Scientific Glossary

GIP is the other major incretin hormone. Like GLP-1, it stimulates insulin secretion in response to blood glucose elevation. However, GIP receptors are also expressed in adipose tissue and bone. The GIP receptor's role in fat metabolism is complex — it may promote fat storage in some contexts. Dual GIP/GLP-1 receptor agonists (like tirzepatide) leverage both pathways, potentially explaining their superior weight reduction vs. GLP-1 monotherapy.

IGF-1 is a 70-amino-acid protein that mediates most of growth hormone's anabolic and mitogenic effects. GH stimulates hepatic IGF-1 production; IGF-1 then travels systemically to promote protein synthesis, cell proliferation, and glucose uptake. IGF-1 provides negative feedback on GH secretion. Serum IGF-1 is used as a clinical marker for GH status because it reflects integrated GH secretion over time (more stable than measuring pulsatile GH directly).

The two main incretins are GLP-1 (from L-cells) and GIP (from K-cells). The incretin effect accounts for 50-70% of postprandial insulin secretion in healthy individuals — explaining why oral glucose produces more insulin than IV glucose (the "incretin effect"). This glucose-dependence is therapeutically important: incretins only stimulate insulin when blood glucose is elevated, dramatically reducing hypoglycemia risk vs. sulfonylureas. Incretin-based therapies (GLP-1RAs, DPP-4 inhibitors, dual/triple agonists) are now foundational to metabolic disease research.

Pulsatile secretion is physiologically important because continuous receptor stimulation leads to desensitization. The pituitary gland releases GH in 6-12 pulses per day (largest pulse shortly after sleep onset). GnRH must be pulsatile to maintain gonadotropin secretion — continuous GnRH agonist exposure paradoxically suppresses LH/FSH. Research into GHRH and GHRP combinations aims to amplify these natural GH pulses rather than create continuous supraphysiological exposure. Mimicking natural pulsatile patterns is a key design principle in hormone research.

Somatostatin (also called growth hormone-inhibiting hormone, GHIH) provides the off-signal in GH regulation, balancing GHRH's stimulatory effects. Endogenous somatostatin release is triggered by high GH levels (negative feedback), hyperglycemia, and high fat/protein intake. Pharmaceutical somatostatin analogues (octreotide, lanreotide) suppress GH in acromegaly treatment. Understanding somatostatin helps explain why timing of research protocols matters — factors that elevate somatostatin (glucose, fatty acids) theoretically blunt GH responses to secretagogues.