Chapter 13 — Exercises

C5 — writing down everything you take and putting it in your phone — takes four minutes and is the single most useful clinical act in this chapter.

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

Selected answers in Appendix J.

Part A — Conceptual Understanding ⭐

A1. Define "vitamin." How many are there, and how are they divided?

A2. Give four differences between fat-soluble and water-soluble vitamins. Which vitamin is the exception, and in what way?

A3. Define EAR, RDA, AI, and UL. Which is used for populations and which for individuals?

A4. Explain why "most people don't get enough" is usually an artefact. What's the correct comparator?

A5. State the threshold concept. Name the four routes by which status and intake diverge.

A6. Describe the shape of the intake–outcome curve for a nutrient. What shape does "more is better" implicitly assume?

A7. Name the deficiency disease for: vitamin C, thiamin, niacin, vitamin D, vitamin A, B12.

A8. Why do vegans require B12 supplementation? Why is "I feel fine" uninformative?

A9. Why must folic acid be taken before conception rather than after a positive test?

A10. Fill in the isolated-nutrient template with four examples from this chapter.

A11. What are the two competing thresholds for vitamin D sufficiency, and why does the choice matter?

A12. Name four vitamins where high intake causes real harm, and describe the harm for each.

Part B — Applied Analysis ⭐⭐

B1. Walt takes a multivitamin containing 6 µg of B12, has been on metformin nine years, and has low B12. Explain the mechanism and why the multivitamin didn't prevent it.

B2. For each person, say whether they need a supplement, a blood test, both, or neither: (a) a 28-year-old vegan of three years who feels well; (b) a 45-year-old with fatigue and a mixed diet; (c) a 70-year-old on omeprazole with new numbness in the hands; (d) a 31-year-old planning pregnancy in six months; (e) a 22-year-old who reads that everyone is vitamin D deficient.

B3. A supplement provides 3,000 µg of preformed vitamin A. Assess this for (a) a healthy adult man and (b) a woman in the first trimester of pregnancy. What changes?

B4. Explain why the folic-acid-lowers-homocysteine trials are the cleanest surrogate endpoint failure in the book. Which Chapter 2 section does this illustrate?

B5. A direct-to-consumer company offers a "comprehensive 40-nutrient panel" for $299 and provides a personalized supplement subscription based on results. Evaluate, using §13.11 and Chapter 1 §1.5.

B6. Someone takes 5,000 IU of vitamin D daily with no testing. Under what circumstances is this reasonable, and under what circumstances would you change it? What would you want to know?

B7. Explain why the vitamin D observational literature is so vulnerable to reverse causation. Name two specific mechanisms.

B8. A patient reports tingling feet and takes a "nerve support" supplement containing 100 mg of B6. Work through what could be happening — there are at least two possibilities and they compound.

Part C — Skills & Practice ⭐⭐–⭐⭐⭐

C1. Do the vitamin gap analysis. The Project Checkpoint: risk-group check first, then rough intake estimates for D, folate, C and B12, then your own labels against the UL.

C2. Check the risk groups honestly. Go through §13.5 for yourself and for anyone you feed — particularly anyone over 65, anyone on metformin or long-term acid suppression, and anyone eating plant-based. This is the highest-yield two minutes in the chapter.

C3. Read your own labels against the UL. Not the RDA — the UL, in Appendix A. Pay attention to B6, preformed vitamin A, and biotin. Note anything above it.

C4. Find the % Daily Value trick. Look at three supplement labels. For each, find the highest percentage figure. Then ask: is this nutrient water-soluble? Is the surplus usable? Is there a UL concern? Percentages above ~500% are a marketing decision, not a nutritional one.

C5. Write the list. Every supplement, vitamin, herbal product and fortified functional food you take, with doses. Put it in your phone. Take it to your next medical appointment. (Four minutes. Walt's list contained a biotin dose that nearly cost him a correct emergency diagnosis.)

C6. Audit a "most people are deficient" claim. Find a marketing claim of this form. Determine: what reference value was used — RDA or EAR? Was intake or status measured? What population? How many of those three questions can you answer from the source?

C7. Ask about B12. If you or a relative is over 65, on metformin, on long-term acid suppression, or eating plant-based — raise B12 at the next appointment. Note what happens. (This exercise has changed outcomes for readers of drafts of this book, which is not something I can say about many exercises.)

Part D — Synthesis & Critical Thinking ⭐⭐⭐

D1. The chapter gives three ✅ verdicts after twelve chapters with none. Is that suspicious timing, or does the material genuinely differ? Examine what the three affirmed claims have in common, and whether that shape is what should earn a ✅.

D2. The vitamin D sufficiency threshold is genuinely disputed between expert bodies. How should a clinician act under that disagreement? How should a patient? Are those the same answer?

D3. Fortification — folic acid in flour, vitamin D in milk, niacin in bread — is mass medication without individual consent, and it has prevented an enormous amount of disease. Construct the strongest objection to it, then the strongest defence. Where do you land?

D4. The multivitamin gets 🟠 for the well-nourished and 🟢 for those with poor intake. But the people with poor intake are least able to afford it and least likely to buy it. What follows for policy?

D5. §13.7 argues the antioxidant trials refuted "a specific inference from the food finding" rather than the food finding itself. Is that a fair reading or special pleading? What would count as evidence against the food finding?

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

M1. (With Chapter 2.) The isolated-nutrient template has now been filled seven times across this book. List all seven, and identify which Chapter 2 concept each one illustrates.

M2. (With Chapter 3.) B12 absorption requires two things in two places. Name them, and list every way from Chapter 3 §3.8 that either can fail.

M3. (With Chapter 11.) Walt's audit kept vitamin D and magnesium. Using §13.8, justify the vitamin D decision and the dose change. Then say what would have changed your mind.

M4. (With Chapters 5 and 11.) Compare the marketing structure of the metabolism aisle (Ch 5), the protein aisle (Ch 8), the fiber aisle (Ch 11), and the vitamin aisle. What's the common pattern, and where does the vitamin aisle differ?

M5. (With Chapter 8.) Ruth Kaminsky is 79, eats 42 g of protein, and was told years ago to cut eggs and cheese. Which vitamin deficiencies is she at risk of, and why does her age create risk through two independent mechanisms?

M6. (With Chapter 10.) A person on a strict carnivore diet — which Chapter 10 gave ⚗️ — is at risk of which vitamin deficiencies? Which are they not at risk of, and does that surprise you?

M7. (With Chapter 12.) Why is thiamin deficiency specifically associated with alcohol use disorder? Name three contributing mechanisms.

M8. (Revisit at Chapter 16.) Predict now how many of the supplements in Walt's original nine will receive a ✅ or 🟢 verdict in Chapter 16. Write your number.

Part E — Research & Extension ⭐⭐⭐⭐

E1. Read the MRC Vitamin Study (1991) on folic acid and neural tube defect recurrence, and find data on neural tube defect rates before and after mandatory fortification in a country that adopted it. Write a page on why this is nutrition's clearest success story.

E2. Read the VITAL trial (vitamin D and omega-3) in full, including the subgroup analyses. Then find at least two commentaries arguing that it was misinterpreted. Where does the disagreement sit?

E3. Investigate the vitamin D threshold dispute — the National Academies position versus the Endocrine Society position. Read both organizations' reasoning. What are they each optimizing for, and is the disagreement empirical or definitional?

E4. Read the SELECT trial (selenium and vitamin E, prostate cancer). What was expected, what was found, and what happened to the trial? Compare its structure to ATBC and CARET.

E5. Research biotin interference with immunoassays — the FDA safety communications and the clinical literature. Which assays are affected, at what doses, and what should clinicians and patients do? (This directly underlies Chapter 16's central story.)

E6. Investigate the history of a fortification programme — folic acid, iodized salt, vitamin D in milk, or niacin in flour. What was the disease burden before? What opposition was there? What has it prevented? These are among public health's largest and least-known achievements.