Chapter 3 — Exercises

Anatomy is the easiest material in Part I to read and the easiest to think you've learned. Part C is where you find out.

Difficulty legend: ⭐ basic recall · ⭐⭐ applied · ⭐⭐⭐ synthesis and judgment · ⭐⭐⭐⭐ extension

Selected answers in Appendix J.

Part A — Conceptual Understanding ⭐

A1. Explain what is meant by "you are a donut," and give three practical consequences.

A2. List the six stages of the digestive route in order, with the approximate time spent in each and what is absorbed there.

A3. Name four things the stomach does. Then name one important thing it does not do.

A4. What is intrinsic factor, where is it made, what is it for, and where is it used?

A5. Bile is often described as a digestive enzyme. Why is that wrong, and what is bile actually?

A6. What is the brush border, and why does its location explain lactose intolerance?

A7. Trace the route of (a) absorbed glucose and (b) absorbed long-chain fat, from the intestinal cell to the general circulation. Why does the difference matter?

A8. Name three functions of the liver relevant to nutrition. Explain how any one of them makes the "detox" claim untenable.

A9. What are short-chain fatty acids, where do they come from, and why is butyrate specifically notable?

A10. Name five gut hormones, where each is released, and what each does.

A11. What are the Atwater factors, and why are label calories described in §3.9 as estimates rather than measurements?

A12. Explain the difference between the enteric nervous system and the gut–brain axis.

Part B — Applied Analysis ⭐⭐

B1. A patient has had their gallbladder removed. Predict what symptoms they might report and after what kind of meal. Explain the mechanism.

B2. Explain, using two distinct mechanisms from this chapter, why 300 calories of apple juice leaves you hungry sooner than 300 calories of whole apples plus peanut butter.

B3. A friend says: "I did a three-day juice cleanse and I feel amazing — clearer skin, more energy, less bloated. So it must be doing something." Without dismissing their experience, identify at least four things that changed simultaneously during those three days that are not "toxin removal."

B4. An older adult on long-term acid-suppressing medication develops fatigue and is found to have low B12 despite eating meat and dairy daily. Explain the mechanism in three sentences.

B5. Why does grinding almonds into almond butter increase the energy you actually absorb, when the almonds are chemically identical? Name the principle, and give two other examples of it from the chapter.

B6. A supplement is marketed as "sealing your leaky gut." Using the 🔬 verdict in §3.8, write the three specific questions you'd ask before spending money on it.

B7. Someone tells you that gut-produced serotonin explains their depression and that a probiotic will fix it. Explain, in three sentences a non-scientist could follow, why the anatomy doesn't support the reasoning — while being fair about what remains genuinely open.

B8. A person has surgery removing a substantial portion of their ileum. List four specific nutritional consequences and explain each mechanistically. Then say which of the four cannot be addressed by changing what they eat, and why.

Part C — Skills & Practice ⭐⭐–⭐⭐⭐

C1. Trace your own dinner. Take last night's meal. Write out, stage by stage, what happened to each component: which enzymes acted where, what form each nutrient was in when it crossed the gut wall, which route it took out, and what reached the colon. Where you don't know, write "don't know" — the gaps are the useful output.

C2. The amylase demonstration. Put a plain cracker or a piece of bread in your mouth and hold it, chewing slowly, without swallowing, for at least ninety seconds. Record when it starts tasting sweet. Then explain what you just tasted. (This is the cheapest biochemistry experiment available and almost nobody has done it.)

C3. Audit a detox product. Find a commercial cleanse, detox, or "gut reset" product. Record: what specific toxins does it claim to remove? What evidence is offered? What does it cost per month and per year? What would happen physiologically if you did nothing? Write a paragraph you could actually say to a friend without being insufferable — that last constraint is the hard part.

C4. The satiety experiment. Over one week, note three meals that kept you comfortable for three or more hours and three that didn't. For each, record the approximate fat, protein, and fiber content (rough estimates are fine). Then check your findings against §3.3 and §3.7. Does your own data fit the mechanism?

C5. Explain it to a nine-year-old. Write an explanation of digestion, mouth to colon, that Nico could follow. Maximum 200 words, no jargon, at least one analogy. (Teaching is the strongest test of whether you actually understand something — if you find yourself hedging, that's where your model is thin.)

C6. Find the enzyme. For each of these conditions, identify what is missing or damaged and where: lactose intolerance · celiac disease · pancreatic insufficiency · pernicious anemia. Then say which can be managed by avoiding a food and which cannot.

Part D — Synthesis & Critical Thinking ⭐⭐⭐

D1. §3.9 shows that label calories are estimates with real error. Does this support or undermine calorie counting as a practice? Argue both, then state where you land and why. (You'll revisit this after Chapter 4 and possibly change your answer.)

D2. GLP-1 agonist medications work by mimicking a gut hormone, and they produce substantial weight loss. What does that fact imply about the long-standing framing of body weight as a matter of willpower? Be careful: does it imply willpower is irrelevant, or something more specific?

D3. The chapter argues that "cleaning out your colon" is a category error because the colon is a bioreactor rather than a sewer. But bowel preparation before a colonoscopy does exactly that, and is medically necessary. Reconcile these. What's the principle that makes one legitimate and the other not?

D4. The Ortiz-Lindqvists eliminated gluten before testing, making the celiac test invalid. Whose failure was that? The neighbor's, for confident advice? The parents', for acting on it? The healthcare system's, for not being reachable? Argue a position, then argue that the question itself is badly framed.

D5. Roughly 70% of immune tissue sits along the gut, and the gut is where "outside" comes closest to "inside." Use this to explain why food allergy exists at all — why would a body evolve to attack food? What does the existence of oral tolerance suggest about how the system normally works, and what does that imply for the allergen-introduction guidance you'll meet in Chapter 25?

Part M — Mixed & Interleaved Practice ⭐⭐–⭐⭐⭐

M1. (With Chapter 2.) The "leaky gut" verdict rests on a Chapter 2 distinction. Name it, then find two other claims in this chapter that fail the same way.

M2. (With Chapter 2.) Place each of the following on the evidence ladder: the almond metabolizable-energy feeding studies; the claim that probiotics treat depression; the mechanism by which bile emulsifies fat; a testimonial about a cleanse. Which of these is not a study at all, and what is it instead?

M3. (With Chapter 1.) The neighbor's gluten advice used at least two myth shapes from §1.6. Identify them. Then explain why her sincerity made the advice more persuasive, using Chapter 1's "who benefits?" filter.

M4. (With Chapter 2.) §3.9 states that whole nuts deliver less energy than the label predicts. Before accepting this, run it through the Claim Filter. What design produced this finding? What comparison? How big, and does the size matter for you?

M5. (With Chapters 1 and 2.) Theo's 2022 juice cleanse cost him $150–$400 and ten days. Using Chapter 1's incentive analysis and Chapter 2's confounding tools, write the three-sentence explanation you'd give him for why it felt like it worked.

M6. (Revisit at Chapter 11.) Predict, from §3.6 alone, three specific health effects of eating substantially more fiber. Seal your prediction; check it in Chapter 11.

Part E — Research & Extension ⭐⭐⭐⭐

E1. Look up the research on metabolizable energy of whole nuts — much of it conducted through USDA Agricultural Research Service feeding studies. How were these studies designed? What makes measuring this so difficult, and what would you have to control? Then consider whether the finding should change food labelling, and what would be lost if it did.

E2. Read about refeeding syndrome — what happens when severely malnourished people are fed too quickly. Work out, from the physiology in this chapter plus what you can find, why the danger is electrolytes rather than the food itself. (This is Chapter 29's material; arriving at it yourself from first principles is a good test of whether §3.4 landed.)

E3. Investigate the evidence on fecal microbiota transplantation for recurrent C. difficile infection, and contrast it with the evidence for FMT in any other condition. This is one of the clearest examples in medicine of a treatment with strong evidence in one narrow indication being extrapolated far beyond it — a pattern you'll see repeatedly.

E4. The claim that the small intestine's surface area equals a tennis court appears in many textbooks; more recent estimates are considerably smaller. Trace where the tennis-court figure came from and how it propagated. Write a page on what this says about how "facts" persist in textbooks — and note that this book repeated the correction rather than the original, which you should also check.