Understanding GLP-1 Research

When glucose is taken orally, the insulin response is larger than when the same amount of glucose is delivered intravenously. That difference is called the incretin effect, and it is caused by gut hormones released in response to food.

The two principal incretin hormones are glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Both are peptides, and both are broken down within minutes by the enzyme DPP-4.

Native GLP-1 has a half-life of a few minutes, which makes it useless as a research or therapeutic agent in its raw form. Analogues are engineered versions with modifications that resist DPP-4 cleavage and bind to albumin, extending circulation from minutes to days.

That structural change is why once-weekly dosing schedules are possible with modern analogues, and why comparing an analogue to native GLP-1 on a milligram basis is meaningless.

Single agonists act at the GLP-1 receptor alone. Dual agonists such as tirzepatide add GIP receptor activity. Triple agonists such as retatrutide add glucagon receptor activity on top of both.

Each additional receptor is a research hypothesis, not simply an incremental improvement. Glucagon receptor agonism, for instance, is studied for effects on energy expenditure and hepatic fat that GLP-1 activity alone does not produce.

Nearly every published incretin trial uses a stepwise escalation schedule with four-week intervals. This is a tolerability design, not a therapeutic ramp — the starting dose in most trials is explicitly described as sub-therapeutic.

When comparing published results, the maintenance dose and the trial duration matter far more than the starting dose.