Ghrelin vs IGF-1

Must be acylated at Ser3 (by GOAT enzyme) for GHSR-1a binding. Receptor activation in the pituitary stimulates GH release; hypothalamic action via NPY/AgRP neurons increases appetite and reduces energy expenditure.

Ghrelin is an endogenous 28-amino-acid peptide hormone produced primarily by X/A-like cells of the gastric fundus, characterized by a unique octanoyl modification at Ser3 required for GHS-R1a receptor binding; it is the endogenous ligand for the growth hormone secretagogue receptor and functions as a dual regulator of GH secretion and energy homeostasis. Ghrelin acts centrally via hypothalamic GHS-R1a receptors to potently stimulate GH release from the pituitary and to promote appetite through NPY/AgRP pathway activation, and has peripheral effects on gastric motility and insulin secretion, establishing it as a key integrator of nutritional status, GH axis activity, and energy balance. The pharmacology of GHS-R1a activation in humans is validated through macimorelin (Macrilen), an FDA-approved oral ghrelin receptor agonist; Phase 1 and Phase 2 randomized controlled trials of macimorelin demonstrated robust and reliable GH stimulation in adults, supporting FDA approval in 2017 for the diagnosis of adult GH deficiency and confirming the human physiological relevance of ghrelin receptor activation. Ghrelin itself is not therapeutically administered; it has a very short plasma half-life and the active acylated form is rapidly degraded in circulation; FDA-approved ghrelin receptor agonists require prescription and are indicated for diagnostic rather than therapeutic use, while ghrelin peptide is used exclusively as a research tool compound in neuroendocrine pharmacology studies.

Binds IGF-1R with high affinity, activating PI3K/Akt and MAPK pathways promoting protein synthesis, satellite cell activation, and glucose uptake. Much shorter half-life (~15 min) than the LR3 analog (~20-30 hr) due to binding protein interactions.

IGF-1 (insulin-like growth factor 1; somatomedin C) is an endogenous 70-amino-acid polypeptide produced primarily in the liver in response to growth hormone signaling, functioning as the principal mediator of GH anabolic effects through its receptor (IGF-1R) expressed in virtually all tissues, with roles in muscle protein synthesis, bone growth, and cellular proliferation. IGF-1 activates IGF-1R tyrosine kinase to stimulate PI3K/Akt and MAPK/ERK signaling cascades, promoting protein synthesis, cell survival, and glucose uptake in muscle and bone; endogenous IGF-1 levels peak during puberty and decline with age, and GH-stimulating interventions exert many of their effects by elevating circulating IGF-1. The FDA-approved recombinant form, mecasermin (Increlex), is indicated for primary IGF-1 deficiency in children with GH receptor insensitivity syndrome, with multicenter controlled clinical trials demonstrating significant height velocity improvements in this rare pediatric population; this approved indication is specific to a defined hormonal insufficiency condition and is distinct from performance-enhancing or anti-aging uses of exogenous IGF-1. Research-grade IGF-1 is available as a compounded or gray-market compound used outside its approved indication; it is not approved for adult use or performance applications, and risks associated with IGF-1 excess — including effects on glucose homeostasis, potential oncogenic cell signaling, and acromegaly-related comorbidities — are relevant safety considerations for any off-label use.