin keeps the system in check.

A second signal, ghrelin, also pushes the pituitary to release GH through its own receptor. Research peptides target this axis at two points: GHRH analogs (sermorelin, CJC-1295, tesamorelin) and ghrelin mimics, often called GH-releasing peptides or GH secretagogues (ipamorelin, GHRP-2, GHRP-6).

Hypothalamus(brain)GHRHPituitary gland(releases GH)GHLiver(makes IGF-1)IGF-1 → tissuesGhrelin (from the stomach)GHRELIN MIMICSIpamorelin, GHRP-2, GHRP-6Somatostatin(the brake)GHRH ANALOGSSermorelin, CJC-1295, tesamorelin

Swipe to see the full diagram →

The growth hormone axis and where research peptides act on it.

Key terms

GLP-1
A gut hormone released after eating that signals insulin release and fullness.
GIP
A second gut hormone that also signals insulin release.
Amylin
A hormone released with insulin that adds a fullness signal.
GHRH
Growth hormone-releasing hormone, the brain's signal to release GH.
Secretagogue
A substance that causes a gland to release (secrete) a hormone.
IGF-1
Insulin-like growth factor 1, made by the liver in response to GH.

Remember

  • GLP-1, GIP, amylin and glucagon form the core of metabolic peptide research.
  • GH release runs hypothalamus → pituitary → liver (IGF-1).
  • GHRH analogs and ghrelin mimics act at different points on the same axis.