Chapter 15 — Exercises

C5 — buying a box of ORS sachets — costs about eight dollars and is the single highest-value action in this chapter for any household with children or older adults.

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

Selected answers in Appendix J.

Part A — Conceptual Understanding ⭐

A1. What proportion of adult body mass is water, and what three factors shift it?

A2. Name five jobs water does in the body.

A3. Describe the osmoreceptor–ADH–thirst system. What size change does it detect?

A4. What is the approximate maximum rate at which healthy kidneys can excrete water? Why does that number matter?

A5. Explain the asymmetry between under-drinking and over-drinking.

A6. Where did "eight glasses a day" come from? What was dropped?

A7. State the Adequate Intakes for total water. What three things about those numbers are commonly missed?

A8. Give the sweat rate formula, and work an example.

A9. List five situations where electrolyte replacement is genuinely warranted, and three where it isn't.

A10. Explain the SGLT1 mechanism and why it makes ORS work in diarrhoeal illness.

A11. Define exercise-associated hyponatremia. Name six risk factors.

A12. Why is the intuitive treatment for a hyponatremic athlete dangerous?

A13. Does coffee dehydrate you? Explain.

Part B — Applied Analysis ⭐⭐

B1. Priya drank ~4.5 L over 5 hours against a clearance ceiling of 0.8–1 L/hr. Explain, using at least three mechanisms, why she still became hyponatremic.

B2. Calculate sweat rate: weight before 72.4 kg, weight after 71.1 kg, fluid consumed 750 mL, duration 90 minutes. Express as L/hr.

B3. Two athletes finish a marathon feeling unwell. One has lost 2.5 kg, the other has gained 1.2 kg. Both are nauseated with a headache. What do you do differently for each?

B4. Someone drinks 3 L of water daily, takes a B-complex, and reports their urine is "always bright yellow so I must need more." Correct this.

B5. A parent asks whether a sports drink is suitable for a toddler with gastroenteritis. Answer properly, including why the formulation matters.

B6. Camila works 12-hour night shifts and goes 6–7 hours without fluid. Design an intervention that addresses the actual constraint, and say why a 2 L target would fail.

B7. An 82-year-old is admitted confused. Why is thirst an unreliable guide in this population, and what two physiological changes are responsible?

B8. A patient with heart failure has been told by a friend to drink more water. What's the problem, and what would you say?

Part C — Skills & Practice ⭐⭐–⭐⭐⭐

C1. Do the hydration self-check. The Project Checkpoint: three days of urine colour, thirst observation, the situational audit, and the cost total.

C2. Measure your sweat rate. If you train, do §15.8 once. Weigh before, record fluid, weigh after. Then repeat in different conditions — cool versus hot — and note how much the number moves.

C3. Run the situational audit honestly. Seven boxes in the checkpoint. Any tick means "drink to thirst and forget it" isn't your answer — and the fluid-restriction box overrides the entire chapter.

C4. Cost the market. Total your annual spend on bottled water, electrolytes, and any "functional" water. Compare to §15.13's table. Then ask which justifying condition from §15.9 actually applies.

C5. Buy ORS. ⚠️ A box of WHO-formula oral rehydration sachets, for the cupboard. Especially if you have children or care for an older adult. Under ten dollars, and it is the best-evidenced item in this chapter.

C6. Trace the eight-glasses claim. Find three current sources repeating it. For each: do they cite anything? Does any of them mention that food contributes ~20%? Does any distinguish total water from drinking water?

C7. Read three hydration product labels. Record electrolyte content per serving, sugar, and price per serving. Compare the sodium content to what's in one slice of bread.

Part D — Synthesis & Critical Thinking ⭐⭐⭐

D1. Priya said every mistake came from following advice. What obligation does that place on people who give hydration advice to endurance athletes? How should guidance be worded to avoid this failure mode?

D2. The dehydration-and-performance literature is contested largely because of blinding problems. Design a study that would settle it. What would it require, and would it be ethical?

D3. ORS is one of the great medical advances and almost nobody in wealthy countries knows about it. Compare this to Chapter 13's folic acid and Chapter 14's iodized salt. What do all three have in common, and what does that tell you about which health stories propagate?

D4. The chapter argues the hydration market's claims are exotic because the product competes with a free public utility. Test that theory against two other nutrition categories. Does it hold?

D5. "Drink to thirst" is good advice for most people and dangerous advice for some — older adults, young children, people in extreme heat. How should a general recommendation handle a population where the default guidance inverts? Write the actual wording.

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

M1. (With Chapter 13.) The 8×8 rule and "most people don't get enough vitamin X" share a structural flaw. Name it precisely.

M2. (With Chapter 3.) Explain why alkaline water's claim fails, using stomach physiology from Chapter 3.

M3. (With Chapter 14.) Sweat sodium varies roughly tenfold between individuals. Connect this to Chapter 14's argument about why population mineral advice is weak.

M4. (With Chapter 14.) The chapter says tea and coffee count toward fluid intake — but Chapter 14 says move them away from meals. Reconcile these, and explain why they're not in conflict.

M5. (With Chapter 4.) A low-carbohydrate diet produces rapid early weight loss. Using Chapter 4 §4.9 and this chapter, explain the fluid mechanism and what happens to hydration status.

M6. (With Chapter 12.) Both alcohol and caffeine are described as diuretics. Compare their actual effects on fluid balance. Which one genuinely matters, and why?

M7. (With Chapters 6 and 23.) Devi's sweat rate is 1.3 L/hr and she runs 60 miles a week. Estimate her weekly sweat losses and say what that implies for sodium, and for the RED-S picture.

M8. (Revisit at Chapter 23.) Predict what Chapter 23 will add about hydration for athletes beyond this chapter.

Part E — Research & Extension ⭐⭐⭐⭐

E1. Read Valtin's 2002 American Journal of Physiology review, "Drink at least eight glasses of water a day. Really?" Then find three current sources still repeating 8×8. Write a page on why the correction hasn't propagated in twenty years — and compare it to Chapter 12's asymmetric correction.

E2. Investigate exercise-associated hyponatremia: the consensus statements from endurance medicine conferences, documented cases, and how event guidance has changed. What did organizers do differently, and did it work?

E3. Read the debate on dehydration and endurance performance — particularly studies using blinded fluid administration, and field studies of finishing times versus body mass loss. Where does the disagreement sit? Search for the exchange between proponents of prescribed drinking and of drinking to thirst.

E4. Research the history of oral rehydration therapy — the discovery of sodium-glucose cotransport, the trials during the 1971 Bangladesh refugee crisis, and the global rollout. This is one of the great stories in medicine and it takes an hour to read.

E5. Investigate hydration assessment in older adults: why standard clinical signs perform poorly, what markers are used instead, and how dehydration contributes to hospital admissions. Then consider what a practical prevention strategy in a care setting would look like.

E6. Look into the bottled water market — its size, its environmental footprint (Chapter 36), and the evidence on whether bottled water differs meaningfully from tap in wealthy countries. Then consider where it genuinely does matter.