Insulin Sensitivity

For two decades GIP (glucose-dependent insulinotropic polypeptide) was written off as a therapeutic target. In uncontrolled type-2 diabetes, GIP's insulin-stimulating effect looked broken — "severely impaired", as the literature put it. The pharma industry moved on to GLP-1.

The breakthrough was that GIP wasn't broken, it was dormant. Pair it with a GLP-1 agonist and GIP signalling reactivates. This is the mechanism behind tirzepatide (Mounjaro/Zepbound) — the first clinically successful "twincretin". The GLP-1 signal appears to wake up the GIP pathway that metabolic disease had silenced, and the two hormones act synergistically on insulin sensitisation.

The framing shift matters. Instead of a single receptor hit with overwhelming force, the dual agonist approach better mimics the natural hormonal symphony the body uses to handle a meal. The strategy is closer to restoration than to override.

The most clinically important observation about tirzepatide is not the weight loss number. It is that insulin sensitivity improves faster than weight does, and more than weight loss alone would predict.

From the source literature: weight loss explains only about 13–21% of the observed improvement in HOMA-IR (the standard insulin-resistance index). The remaining ~80% is direct metabolic reprogramming — independent of how much the patient has lost on the scale.

Mechanistically, this direct reprogramming works at several levels:

• Ectopic fat clearance — reduction in fat accumulation in the liver and skeletal muscle, where it drives insulin resistance at the cellular level.

• Branched-chain amino acid clearance — improved clearance of BCAAs from the blood, which are an under-discussed driver of insulin resistance.

• Adiponectin — higher circulating adiponectin, a hormone released from healthy fat tissue that directly sensitises the liver and muscle to insulin.

The clinical takeaway: insulin sensitivity improves before the weight does. It decouples the weight-loss story from the metabolic-health story, and it means that even patients who respond modestly on the scale may be gaining substantial metabolic benefit. The scale is a lagging indicator here, not the primary outcome.