Chapter 15 — Quiz

Twenty questions. Attempt before opening.

Multiple Choice

1. Osmoreceptors detect changes in plasma osmolality of approximately:

  • a) 0.01%
  • b) 1–2%
  • c) 10%
  • d) 25%
Answer **b.** A more sensitive instrument than any tracking app, calibrated to your specific body since birth.

2. Healthy adult kidneys can excrete water at roughly:

  • a) 0.1 L/hour
  • b) 0.8–1 L/hour
  • c) 3 L/hour
  • d) There is no limit
Answer **b.** And this is a *resting* figure — **exercise raises ADH, which lowers it further**, which is why Priya's arithmetic looked survivable and wasn't.

3. The asymmetry the chapter identifies is that:

  • a) Over-drinking has an alarm and under-drinking doesn't
  • b) Under-drinking has an alarm (thirst); over-drinking has none
  • c) Both have equally strong alarms
  • d) Neither has an alarm
Answer **b.** The body defends against the risk it evolved with, and unlimited palatable fluid wasn't it. **Advice that overrides thirst is overriding the only signal in the system.**

4. "Eight glasses a day" originates from a 1945 recommendation that:

  • a) Was based on a randomized trial
  • b) Recommended ~1 mL water per calorie and noted that most comes from food — the second clause was dropped
  • c) Was about athletes only
  • d) Never existed
Answer **b.** Valtin's 2002 review went looking for the evidence base and reported he couldn't find one. **The number survived; the qualifier didn't.**

5. The Adequate Intake for water (~3.7 L men / ~2.7 L women) refers to:

  • a) Plain water only
  • b) Total water from all beverages and food
  • c) Water plus electrolyte drinks
  • d) Water excluding caffeinated drinks
Answer **b.** Food contributes roughly **20%**. And it's an **AI** — derived from observed intakes in apparently healthy people, not from a measured requirement.

6. "By the time you're thirsty you're already dehydrated" is:

  • a) Well supported
  • b) Technically true about a 1–2% osmolality shift, and misleading — that's the normal operating range of a feedback system
  • c) Completely fabricated
  • d) True only for athletes
Answer **b.** Thirst is an *early* signal, not a late one — like hunger not meaning malnutrition. And this framing contributed to real harm; endurance guidance has largely shifted toward **drinking to thirst.**

7. The most useful everyday hydration marker is:

  • a) Counting glasses
  • b) Urine colour — pale straw to light yellow
  • c) A hydration app
  • d) Skin turgor
Answer **b.** ⚠️ With a major confounder: **riboflavin (B2) turns urine bright yellow**, so anyone on a B-complex or multivitamin gets an uninterpretable reading. (d) performs poorly in older adults because skin elasticity declines with age.

8. Sweat rate is calculated as:

  • a) Weight before − weight after
  • b) (Weight before − weight after) + fluid consumed, divided by hours
  • c) Fluid consumed ÷ hours
  • d) Body mass × exercise intensity
Answer **b.** Typical rates run **0.5–2.0 L/hour**, varying by an order of magnitude between individuals — which is why population hydration advice is so weak.

9. Oral rehydration solution works because:

  • a) It tastes better than water
  • b) The SGLT1 transporter moves sodium and glucose together, and water follows osmotically — and it keeps working during many diarrhoeal illnesses
  • c) It sterilizes the gut
  • d) It slows intestinal transit
Answer **b.** You pump solute and let water chase it. And the ratio matters — **too much glucose draws water the wrong way**, which is why a sugary drink is not a substitute.

10. Exercise-associated hyponatremia is caused by:

  • a) Excessive sweating without replacement
  • b) Drinking more fluid than is lost, diluting blood sodium
  • c) Too much salt intake
  • d) Insufficient carbohydrate
Answer **b.** Dilutional. Water then moves *into* cells — tolerable in most tissues, **dangerous in the brain**, which is enclosed in a rigid skull.

11. Which is NOT a risk factor for EAH?

  • a) Slower finishing time
  • b) Smaller body size
  • c) NSAID use
  • d) Elite finishing time
Answer **d.** The risk pattern **inverts expectations** — it falls on smaller, slower, first-time participants with abundant aid stations who have been told to drink a lot and have done so diligently. **Priya ticked six of seven.**

12. The single most useful sign distinguishing hyponatremia from dehydration at a finish line is:

  • a) Heart rate
  • b) Body weight change during the event
  • c) Skin colour
  • d) Thirst
Answer **b.** ⚠️ Someone who has **gained** weight during an endurance event has drunk more than they lost, and dehydration is not the problem. **The scale settles it in ten seconds** — and the intuitive treatment, more fluid, makes hyponatremia worse.

13. Coffee and tea:

  • a) Dehydrate you and don't count toward fluid intake
  • b) Count toward fluid intake — tolerance develops to the diuretic effect and net fluid balance is positive
  • c) Are equivalent to water in every respect
  • d) Should be avoided entirely
Answer **b.** Two caveats: very high doses in non-habitual consumers do increase output, and tea/coffee with meals inhibit **non-heme iron absorption** (Ch 14) — a genuine reason to move them, just not a hydration one.

True / False

One-line justification.

14. Alkaline water changes your blood pH.

Answer **False.** Stomach pH is roughly 1.5–3.5 and neutralizes it essentially on arrival; blood pH is regulated tightly by lungs and kidneys. **You cannot drink your way to a different blood pH — and if you could, it would be a medical emergency.**

15. Mild dehydration has been clearly shown to impair cognition.

Answer **False — 🟡.** The literature is small, heterogeneous, and **largely unblinded**, which is the core problem: you can't easily blind someone to whether they've had a drink, and expectancy effects on subjective measures are exactly what you'd predict. Severe dehydration unambiguously impairs cognition; the 1–2% range is much less clear.

16. Electrolyte drinks are better than water for everyday hydration.

Answer **False for everyday use.** 🟢 genuinely useful beyond ~60–90 minutes of hard exercise, in heat, or for salty sweaters. ❌ at a desk — you already consume 3,000–4,000 mg of sodium daily (Ch 14), and most products are a dilute sugar solution with some sodium.

17. For a toddler with gastroenteritis, a sports drink is an acceptable substitute for ORS.

Answer **False.** ⚠️ Sports drinks have a different sugar and sodium profile, designed for a different purpose. **Too much glucose relative to sodium draws water into the gut and can worsen diarrhoea.** The WHO-formula sachet is a formula, not a suggestion.

18. "Drink to thirst" is appropriate advice for everyone.

Answer **False.** ⚠️ Genuine exceptions: **older adults** (reduced thirst sensation *and* reduced renal concentrating ability), **infants and young children**, **illness**, **extreme conditions**, and people who **can't act on thirst** when they feel it. And ⚠️ anyone given a **fluid restriction** by a clinician — that overrides everything in the chapter.

Short Answer

19. Explain why Priya became hyponatremic despite drinking less than her kidneys should have been able to clear.

Answer **Four mechanisms, compounding.** **Exercise raises ADH**, which tells the kidneys to *retain* water — so the 0.8–1 L/hour ceiling is a resting figure and was substantially lower during a five-hour event. **She was losing sodium in sweat while replacing only water**, so total body sodium fell as the volume it was dissolved in rose — two mechanisms pushing the same direction. **Her intake was in boluses** at fifteen aid stations, so instantaneous rates far exceeded the average. And **if she took NSAIDs**, those further impair renal water excretion. **And the reason it looked survivable: she was slow.** Five hours meant maximum drinking opportunity and a relatively low sweat rate — which is why the risk falls on the people who look least at risk.

20. ORS is described as possibly the most important medical advance of the twentieth century. Justify that, and explain what it demonstrates about mechanism-based science.

Answer **The justification:** diarrhoeal disease was, and in places remains, a leading cause of childhood death. Before ORS, treating significant dehydration required **intravenous fluids** — equipment, sterile supplies, trained staff, a facility. ORS is **a sachet of salt and sugar in the right ratio, dissolved in clean water, administered by anyone, anywhere, with no equipment and no training.** Its global dissemination is credited with saving an enormous number of lives. **What it demonstrates:** this book spends a great deal of time on mechanisms that failed to translate — beta-carotene, vitamin E, L-carnitine, homocysteine. **ORS is the counterexample.** Understanding one transporter precisely enough — that SGLT1 moves sodium and glucose only together, and that it survives in illness when other absorptive routes fail — produced an intervention that is cheap, robust, and enormously effective. **Mechanistic biology does deliver.** It delivers when the mechanism is understood well enough to specify a *ratio* rather than an ingredient, and when it's tested in the population that actually has the problem — which is the same distinction Chapter 13 §13.6 drew between folic acid and antioxidants.

Applied Scenario

21. You're the medical volunteer at a marathon finish. Two runners present within minutes of each other, both nauseated with headaches, both saying they feel "terrible."

Runner A: finished in 3:05, weighs 2.8 kg less than at the start, urine very dark, feels dizzy on standing. Runner B: finished in 5:20, weighs 1.4 kg more than at the start, fingers visibly swollen, seems slightly confused about which race she's in.

What do you do?

Answer **These look similar and require opposite responses. The weights settle it.** **Runner A — dehydration.** Lost 2.8 kg (~4% of body mass if ~70 kg), dark urine, orthostatic dizziness. **Fluid is appropriate**, with electrolytes given the duration and losses. Monitor, and escalate if not improving. **Runner B — ⚠️ presume exercise-associated hyponatremia until proven otherwise.** She has **gained weight during a marathon**, she is puffy, she is a slow finisher (maximum aid-station exposure, relatively low sweat rate), and she is **confused** — which moves this from mild to potentially severe. **⚠️ Do NOT give her fluid.** This is the diagnostic trap: her symptoms look like dehydration and the intuitive response would make cerebral oedema worse. Every cup steepens the gradient driving water into her brain cells. **She needs urgent medical assessment with sodium measurement**, and if confirmed and symptomatic, the treatment is hypertonic saline under medical supervision — not water, and not withholding treatment either. **The two things that made this diagnosable in seconds:** the scale, and the ring. **Which is why "weigh yourself before and after" is the most useful line in §15.10** — and why an event that weighs its runners catches this and one that doesn't, doesn't.

Scoring

Score Reading
18–21 Strong. You'd have caught Runner B.
14–17 Good. Reread §15.10 (hyponatremia) and §15.2 (the regulatory system).
10–13 Reread §15.2, §15.5 and §15.10 — the asymmetry, thirst, and the risk.
Under 10 Reread the chapter. Chapters 23 and 29 build on it.

And buy the ORS. Under ten dollars, cupboard, done.