Chapter 7 — Further Reading

Tier 1 — Verified canonical

Endocrine and gastrointestinal physiology. Guyton and Hall's Textbook of Medical Physiology and Williams Textbook of Endocrinology both cover incretin physiology, though at less depth than this chapter in places — gut endocrinology is still under-represented in general physiology texts relative to its clinical importance.

Bayliss and Starling (1902) — the secretin experiment, the origin of both endocrinology and gut hormone research. Freely available; see Chapter 3's reading list.

The proglucagon gene and its tissue-specific processing to yield glucagon in pancreatic alpha cells and GLP-1 in intestinal L cells is standard molecular endocrinology, covered in the references above and in any current review of incretin biology.

The IUPHAR/BPS Guide to Pharmacology (guidetopharmacology.org) has curated entries for the GLP-1 receptor and the GIP receptor, including their classification as class B GPCRs, their endogenous ligands, and their known pharmacology. Free, expert-maintained, and the right place to check any receptor claim.

Approved drug labels for the GLP-1 receptor agonists (DailyMed) contain a Clinical Pharmacology section describing the mechanism in regulatory language. Comparing that language to a marketing claim is instructive — regulators are notably more careful about what is and is not established.

Bariatric surgery. The metabolic effects of bariatric procedures, including glycemic improvement preceding substantial weight loss and exaggerated postprandial GLP-1 responses, are well documented in the surgical and endocrine literature and covered in major textbooks of both fields.

Tier 2 — Attributed, specifics unverified

On the magnitude of the incretin effect. The proportion of the insulin response to an oral glucose load attributable to incretins is commonly cited as roughly half or more, varying substantially with the size of the load and between individuals. This book states it as a range.

On the reduced incretin effect in type 2 diabetes. That the incretin effect is substantially diminished in type 2 diabetes is well established. Whether this is a cause or a consequence of the disease is genuinely debated, and this book states that it is probably some of both rather than taking a side.

On GIP's blunted insulinotropic effect in type 2 diabetes and its apparent role in promoting fat storage: both are well described. The reconciliation with tirzepatide's clinical results is unresolved, and the four candidate explanations in §7.5 are presented as candidates, not findings. That both GIP receptor agonists and antagonists are being pursued for obesity is accurate and is the strongest available indication that the mechanism is not understood.

On the routes of GLP-1 action. That endogenous GLP-1 acts substantially through local vagal afferent signaling, given how little survives DPP-4 and hepatic extraction, is a well-supported account. The relative contribution of neural, humoral, and central routes to the effects of long-acting agonists — particularly the weight effect — is not established, and this chapter says so repeatedly.

On brain GLP-1 receptor distribution. GLP-1 receptors in the hypothalamic arcuate nucleus, the area postrema, and the nucleus tractus solitarius are well described, as is the area postrema's status as a circumventricular organ with an incomplete blood-brain barrier. Precise receptor densities and species differences vary across the literature.

On "food noise." This term originated in patient communities rather than in research, describes a real and consistently reported phenomenon, and lacks a validated measurement instrument. Research using it is emerging. This book treats the report as genuine and the evidence as early.

On the historical timeline. The sequence in Case Study 1 is compressed and flattens a much messier reality in which multiple groups worked in parallel; priority disputes exist for several of the steps described. Specific dates for individual discoveries vary by source and by what counts as the discovery.

Tier 3 — Illustrative and constructed

• All ASCII diagrams in this chapter — the incretin effect curves, the L cell schematic, the four actions, the glucose-dependence comparison, the two-minute problem, the three routes, and the bariatric rerouting diagram — are schematic teaching devices, explicitly not to scale.
• The worked deepened Field 3 entry for GLP-1 is a demonstration constructed for this book.
• Figure 7.CS1 renders the real GLP-1 infusion literature in this book's six-field format; the format is the book's own and the studies described are a characterization of a body of work rather than a single paper.

If you only read one thing

Look up the GLP-1 receptor on the IUPHAR/BPS Guide to Pharmacology.

You will find: the receptor's classification, its endogenous ligands, the drugs that act on it, and — in the comments — an expert-written summary of what is established. It takes five minutes.

Then read the Clinical Pharmacology section of any GLP-1 receptor agonist label on DailyMed and compare. Two carefully written descriptions of the same system by people with different obligations, and the differences between them are instructive.

Looking ahead

Chapter 8 covers semaglutide's trial programs. The single most useful preparation is to look up one of them on ClinicalTrials.gov — search "STEP semaglutide" or "SELECT semaglutide" — and read the registered primary endpoint, the eligibility criteria, and the enrollment before you read what the chapter says the trial showed.

Reading the eligibility criteria first is the habit worth building. It tells you the population before anyone tells you the result, and Chapter 5's population-swap trap becomes much harder to fall into once you have made it a reflex.