Why Ghrelin Is Called the “Hunger Hormone” — and Why That Name Is Incomplete
Ghrelin was named in 1999 after the root of the word “grow,” because its discoverers found it as a growth hormone-releasing peptide from the stomach. The hunger label came later, from levels that nearly double before meals and an infusion study that raised food intake by 28%. Yet adult mice with 80–95% less ghrelin eat and weigh the same as controls, while mice unable to make active ghrelin become severely hypoglycemic within 7 days on 60% of their usual calories.
Ask almost any health site what ghrelin does, and the answer fits in two words: hunger hormone. The scientists who found it had a different idea. They named it for growth, and two decades of genetic studies suggest their instinct holds up better than the nickname. In work with peptides for scientific research, ghrelin sits where appetite, growth hormone, and glucose control meet, so the label shapes how studies get framed. The sections below weigh what supports the ghrelin hunger hormone label, what contradicts it, and what the molecule turns out to be essential for.
Note: This content is provided for educational purposes within a research context only. It does not promote or suggest the use of peptides for personal, medical, or non-research applications.
Where Did the Name Ghrelin Come From?
Ghrelin was not discovered as an appetite signal. In 1999, Kojima and colleagues were looking for the natural ligand of the growth hormone secretagogue receptor, the receptor that synthetic GH-releasing compounds already targeted. They purified it from rat stomach and found a 28-amino-acid peptide with an unusual feature: the serine at position 3 carries an n-octanoyl group, a fatty acid chain the peptide needs in order to be active (Kojima et al., 1999).
The name came from that first result. “Ghre” is the Proto-Indo-European root of the word “grow,” and the peptide’s defining action was releasing growth hormone. Its place among the growth hormone peptides studied today traces back to that discovery, not to anything about food.
The source was the surprise. A ligand for a receptor tied to pituitary GH release might be expected in the brain, yet the main supply came from the stomach (Tschöp et al., 2000). That gut origin is what later pulled ghrelin into appetite research. Inside the cell, its receptor runs a calcium-based cascade that sets the GHRH and ghrelin pathways apart.
The Evidence Behind the Ghrelin Hunger Hormone Label
The hunger label was earned within two years of the discovery. Four findings carry most of the weight behind calling ghrelin the hunger hormone:
- Rodent levels follow feeding. Serum ghrelin rose with fasting and fell with refeeding or oral glucose, and ghrelin injected into the brain ventricles increased food intake in a dose-dependent way (Tschöp et al., 2000).
- Infusion raises intake in humans. In 9 healthy volunteers, intravenous ghrelin at 5.0 pmol/kg/min increased energy intake from a free-choice buffet by 28% (± 3.9%), with no change in gastric emptying. That made ghrelin the first circulating hormone shown to stimulate food intake in humans (Wren et al., 2001).
- Levels rise before meals. Sampling 10 people 38 times over 24 hours, Cummings and colleagues saw plasma ghrelin nearly double before each meal and fall to a trough within an hour of eating (Cummings et al., 2001).
- No other hormone does the same job. A later review called ghrelin the only known circulating orexigenic hormone and placed its action in the hypothalamus, the hindbrain, and the mesolimbic reward system (Cummings, 2006).
Together, these results make the case for the ghrelin hunger hormone label: levels track the meal clock, extra ghrelin makes people eat more, and the brain areas involved cover both need and reward. The reward part matters because appetite drugs also act on reward circuitry, which helps explain why changing an appetite signal can change more than appetite.
Why Don’t Mice Without Ghrelin Lose Their Appetite?
If ghrelin drove hunger, removing it should reduce eating. It mostly does not. McFarlane and colleagues set out the paradox plainly: injected ghrelin stimulates food intake, yet mice born without ghrelin show no significant loss of appetite. That left two readings. Either other systems compensate during development, or ghrelin is not essential for normal eating (McFarlane et al., 2014).
To separate the two, the team used diphtheria toxin to destroy ghrelin-producing cells in adult mice, after development was complete. Plasma ghrelin fell by 80–95%. The mice did not eat less, did not lose weight, and responded to a high-fat diet like controls. To make them eat more by injection, the researchers had to push plasma ghrelin many-fold above its normal range (McFarlane et al., 2014).
The genetic picture matches. Zhao and colleagues summarized it in one line: “Elimination of the gene encoding ghrelin or its receptor produces only mild phenotypes in mice” (Zhao et al., 2010). Under everyday conditions, the ghrelin hunger hormone behaves like one input among several, not a switch.
The evidence does not all point one way. Cummings (2006) reported that congenital knockout mice resist diet-induced obesity and that drugs blocking ghrelin reduce food intake. One way to hold both sets of results is that ghrelin can push intake up at high levels and contributes at the margins, without being required for normal daily eating.
What Is Ghrelin Actually Essential For?
The clearest answer came from an enzyme. Ghrelin only works once ghrelin O-acyltransferase (GOAT) attaches its octanoyl group, so mice lacking GOAT cannot make active ghrelin. Zhao and colleagues found that these mice grew normally on regular food. Fed 60% of their usual calories, they became severely hypoglycemic and were moribund by day 7. Infusing either ghrelin or growth hormone rescued them (Zhao et al., 2010).
The adult-ablation mice told the same story. They ate normally, but under prolonged calorie restriction they developed profound hypoglycemia, “confirming that ghrelin acts to maintain blood glucose under famine conditions” (McFarlane et al., 2014). Ge and colleagues went a step further and proposed that GH release through the ghrelin receptor “evolved to protect against starvation-induced hypoglycemia” (Ge et al., 2018).
The pattern fits what is known about fasting, when GH secretion rises while IGF-1 falls. Ghrelin appears to be one of the signals that keeps GH coming when food runs out, and its loss only becomes visible once blood glucose is under real pressure.
Acyl Ghrelin, Des-Acyl Ghrelin, GOAT, and LEAP2
| Component | What it is | Where it is made or found | Effect on GHSR-1a |
|---|---|---|---|
| Acyl ghrelin | 28-amino-acid peptide with octanoyl on serine 3 | Mainly the stomach | Activates the receptor |
| Des-acyl ghrelin | Same chain without the octanoyl group | Circulation | No activation |
| GOAT | Enzyme that attaches the octanoyl group | Largely stomach and intestine | None directly; makes activation possible |
| GHSR-1a | Ghrelin receptor | Pituitary, hypothalamus, hindbrain, reward areas | About 50% active with nothing bound |
| LEAP2 | Endogenous peptide antagonist | Liver and small intestine; falls with fasting | Blocks activation |
The octanoyl group is the reason one peptide behaves like two. Yang and colleagues identified GOAT as the enzyme that attaches it and showed the fatty acid is required for ghrelin’s endocrine actions. GOAT mRNA is largely restricted to the stomach and intestine, matching where active ghrelin is made (Yang et al., 2008). A single lipid modification deciding whether a peptide works at all is a sharp example of peptide structure setting function.
The receptor adds its own baseline. GHSR-1a shows about 50% constitutive activity, meaning it signals even with nothing bound, so selective agonists such as Ipamorelin act on a receptor that is already partly on.
The newest piece is LEAP2. Ge and colleagues showed it is an endogenous antagonist of the ghrelin receptor: it fully blocked GHSR-1a activation and ghrelin’s effects on food intake, GH release, and blood glucose. Fasting suppresses LEAP2, so during a fast the receptor meets more ghrelin and less of its blocker at the same time (Ge et al., 2018).
What Else Does Ghrelin Do Beyond Hunger?
Hunger and GH are not the full list. Reviews of ghrelin function describe roles in (Müller et al., 2015):
- learning and memory
- gut motility and gastric acid secretion
- sleep and wake rhythm
- reward seeking
- taste
- glucose metabolism
Some of these run through other hormones. Ghrelin raises ACTH, cortisol, catecholamines, and prolactin as well as plasma glucose, and its effects on motility and acid depend on vagal cholinergic pathways (Hagemann et al., 2003). Inui and colleagues described ghrelin and leptin as complementary yet antagonistic signals, a pairing that places the stomach among the body’s endocrine organs (Inui et al., 2004).
How Do Ghrelin Receptor Agonists Separate Appetite From GH Release?
| Compound | Type | Reported profile | Status |
|---|---|---|---|
| GHRP-6 | Synthetic peptide | Strongest appetite effect in the GHRP family; ACTH, cortisol, prolactin co-release | Research compound |
| GHRP-2 | Synthetic peptide | Moderate ACTH co-release | Research compound |
| Hexarelin | Synthetic peptide | Higher GHSR-1a affinity; also binds CD36 | Research compound |
| Ipamorelin | Synthetic peptide | Most GHSR-1a-selective; essentially no ACTH or prolactin | Research compound |
| MK-677 | Non-peptide, oral | Half-life about 24 hours | Research compound |
| Macimorelin | Oral agonist | Tests GH reserve | FDA-approved, 2017, adult GH deficiency diagnosis |
| Anamorelin | Receptor agonist | Targets cachexia | Approved in Japan, 2021, cancer cachexia |
The spread across this class is what makes it useful in study design. GHRP-6 drives its appetite effect through NPY/AgRP orexigenic signaling, while Ipamorelin releases GH with the side signals largely stripped away. Running both in one model is a way to test which outcomes follow GH and which follow the wider ghrelin signal.
MK-677 shows that the receptor does not need a peptide at all. A small non-peptide molecule is enough to activate it, which makes the receptor, not the ghrelin sequence, the common thread across the class.
A shared receptor does not mean shared status. Only two ghrelin receptor agonists are approved medicines, each for one narrow use, and every other compound in the class sits on the research side of the line between approved and research-use peptides.
A Hormone Named for Growth, Remembered for Hunger
The ghrelin hunger hormone label is not wrong. Ghrelin levels rise before meals, infusion makes people eat about 28% more, and no other circulating hormone stimulates intake the same way. What the label misses is how little normal eating depends on it: adult mice with most of their ghrelin removed eat and grow as usual. The job ghrelin cannot hand off appears only under severe calorie restriction, when its push on GH keeps blood glucose from collapsing. For study design, that means appetite readouts and GH or glucose readouts answer different questions about the same peptide.
References
Cummings, D. E. (2006). Ghrelin and the short- and long-term regulation of appetite and body weight. Physiology & Behavior, 89(1), 71–84. https://doi.org/10.1016/j.physbeh.2006.05.022
Cummings, D. E., Purnell, J. Q., Frayo, R. S., Schmidova, K., Wisse, B. E., & Weigle, D. S. (2001). A preprandial rise in plasma ghrelin levels suggests a role in meal initiation in humans. Diabetes, 50(8), 1714–1719. https://doi.org/10.2337/diabetes.50.8.1714
Ge, X., Yang, H., Bednarek, M. A., [AUTHORS TO ADD], & Kaplan, D. D. (2018). LEAP2 is an endogenous antagonist of the ghrelin receptor. Cell Metabolism, 27(2), 461–469.e6. https://doi.org/10.1016/j.cmet.2017.10.016
Hagemann, D., Meier, J. J., Gallwitz, B., & Schmidt, W. E. (2003). Appetite regulation by ghrelin: A novel neuro-endocrine gastric peptide hormone in the gut-brain-axis. Zeitschrift für Gastroenterologie, 41(9), 929–936. https://doi.org/10.1055/s-2003-41853
Inui, A., Asakawa, A., Bowers, C. Y., Mantovani, G., Laviano, A., Meguid, M. M., & Fujimiya, M. (2004). Ghrelin, appetite, and gastric motility: The emerging role of the stomach as an endocrine organ. The FASEB Journal, 18(3), 439–456. https://doi.org/10.1096/fj.03-0641rev
Kojima, M., Hosoda, H., Date, Y., Nakazato, M., Matsuo, H., & Kangawa, K. (1999). Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature, 402(6762), 656–660. https://doi.org/10.1038/45230
McFarlane, M. R., Brown, M. S., Goldstein, J. L., & Zhao, T. J. (2014). Induced ablation of ghrelin cells in adult mice does not decrease food intake, body weight, or response to high-fat diet. Cell Metabolism, 20(1), 54–60. https://doi.org/10.1016/j.cmet.2014.04.007
Müller, T. D., Nogueiras, R., Andermann, M. L., Andrews, Z. B., Anker, S. D., Argente, J., … Tschöp, M. H. (2015). Ghrelin. Molecular Metabolism, 4(6), 437–460. https://doi.org/10.1016/j.molmet.2015.03.005
Tschöp, M., Smiley, D. L., & Heiman, M. L. (2000). Ghrelin induces adiposity in rodents. Nature, 407(6806), 908–913. https://doi.org/10.1038/35038090
Wren, A. M., Seal, L. J., Cohen, M. A., Brynes, A. E., Frost, G. S., Murphy, K. G., Dhillo, W. S., Ghatei, M. A., & Bloom, S. R. (2001). Ghrelin enhances appetite and increases food intake in humans. The Journal of Clinical Endocrinology & Metabolism, 86(12), 5992. https://doi.org/10.1210/jcem.86.12.8111
Yang, J., Brown, M. S., Liang, G., Grishin, N. V., & Goldstein, J. L. (2008). Identification of the acyltransferase that octanoylates ghrelin, an appetite-stimulating peptide hormone. Cell, 132(3), 387–396. https://doi.org/10.1016/j.cell.2008.01.017
Zhao, T. J., Liang, G., Li, R. L., Xie, X., Sleeman, M. W., Murphy, A. J., Valenzuela, D. M., Yancopoulos, G. D., Goldstein, J. L., & Brown, M. S. (2010). Ghrelin O-acyltransferase (GOAT) is essential for growth hormone-mediated survival of calorie-restricted mice. Proceedings of the National Academy of Sciences, 107(16), 7467–7472. https://doi.org/10.1073/pnas.1002941107
Frequently Asked Questions
The ghrelin hunger hormone nickname comes from blood levels that nearly double before meals and fall within an hour after eating, and from a study in which intravenous ghrelin raised food intake by 28% in healthy volunteers. It is also the only known circulating hormone that stimulates appetite. The name itself refers to growth: its discoverers named it in 1999 for its ability to release growth hormone.
Ghrelin and leptin are complementary yet opposing appetite signals. Ghrelin, made mainly in the stomach, is the only known circulating hormone that stimulates food intake, while leptin works against it. Ghrelin also releases growth hormone through its receptor, GHSR-1a.
Ghrelin is produced mainly in the stomach, where it was first purified in 1999. The enzyme that activates it, GOAT, is largely restricted to the stomach and intestine. Its natural blocker, LEAP2, comes from the liver and small intestine.
Acyl ghrelin carries an octanoyl fatty acid on serine 3 and activates the ghrelin receptor, GHSR-1a. Des-acyl ghrelin is the same 28-amino-acid peptide without that group and does not activate the receptor. The enzyme GOAT attaches the octanoyl group.
Yes. Ghrelin was discovered as the natural ligand of the growth hormone secretagogue receptor and releases GH through it. Mouse studies suggest this is its essential role: animals unable to make active ghrelin become severely hypoglycemic under heavy calorie restriction, and GH infusion rescues them.
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