Ghrelin Is Far More Than a Hunger Hormone — Here Is What the Science Shows

Ghrelin was discovered in 1999 as a 28-amino acid peptide from the gut that activated the orphan growth hormone secretagogue receptor GHSR1a. Since then, its biological footprint has expanded dramatically. This comprehensive 2025 review from researchers at Jagiellonian University synthesizes decades of molecular, cellular, and systems-level research to reframe ghrelin as a pleiotropic regulator of human physiology, touching nearly every major organ system and offering multiple drug targets.

At the molecular level, ghrelin's maturation requires a tightly regulated sequence: the 117-amino acid preproghrelin precursor is cleaved and then O-octanoylated at Serine-3 by ghrelin O-acyltransferase (GOAT), the only known enzyme with this substrate specificity. This acylation is essential for high-potency GHSR1a binding, with acyl-ghrelin achieving EC50 values of 2–2.6 nM. Des-acyl ghrelin (DAG), which constitutes roughly 90% of circulating ghrelin, has an EC50 three orders of magnitude weaker (1.6–2.4 µM) at GHSR1a, yet still exerts significant biological activity via ERK1/2 and PI3K/Akt pathways — including cardioprotection, adipogenesis support, macrophage polarization toward M2 phenotypes, and anxiety modulation. Mini-ghrelins, truncated isoforms generated by plasma proteases (including activated protein C), retain N-terminal acylation but act as competitive GHSR1a antagonists, blocking orexigenic signaling in vivo despite partial receptor binding in vitro.

Recent cryo-EM structural work on GHSR1a has revealed a bipartite ligand-binding pocket that accommodates both peptide and small-molecule ligands, providing a structural basis for biased signaling, constitutive receptor activity (GHSR1a is 50% constitutively active even without ligand), and differential coupling to Gq, Gi, and β-arrestin pathways. The endogenous antagonist LEAP-2, which rises with feeding and obesity, competes with ghrelin at GHSR1a and has emerged as a therapeutic target. Synthetic ligands — agonists, antagonists, inverse agonists — exploiting these structural features are in various stages of development.

Systemically, ghrelin's roles extend well beyond appetite stimulation and growth hormone secretion. In metabolism, ghrelin suppresses insulin secretion and promotes hepatic glucose production, with plasma levels rising during fasting and falling post-meal — patterns disrupted in obesity, polycystic ovary syndrome, and Prader-Willi syndrome. In the cardiovascular system, ghrelin exerts vasodilatory and cardioprotective effects, reducing afterload and protecting cardiomyocytes from apoptosis. In bone, ghrelin promotes osteoblast activity and inhibits osteoclastogenesis. In the kidneys, ghrelin regulates renal hemodynamics and shows cytoprotective effects. Immunologically, ghrelin broadly dampens pro-inflammatory cytokine production and shifts macrophages toward anti-inflammatory M2 phenotypes, suggesting relevance in inflammatory and autoimmune diseases.

In the central nervous system, ghrelin influences neuroprotection, stress reactivity, sleep architecture (plasma levels peak during sleep), and circadian rhythms. It has been implicated in depression, Alzheimer's disease pathology, and — critically — substance-use disorders, particularly alcohol and opioid addiction, via dopaminergic signaling in the ventral tegmental area. Translational strategies highlighted include GOAT inhibitors to reduce acyl-ghrelin production, LEAP-2-based GHSR1a antagonism for obesity, and biased agonists to selectively harness ghrelin's beneficial signaling arms while avoiding orexigenic side effects. Ghrelin stabilization in blood (its serum half-life is very short due to rapid de-acylation) remains a formidable pharmacological challenge.