Ghrelin

Ghrelin is a 28-amino-acid hormone released mainly by the stomach and the only circulating signal known to actively stimulate hunger. It was discovered in 1999 as the natural ligand for GHS-R1a, the receptor that synthetic growth hormone secretagogues target. It rises before meals, climbs during fasting and weight loss, and both raises appetite and releases growth hormone.

Ghrelin is the endogenous molecule; the others are synthetic drugs built to copy it. Ipamorelin, GHRP-6, and MK-677 all agonize the same GHS-R1a receptor, but they are engineered for stability and selectivity, whereas native ghrelin has a short half-life and a fragile acyl group. In practice the mimetics are what researchers actually dose. Ghrelin itself is studied as the reference ligand and in wasting conditions.

Because it is the appetite hormone, understanding it is central to weight regulation, but its direct effect is the opposite of a fat-loss drug. Ghrelin increases food intake, so it is researched for cachexia, anorexia, and muscle wasting, not for shedding weight. GLP-1 agonists like semaglutide suppress appetite; ghrelin drives it. The honest framing is that ghrelin is the target to understand, not a compound to take for fat loss.

Yes. Acyl-ghrelin binds GHS-R1a on pituitary somatotrophs and produces a GH pulse, and it works in synergy with GHRH so the combined release is larger than either signal alone [1,3]. This is exactly the pathway Ipamorelin and the GHRPs exploit. In native ghrelin the GH effect and the hunger effect are inseparable, which is one reason the selective mimetics were developed.

Cachexia in heart failure, COPD, and cancer combines anorexia with muscle loss, and ghrelin counters both by driving appetite and nudging anabolic GH/IGF-1 signaling. Ghrelin levels are actually elevated in cachectic patients as a compensatory response [6], and short intravenous infusions have been tested to improve intake and body composition. This is the setting where raising appetite is the therapeutic goal rather than a side effect.

The enzyme ghrelin O-acyltransferase attaches an octanoyl fatty acid to the serine-3 residue, and only this acylated form activates GHS-R1a [1]. The des-acyl form circulates in larger quantities but does not trigger the classic GH and appetite responses. The acyl group is chemically labile and hydrolyzes off over time, which is why reconstituted ghrelin must be kept cold, protected from light, and used quickly.