Chapter 3 — Further Reading

Real sources only. Where I'm unsure of an exact edition or figure, I describe the source rather than inventing a citation.

The anatomy and physiology, properly

Any current human physiology textbook — Guyton and Hall's Textbook of Medical Physiology (Elsevier), Silverthorn's Human Physiology: An Integrated Approach (Pearson), or Marieb's Human Anatomy & Physiology (Pearson) are the standards.

I'm being deliberately non-prescriptive here because these books are largely interchangeable for our purposes and because the honest advice is: use whichever one your library has, or whichever edition is cheapest secondhand. The GI chapter of any of them will give you a far more detailed version of this chapter, with the diagrams I could only describe. Older editions are fine — digestive anatomy has not changed. For: students, and anyone who wants the pictures.

OpenStax Anatomy and Physiology — openstax.org. Free, peer-reviewed, openly licensed, with proper illustrations. The digestive system chapter covers everything in §3.1–§3.6 at greater depth than this chapter does, at no cost. For: everyone. If you only follow one link from this page, make it this one.

On gut hormones and appetite

The research literature on GLP-1, PYY, ghrelin, and appetite regulation is large and moving fast. Rather than a single citation, search "gut hormones appetite regulation review" on PubMed and read a recent review article. The field has changed substantially since GLP-1 agonists entered widespread clinical use, and anything written before roughly 2020 will feel dated on the clinical side while remaining sound on the physiology. For: students, clinicians, and anyone who found §3.7 the most surprising part of the chapter.

Stephan Guyenet, The Hungry Brain (Flatiron, 2017).

A neuroscientist's account of how the brain regulates food intake, including the gut–brain signalling in §3.7 and a great deal of what Chapter 33 will cover. Careful with the evidence, readable, and unusually good at explaining why "just eat less" is a description of an outcome rather than a mechanism. For: general readers. One of the better popular science books in this whole area.

On energy availability from food

USDA Agricultural Research Service work on metabolizable energy of nuts — search for the feeding studies on almonds, walnuts, pistachios, and cashews. These are careful, controlled human feeding studies, and reading one is a good education in how hard it is to measure something as apparently simple as "how many calories are in this." For: anyone who found §3.9 surprising, and anyone who counts calories.

Richard Wrangham, Catching Fire: How Cooking Made Us Human (Basic Books, 2009).

The argument that control of fire — by increasing the energy return on food through cooking — was a precondition for human brain expansion. The broad thesis is contested among anthropologists; the underlying digestibility evidence is not. Read it for the digestibility material and hold the evolutionary argument loosely. For: the curious. Genuinely enjoyable.

USDA FoodData Central — fdc.nal.usda.gov. The reference database behind essentially every nutrition label and app you have ever used. Free, searchable, and worth understanding as a source rather than treating as an oracle — the entries carry their own uncertainty.

On the gut–brain axis, held at the right distance

Emeran Mayer, The Mind-Gut Connection (Harper Wave, 2016).

Mayer is a gastroenterologist and a serious researcher in this area, and the book is a good introduction to gut–brain signalling from someone who actually works on it. It is, in places, more confident than the current evidence supports — read it alongside §3.10's 🟡 verdict and Chapter 27. For: general readers, with the calibration caveat.

On probiotics specifically: the NIH Office of Dietary Supplements probiotics fact sheet (ods.od.nih.gov) and any current Cochrane review on probiotics for a specific condition. Both are notably more cautious than the popular literature, and the gap between them is instructive in itself. For: anyone considering buying a probiotic.

On coeliac disease and testing before eliminating

Coeliac UK (coeliac.org.uk) and the Celiac Disease Foundation (celiac.org).

Both state clearly and prominently that testing requires gluten in the diet. If you take one practical thing from this chapter, take this — and if someone in your life is about to go gluten-free for symptoms, send them one of these links first. It takes thirty seconds and it can save a diagnosis. For: everyone with gut symptoms, and everyone who has a friend with gut symptoms.

Full treatment in Chapter 28.

On why detoxes don't work

The published critical reviews of commercial detox diets. Search for "detox diets systematic review" — there is a small but useful literature, and its consistent finding is that the commercial claims lack supporting clinical evidence in humans.

The FDA's actions on detox and cleanse products (fda.gov) — searchable warning letters and consumer updates. Reading a few is more persuasive than any argument I can make, particularly the cases where products marketed as natural cleanses were found to contain undeclared pharmaceutical ingredients. For: anyone still tempted, and anyone who needs to talk someone else out of it.

A note on what's absent

There is no shortage of popular books about gut health, and I've named only two. That's deliberate.

This is currently the most oversold area in all of nutrition — the gap between what is genuinely established (the gut–brain axis exists; fiber feeds bacteria; SCFAs matter) and what is being sold (a microbiome test, a protocol, a supplement, a cure for your fatigue) is as wide as anywhere in this book.

Come back to that genre after Chapter 27, where the evidence gets laid out properly. You'll read them very differently — and the test from Chapter 2 applies: does the author ever apply their own skepticism to a claim they like?