I wanted to write up a mechanistic explainer on triple agonism since I keep seeing confusion about why adding a glucagon receptor agonist to a GLP-1/GIP backbone is beneficial rather than counterproductive. After all, glucagon raises blood glucose, so why would you want that in an anti-obesity/anti-diabetes drug?

The Three Receptors

GLP-1 (glucagon-like peptide-1): Stimulates insulin secretion, suppresses glucagon, delays gastric emptying, reduces appetite via hypothalamic signaling. This is the workhorse we already know from semaglutide and liraglutide.

GIP (glucose-dependent insulinotropic polypeptide): Enhances insulin secretion synergistically with GLP-1, may promote adipocyte lipid uptake (debated), appears to reduce nausea compared to GLP-1-only agonism. The addition of GIP is what makes tirzepatide more effective than semaglutide.

Glucagon (GCG): Here's where it gets interesting. Glucagon at the hepatic level promotes glycogenolysis and gluconeogenesis (raising glucose), but it ALSO drives:

• Hepatic lipid oxidation (fat burning in the liver)
• Increased energy expenditure / thermogenesis
• Amino acid catabolism (potentially concerning for lean mass)
• Satiety signaling (independent of GLP-1 pathways)

The insight behind retatrutide is that if you combine GCG agonism with sufficient GLP-1/GIP agonism, the glucose-raising effect of glucagon is counterbalanced by the insulin-stimulating effects of GLP-1 and GIP, while the metabolic benefits (fat oxidation, thermogenesis) are preserved.^[1]

"The rationale for triple receptor agonism is to harness the complementary metabolic effects of each hormone while using the glucose-lowering properties of GLP-1 and GIP to offset the hyperglycemic potential of glucagon."
— Finan B, et al. Nat Rev Drug Discov. 2015;14:45-56.

[1] Coskun T, et al. LY3437943, a novel triple GIP/GLP-1/glucagon receptor agonist for the treatment of type 2 diabetes mellitus. Lancet. 2022.

Excellent write-up. I want to add some quantitative context on the thermogenesis claim because it's one of the most exciting and under-discussed aspects.

In preclinical models, glucagon receptor agonism has been shown to increase resting energy expenditure (REE) by approximately 10-15%. If that translates to humans even partially, we're talking about an additional 150-250 kcal/day burned at rest for an average adult.

For perspective, the total caloric deficit from GLP-1 agonism (reduced appetite + delayed gastric emptying) is estimated at ~500-700 kcal/day. Adding 150-250 kcal/day from thermogenesis would represent a 25-35% enhancement in total energy deficit.

This is likely why retatrutide's weight loss curve is steeper than tirzepatide's — you're attacking obesity from both the intake side AND the expenditure side simultaneously.

This is a great explanation, thank you! One concern though — you mentioned glucagon promotes amino acid catabolism. Doesn't that mean retatrutide could cause MORE muscle loss than tirzepatide or semaglutide? That's already a big concern with GLP-1 drugs.

I've been lifting 4x/week and eating 140g protein daily on semaglutide specifically to prevent lean mass loss. Would I need to do even more on a triple agonist? 😬

That's the right question to ask. The amino acid catabolism concern is legitimate but appears to be dose-dependent and potentially mitigated by the GIP component.

There's actually some preclinical evidence that GIP receptor agonism has anabolic effects on bone and potentially muscle tissue. One hypothesis is that the GIP component in both tirzepatide and retatrutide partially counteracts glucagon-driven protein catabolism.

From the Phase 2 retatrutide data, the lean mass loss as a proportion of total weight loss was actually similar to what's seen with semaglutide (~25-35% lean mass). So the additional weight loss doesn't appear to be disproportionately coming from muscle. But we need DEXA sub-study data from Phase 3 to be confident about this.

Your approach of high protein + resistance training is the right strategy regardless of which drug you're on.

I think people also overlook the hepatic fat angle. Glucagon is arguably the most potent endogenous signal for hepatic lipid mobilization. The Phase 2 retatrutide data showed liver fat reductions that were frankly unprecedented — over 80% of participants on the highest dose achieved ≥70% relative reduction in liver fat content.

Compare that to semaglutide's STEP-liver data (around 50% relative reduction at best) and you can see why the MASH indication might end up being just as important as obesity for retatrutide commercially.

The GCG component isn't just a thermogenesis booster — it's specifically targeting hepatic steatosis in a way that GLP-1 alone cannot.