Tirzepatide

Tirzepatide is engineered for imbalanced dual-receptor agonism — favoring GIP over GLP-1. This is deliberate: GIP activation is not dose-limited by GI side effects; GLP-1 activation is.

Why GIP Matters

GLP-1 receptors are not expressed in adipose tissue; GIP receptors are. This enables direct adipocyte signaling unavailable to semaglutide.

Key mechanism: GIPR activation triggers SERCA-mediated futile calcium cycling — ATP consumption that generates heat without productive work (³¹

Body Composition (DXA Substudy)

DXA analysis of 160 SURMOUNT-1 participants showed ~75:25 fat-to-lean mass loss ratio at the 15 mg dose — fat mass dropped 34%, lean mass dropped 11%. This ratio remained consistent across weight loss categories. In comparison, semaglutide produces ~60:40 in STEP 1 data, with approximately 40% of weight lost coming from lean mass.⁴

SURMOUNT-5 (Head-to-Head vs Semaglutide)

Direct comparison in 751 adults: tirzepatide (max tolerated 10–15 mg) produced 20.2% weight loss versus semaglutide (max tolerated 1.7–2.4 mg) at 13.7% over 72 weeks. Same dosing frequency, same titration schedule. The 47% greater weight loss reflects the mechanistic difference.²

SURPASS-2 (Type 2 Diabetes)

Open-label comparison against semaglutide 1 mg in 1,879 patients with T2DM: tirzepatide achieved greater HbA1c reduction (2.0–2.3% vs 1.86%) and 1.9–5.5 kg more weight loss depending on dose.

Diabetes Context

In people with type 2 diabetes, the body composition advantage shrinks. The fat:lean ratio becomes nearly identical to semaglutide (~87:13 vs ~86:14). This is attributed to the "incretin defect" — impaired GIP signaling in diabetics. Tirzepatide still produces more total weight loss, but the mechanism shifts from "better ratio" to "more total fat."

Liver Fat

MRI substudies showed ~47% relative liver fat reduction at the highest dose — attributed to weight loss and improved insulin sensitivity rather than direct hepatic effects (unlike glucagon-active compounds).¹