Chapter 13 — Quiz

Twenty-one questions. Attempt before opening.

Multiple Choice

1. How many vitamins are there?

  • a) 8
  • b) 13
  • c) 20
  • d) It depends on the classification system
Answer **b.** Four fat-soluble (A, D, E, K) and nine water-soluble (C plus eight B vitamins). The list hasn't changed since the 1940s — anything marketed as a vitamin that isn't on it ("vitamin B17," "vitamin O") isn't one.

2. The RDA is defined as the intake meeting the needs of:

  • a) 50% of a group
  • b) 75% of a group
  • c) 97–98% of healthy individuals
  • d) 100% of everyone
Answer **c.** Which means **most individuals need less than the RDA**, and eating below it on a given day is usually just Tuesday, not a deficiency.

3. "Most people don't get enough vitamin X" is usually an artefact because:

  • a) The data is fabricated
  • b) It compares population intakes to the RDA, a figure designed to exceed what 97–98% of people need, when the correct comparator for population adequacy is the EAR
  • c) Nobody measures intake
  • d) Vitamins can't be measured in blood
Answer **b.** Comparing everyone to a number set to cover almost everyone will always make a large fraction look inadequate. **It isn't even a lie, which is why it works.**

4. A supplement label showing "100% Daily Value" means:

  • a) 100% of your personal RDA
  • b) 100% of a single simplified labelling figure that doesn't vary by age, sex, or pregnancy
  • c) The Tolerable Upper Intake Level
  • d) The Estimated Average Requirement
Answer **b.** The Daily Value is derived from the DRIs but simplified for a general population. For some nutrients in some groups the gap between DV and personal requirement is substantial.

5. The threshold concept of this chapter is:

  • a) All vitamins are essential
  • b) Nutrient status is not nutrient intake, and supplementing someone already adequate almost never helps
  • c) Fat-soluble vitamins are dangerous
  • d) Supplements never work
Answer **b.** They diverge through impaired absorption, increased requirement, increased losses, and endogenous synthesis — and in the other direction through high intake with adequate status.

6. Walt's B12 deficiency occurred despite a multivitamin because:

  • a) The multivitamin was counterfeit
  • b) Metformin impairs ileal B12 absorption, and a physiological dose can't overcome a malabsorption problem
  • c) He didn't take it consistently
  • d) B12 isn't in multivitamins
Answer **b.** The multivitamin addressed **intake** when the problem was **absorption** — and it created the appearance of coverage, which stopped anyone looking.

7. The intake–outcome curve for a nutrient is best described as:

  • a) A straight line rising forever
  • b) Steep rise, long plateau, eventual fall
  • c) Flat throughout
  • d) Falling from the start
Answer **b.** And **"more is better" implicitly assumes (a)**, a graph that doesn't exist for any nutrient at any dose. Most readers, for most nutrients, are on the long plateau.

8. Folic acid must be taken before conception because:

  • a) It causes nausea later in pregnancy
  • b) The neural tube closes by around week four, often before pregnancy is recognized
  • c) It's cheaper before pregnancy
  • d) It interferes with pregnancy tests
Answer **b.** Which is why the recommendation is for anyone who *could* become pregnant, not for those who already are. Starting after a positive test is frequently too late.

9. Which supplement recommendation in this chapter is described as the clearest in the whole book?

  • a) Vitamin D for everyone
  • b) A daily multivitamin
  • c) Folic acid before conception
  • d) Vitamin C in winter
Answer **c.** Randomized trials (MRC Vitamin Study), population-level fortification data, and a clear mechanism — convergence about as complete as nutrition offers. ✅

10. Vegans require B12 supplementation because:

  • a) Plants block B12 absorption
  • b) B12 is synthesized by bacteria and accumulates in animal tissues; it is essentially absent from unfortified plant foods
  • c) Vegans have impaired absorption
  • d) It's a precautionary recommendation without strong evidence
Answer **b.** And claimed plant sources — spirulina, nori, unwashed vegetables — often contain **inactive analogues** that some assays detect but the body can't use. ✅ Non-negotiable.

11. Which trial found increased lung cancer in the supplemented group?

  • a) VITAL
  • b) SELECT
  • c) CARET
  • d) MRC Vitamin Study
Answer **c.** CARET (beta-carotene, in smokers and asbestos-exposed workers) — stopped early for harm. SELECT found a prostate cancer signal with vitamin E.

12. Which water-soluble vitamin has the clearest toxicity, and what does it cause?

  • a) Vitamin C — kidney stones
  • b) B6 — peripheral neuropathy, sometimes irreversible
  • c) Folate — anemia
  • d) B12 — liver damage
Answer **b.** And it appears routinely in "nerve support" and "energy" formulations — meaning someone taking it *for* tingling feet may be taking a dose that *causes* tingling feet.

13. Biotin at high doses is dangerous primarily because it:

  • a) Is toxic to the liver
  • b) Interferes with immunoassays including cardiac troponin and thyroid function tests
  • c) Causes hair loss
  • d) Blocks B12 absorption
Answer **b.** It isn't toxic — **it makes laboratory results wrong**, which is arguably worse because the harm arrives through a misdiagnosis. Chapter 16 tells Walt's ER story in full.

True / False

One-line justification.

14. A daily multivitamin is good insurance for a well-nourished adult.

Answer **Probably false.** Large trials in well-nourished populations show little or no effect on mortality, cardiovascular disease, or cancer. It mostly produces expensive urine and — more importantly — **the appearance of coverage.** 🟢 Reasonable for genuinely poor or restricted intake.

15. Beta-carotene supplements are a safe way to get vitamin A.

Answer **False as stated.** Beta-carotene from *food* is safe — the body regulates conversion, which is why it isn't teratogenic like preformed retinol. But **high-dose beta-carotene supplements increased lung cancer in smokers** in two large trials, one stopped early. Food and pill are different interventions.

16. Vitamin C prevents colds.

Answer **False** in the general population — Cochrane reviews find no meaningful reduction in incidence, and only a modest reduction in duration (hours, for a week-long cold). 🟡 There is a genuine signal in people under extreme short-term physical stress, which is narrow and real.

17. If your vitamin D is low, supplementation helps.

Answer **True** — ✅ well supported. The claim that fails is a different one: that supplementation helps people who are **already replete**, where large trials have been largely null. **The two get merged constantly.**

18. Vitamin K should be avoided by people on warfarin.

Answer **False.** ⚠️ The rule is **consistency, not avoidance** — sudden large changes in leafy green intake destabilize anticoagulation. Steady intake is manageable; erratic intake isn't.

Short Answer

19. Explain the threshold concept using Walt's case, and name the four routes by which status and intake diverge.

Answer Walt's **intake** was adequate — a multivitamin plus meat and dairy. His **status** was low. They diverged because **metformin impaired ileal absorption**, so normal intake produced abnormal status — and the multivitamin created the *appearance* of coverage precisely because it addressed the variable that was never the problem. **The four routes:** impaired **absorption** (metformin, celiac, IBD, bariatric surgery, reduced stomach acid, lost ileum) · increased **requirement** (pregnancy, growth, illness, smoking) · increased **losses** (some diuretics, dialysis) · **endogenous synthesis** (vitamin D from skin, so intake alone tells you almost nothing). **Plus the reverse direction:** high intake with adequate status — which is the expensive urine.

20. Fill in the isolated-nutrient template and give four examples from this chapter.

Answer > **Observed:** people eating food F have less disease D. > **Inferred:** compound C in food F prevents disease D. > **Tested:** compound C, isolated, at high dose, in a pill. > **Result:** nothing, or harm. **Beta-carotene** — high blood levels → less lung cancer; ATBC and CARET found *more*, CARET stopped early. **Vitamin E** — high intake → less CVD; trials null, SELECT found a prostate cancer signal. **Folic acid/B12 for cardiovascular prevention** — homocysteine fell exactly as designed, events didn't *(the cleanest surrogate-endpoint failure in the book)*. **Antioxidant combinations** — consistently null, some harm signals. **Four for four.**

21. A 71-year-old on metformin and omeprazole reports tingling feet and is told it's diabetic neuropathy. What's your concern, and what would you do?

Answer **The concern is B12 deficiency**, and there are **two independent mechanisms** stacked: metformin impairs ileal absorption of the B12–intrinsic factor complex, and long-term acid suppression prevents B12 being liberated from food protein in the stomach. Add age, which reduces gastric acid production independently. **Three routes, all pointing the same way.** **The trap is that the offered explanation is entirely plausible** — this person almost certainly does have diabetes, and this is exactly what diabetic neuropathy looks like. **That's what makes it dangerous: a correct-sounding diagnosis stops the investigation.** **What I'd do:** ⚠️ **check B12 promptly**, with methylmalonic acid if the result is borderline. The ADA *Standards of Care* recommends periodic B12 measurement in metformin-treated patients, particularly with anemia or neuropathy. Also review every supplement they take — **including checking for high-dose B6**, which causes neuropathy itself and appears in "nerve support" formulations, meaning they may have been sold something that worsens the symptom they're treating. **And the urgency matters:** B12-related neurological damage can become **irreversible** if prolonged. This is not a wait-and-see.

Scoring

Score Reading
18–21 Strong. You can now tell a real deficiency from a marketing claim.
14–17 Good. Reread §13.2 (the DRI framework) and §13.3 (the threshold).
10–13 Reread §13.3 and §13.5 — status vs. intake, and who's actually at risk.
Under 10 Reread the chapter. Chapters 14, 16, 25 and 29 all build on it.

And do Exercise C5 regardless of score — write down everything you take and put it in your phone. Four minutes.