What is GIP?

GIP (glucose-dependent insulinotropic polypeptide) is a 42-amino-acid peptide hormone secreted by K cells in the small intestine in response to nutrient ingestion. Its physiological role is to potentiate glucose-dependent insulin secretion — the beta cell responds more strongly to a given glucose level in the presence of GIP than in its absence — and to promote nutrient uptake and storage in adipose tissue.

For most of the modern history of incretin pharmacology, GIP was dismissed as clinically unimportant. Studies in type 2 diabetes patients showed that GIP infusion produced only modest insulinotropic effect, a phenomenon called 'GIP resistance', and theoretical concerns about GIP promoting fat storage suggested GIP agonism might be harmful in obesity. The clinical success of tirzepatide — a dual GIP/GLP-1 agonist that produces substantially larger effects than pure GLP-1 agonism at head-to-head comparable doses — reversed this picture entirely.

The mechanistic explanation for why GIP receptor agonism helps rather than harms in the tirzepatide context is still being worked out. Hypotheses include: chronic GLP-1 receptor activation may restore GIP responsiveness in beta cells, central GIP receptor activity may contribute to appetite regulation, and the adipose-tissue effects of pharmacologically-sustained GIP activation may differ from the older 'GIP promotes obesity' framing. What is clear is that dual GIP/GLP-1 agonism (tirzepatide) and triple GIP/GLP-1/glucagon agonism (retatrutide) produce larger metabolic effects than pure GLP-1 agonism, and GIP is doing meaningful work in that combination.