Chapter 13 — Key Takeaways

Weight Loss Peptides Beyond GLP-1

The core claims

Appetite is a redundant network defending a set point, not a switch. At least a dozen partly overlapping signals — leptin from fat, GLP-1/GIP/PYY/CCK/ghrelin from gut, insulin and amylin from pancreas, vagal afferents — converge on hypothalamic circuitry. Remove one and the others largely compensate.

Three features, each of which kills a drug class: redundancy (no single signal is necessary); counter-regulation (the set point is not reset); and asymmetry (the system defends much harder against loss than against gain).

The leptin reversal. The model predicted obesity = leptin deficiency. The finding was that common obesity involves elevated leptin and resistance. The biology was right; the model was wrong about which disease it explained. And the resistance phenomenon had been visible in the db/db mouse since the 1960s.

Amylin is co-secreted with insulin, slows gastric emptying, suppresses glucagon, and promotes satiety — overlapping GLP-1's actions from a different tissue. Pramlintide is approved as an insulin adjunct; cagrilintide and CagriSema are in late-stage development.

CagriSema ≠ tirzepatide. Two molecules in one injection versus one molecule hitting two receptors. Tunable ratio versus fixed ratio; two pharmacokinetic profiles versus one. Neither design is obviously better.

The melanocortin pathway is the core appetite circuit: leptin drives POMC neurons (→ α-MSH, activates MC4R, less hunger) and inhibits AgRP neurons (AgRP blocks MC4R, more hunger). Break any component and severe early-onset obesity follows — the monogenic obesities.

Setmelanotide is an 8-amino-acid cyclic MC4R agonist, approved for obesity due to specified rare genetic deficiencies. It works by acting downstream of the broken step, and it is not a general obesity drug because in common obesity there is no broken step.

PYY and ghrelin keep failing because of redundancy. "The hunger hormone" is a compression doing the same damage that "the love hormone" does in Chapter 21.

The chapter's central argument

Why single-target appetite drugs underperform: a redundant network defending a set point is robust against single-node intervention. Leptin targeted a signal already maximal. PYY added one satiety signal to a system with several. Ghrelin blockade removed one hunger signal from a system with several. MC4R agonism works only where the upstream pathway is broken.

Why GLP-1 agonists differed: four simultaneous actions rather than one; a system whose specific job is responding to meals rather than a broad neuromodulatory system; sustained supraphysiological exposure through partly different routes than the endogenous signal uses; and an effect large enough to outweigh counter-regulation while present — but not to reset the set point, which is why weight returns on stopping.

Two honest caveats: this explanation is partly retrospective — it fits what happened without having predicted it — and it does not explain the magnitude of the drug effect.

Evidence ratings issued in this chapter

Leptin's two ratings are the book's cleanest demonstration of rule 6 — the same protein, opposite ends of the scale, and a source omitting the population has destroyed the only information that matters.

The four reasons "GLP-1 boosters" fail

  ① HALF-LIFE     endogenous GLP-1 is gone in ~1–2 minutes; semaglutide lasts ~1 week
  ② MAGNITUDE     a modest postprandial bump vs sustained supraphysiological occupancy
  ③ ROUTE         endogenous acts largely LOCALLY via vagal afferents; the drug acts
                  systemically, including at leaky brain regions
  ④ THE DECISIVE  every human has had normal GLP-1 physiology their whole life, and it
     ONE          has never produced pharmacological weight loss in anyone

④ requires no measurement at all. And the ❌ attaches to the comparison, not the ingredient — fiber has real benefits, and a blanket dismissal would be wrong and would hand the argument away.

Key terms

leptin · leptin resistance · ob/ob mouse · set point · amylin · pramlintide · cagrilintide · melanocortin system · POMC · α-MSH · AgRP · MC4R · setmelanotide · monogenic obesity · peptide YY · ghrelin · orexigenic / anorexigenic · redundancy

What you can now evaluate

• ✅ why a hormone can be dramatic in a deficiency and useless in the disease that resembles it
• ✅ the architectural difference between a dual agonist and a two-molecule combination
• ✅ why a proven mechanism still does not license a broad claim
• ✅ any "boost your own [hormone]" supplement claim, using physiology rather than skepticism
• ✅ why weight returns after any successful appetite intervention, including ones that don't exist yet
• ❌ not yet: what happens when the evidence gets much thinner. Part III.

The one-sentence version

Appetite is defended by a dozen redundant signals, which is why every drug that targeted one of them failed and why the ones that worked hit several at once and pushed far harder than physiology ever does.

Part II ends here. Next: Chapter 14 opens Part III — growth hormone, and the point where the evidence thins and the marketing gets loud.