Chapter 21 — Exercises
Three tiers. 🪞 marks reflection; 🔄 marks retrieval.
Tier 1 — Recall and Comprehension
R1. List the protocols in §21.1. ⚠️ Which is most studied, which has the best mechanistic support, and which does almost nobody practise?
R2. State the four numbered claims with their verdicts. ⚠️ Which word in Claim 2 does the work?
R3. Give the physiological timeline from §21.3. ⚠️ Where does a 16-hour fast fall, and why does that matter?
R4. Explain the ghrelin entrainment finding. ⚠️ Why does a hunger wave pass in 20–30 minutes whether or not you eat?
R5. What is the discriminating trial design for Claim 3, and what have such trials found? Name the two specific trials from §21.5.
R6. Fill in the five slots for "is fasting good for you?" ⚠️ Which row is the whole debate?
R7. What did Ohsumi receive the 2016 Nobel Prize for? ⚠️ State three reasons the "16 hours triggers autophagy" claim doesn't follow from it.
R8. Why don't rodent fasting durations map onto human clock time?
R9. Give four established circadian findings from §21.7. ⚠️ What prediction do they generate that the weight trials weren't testing?
R10. Describe the Sutton et al. (2018) design and finding. ⚠️ What are its two main limitations?
R11. What happens to resting metabolic rate during a 24–72 hour fast, and what hormone is involved? ⚠️ How does that relate to Chapter 5's "starvation mode"?
R12. ⚠️ Name eight entries from §21.12's contraindication list. Which does §21.12 call the single most important?
R13. Give three things worth taking from the Ramadan literature. ⚠️ What does secular IF permit that Ramadan doesn't?
Tier 2 — Application
A1. 🔄 Theo's table. His intake went 3,160 → 2,360 → 3,290 kcal with perfect compliance throughout.
a) Which line items disappeared in months 1–3? b) Which grew in months 8–11? c) ⚠️ Explain the trajectory using Chapter 4 §4.7 and without using the word "willpower." d) Write the sentence you'd have said to him in month 2 that would have prevented him concluding he'd failed.
A2. Which column? For each statement, say whether the speaker is reading §21.5b's column one or column two, and whether they're right:
a) "I lost 12 pounds on 16:8. It absolutely works." b) "The trials show no advantage over calorie restriction." c) "My HbA1c came down after I started fasting." d) "There's nothing special about fasting metabolically." e) "Fasting is the only thing that's ever worked for me."
⚠️ Are any two of these actually in disagreement?
A3. Dose the claim. Convert each into a claim that could be tested, using the five slots:
a) "Fasting triggers autophagy." b) "Fasting boosts your metabolism." c) "Intermittent fasting reverses type 2 diabetes." d) "Fasting is how humans are meant to eat."
A4. The window placement. Two people both do an 8-hour window at 1,800 kcal/day.
- Person A: 8am–4pm
- Person B: noon–8pm
a) ⚠️ What does §21.7 predict differs, and what doesn't? b) How confident should you be? (Check §21.14's confidence table.) c) Which is more practical for someone with a family and a 9–5 job? ⚠️ What does that tension imply about translating this finding into advice?
A5. Screen the list. For each person, say whether §21.12 applies and what you'd advise:
a) A 34-year-old with type 2 diabetes on metformin only b) A 34-year-old with type 2 diabetes on insulin c) A 19-year-old who "used to have issues with food in school but is fine now" d) A 71-year-old wanting to lose 8 kg e) A competitive rower training 14 hours a week f) A 28-year-old who is 8 weeks pregnant g) A 45-year-old airline pilot, no medical history
A6. What breaks a fast? Rank these by their actual effect, then say which one people ask about most:
milk in coffee · a 400 kcal butter coffee · a diet soda · a teaspoon of sugar · bone broth · black coffee
⚠️ What does the gap between "asked about most" and "largest effect" demonstrate?
Tier 3 — Analysis and Synthesis
S1. ⚗️ versus 🟠. §21.6 gives autophagy ⚗️ while Chapter 19 §19.5 gave the omega-6 ratio 🟠.
a) Explain the difference precisely. b) ⚠️ Which is in worse evidential shape, and defend your answer against the opposite one. c) Is ⚗️ a softer verdict than 🟠 or a different one? What would move autophagy off ⚗️ — in either direction?
S2. The argument with no villain. §21.14b argues that over-claiming happened here without commercial interest.
a) ⚠️ If "who profits?" finds nothing, what should replace it? Write a replacement question. b) Test your question against Chapters 18, 19 and 20. Does it still work where money IS involved? c) Find one more claim in this book that would survive the who-profits test and shouldn't.
S3. Design the trial. §21.7's early-TRE finding rests on very small eucaloric samples.
a) Design the study that would settle it. Population, duration, control, primary outcome. b) ⚠️ Free-living or controlled feeding? State the trade-off explicitly. c) What's your dropout estimate for an 8am–3pm window over 12 months, and what does that do to your analysis?
S4. The contraindication problem. §21.12 is longer than the protocols mention.
a) ⚠️ Why don't the protocols mention it? Give a structural reason, not a moral one. b) Design the disclosure you'd want on a fasting app's onboarding screen. Under 60 words. c) ⚠️ What would a streak counter have to do differently to stop reinforcing restriction in vulnerable users? Is that compatible with the app's business model?
S5. Adherence as evidence. §21.10 argues the honest case for IF is adherence, not metabolism.
a) ⚠️ Is "it's easier to stick to" a lesser claim? Argue that it's actually the more important one. b) If adherence is the mechanism, what follows for how we should compare diets at all? ⚠️ Should trials measure adherence as a primary outcome? c) What does this do to the entire "which diet is best" literature (Chapter 10)?
S6. 🔄 The Part IV pattern. This is the fifth chapter in a row where both sides gave the same practical advice.
a) List all five, with the shared recommendation in each. b) ⚠️ What are these arguments actually about, then? Write three sentences. c) Chapter 20 Exercise M5 asked you to write this down. Compare. ⚠️ Has your answer changed? (Chapter 22 asks again.)
🪞 Reflection
M1. ⚠️ Do you know your actual eating window? Guess before you measure, then do the Project Checkpoint. How far off were you, and in which direction?
M2. §21.14b says hunger functions as a perceptible confirmation that the intervention is working. ⚠️ Where else in this book has a perceptible sensation stood in for evidence? (There are at least three.) Have you ever trusted one?
M3. ⚠️ §21.12's eating-disorder subsection asks four questions. If you fast — or have ever restricted deliberately — answer them honestly, in writing, for yourself. You don't have to show anyone.
M4. Theo's compliance was perfect and he concluded he'd failed. ⚠️ What does it cost a person to be given a protocol without its expected trajectory? Have you experienced this?
M5. This chapter has no industry to blame and still finds inflated claims. ⚠️ Does that make you more or less confident in the rest of Part IV? Be specific about why.