What it is
Ghrelin is a hormone of 28 amino acids with a fatty acid attached. It is cut from a 117-residue precursor protein (UniProt Q9UBU3), and its sequence is:
GSSFLSPEHQRVQQRKESKKPPAKLQPR
What makes it unusual is the third amino acid. Serine 3 carries an n-octanoyl group — an eight-carbon fatty acid bonded to the side chain. The 1999 discovery paper found that this modification is essential for activity; UniProt also records rarer hexanoyl (six-carbon) and decanoyl (ten-carbon) versions. A shorter ghrelin-27, missing the last arginine, also exists.
The human evidence in this entry is physiology — how the hormone behaves in people — not trials of a product. Ghrelin itself is not an approved medicine.
How it was found
The order of discovery ran backwards. Pharmacologists had spent years building small molecules that released growth hormone — the growth hormone secretagogues — and had cloned the receptor they acted on, without knowing what the body itself used it for. In December 1999, Masayasu Kojima, Kenji Kangawa and colleagues reported in Nature the purification of that missing natural ligand from rat stomach: a 28-amino-acid peptide with an octanoylated serine 3 that released growth hormone in living rats and in cells. Human ghrelin, they noted, differs from the rat hormone at two amino acids (PMID 10604470). They named it from ghre, a Proto-Indo-European root for "grow".
What it does
UniProt's summary of the literature lists what activating its receptor (GHSR, the growth hormone secretagogue receptor type 1) does:
| Effect | Recorded in |
|---|---|
| Releases growth hormone from the pituitary | Discovery paper (rats); UniProt function annotation |
| Stimulates appetite | UniProt, citing PMID 11739476 |
| Promotes fat gain (adiposity) | UniProt, same source |
| Stimulates stomach-acid secretion | UniProt, same source |
It is one of two hormones that set growth-hormone release from opposite directions: ghrelin pushes it up, and somatostatin — which UniProt records as blunting ghrelin-stimulated release — holds it down.
A second peptide from the same gene
The ghrelin precursor carries more than ghrelin. Further along the same chain, residues 76–98, sits obestatin, a 23-amino-acid peptide. UniProt annotates it with a possible appetite-reducing effect and slower stomach emptying, both marked "by similarity" — that is, inferred from animal work rather than shown in people.
The drugs aimed at its receptor
Ghrelin's receptor is the target of a family of compounds that many readers meet before they meet the hormone:
| Compound | What it is | Entry |
|---|---|---|
| GHRP-6 | Six-amino-acid synthetic peptide | GHRP-6 |
| GHRP-2 | Six-amino-acid synthetic peptide | GHRP-2 |
| Hexarelin | Six-amino-acid synthetic peptide | hexarelin |
| Ipamorelin | Five-amino-acid synthetic peptide | ipamorelin |
| MK-677 (ibutamoren) | An oral small molecule, not a peptide | MuscleLedger's MK-677 page covers it |
| Macimorelin (Macrilen) | An oral small molecule; FDA-approved as a diagnostic test | — |
Macimorelin is the only one with a US approval, and the approval is narrow. Its label (NDA 205598, approved 2017-12-20) describes a single oral dose after which growth hormone is measured at 30, 45, 60 and 90 minutes, to diagnose adult growth-hormone deficiency; it was established against the insulin tolerance test in a cross-over study, and its label notes that performance has not been established above a BMI of 40.
Where it stands
openFDA returns no application with ghrelin itself as the active ingredient (2026-09-30). PubMed indexes 13,872 records for ghrelin, 1,020 of them tagged as randomised controlled trials.
Related reading
The growth-hormone-releasing peptides above are copies of ghrelin's effect rather than of its sequence. The hormone ghrelin releases acts through IGF-1, whose long-acting laboratory analogue is IGF-1 LR3; the growth-hormone-releasing hormone side of the same system is covered in sermorelin and tesamorelin.