Chapter 6 — Exercises

If biochemistry has bounced off you before, do Part A and C1 and skip the rest without guilt. If you're taking a course, Part D is where the exam questions live.

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

Selected answers in Appendix J.

Part A — Conceptual Understanding ⭐

A1. Explain the cash / current account / pension analogy. Which is which, and why doesn't the body store energy as ATP?

A2. Give approximate figures for muscle glycogen, liver glycogen, and body fat, in grams and calories. What is the approximate ratio of fat to carbohydrate stores?

A3. Why can't muscle glycogen raise blood glucose? What follows from that?

A4. Why is there no "protein store," and what does the body actually do when it uses protein for energy?

A5. What does glycolysis produce, where does it happen, and what are its two defining advantages?

A6. Explain, in two sentences, why lactate is not a waste product. What actually causes the burn during hard effort, and what causes next-day soreness?

A7. Describe the two stages inside the mitochondrion. What is oxygen's specific role?

A8. What is acetyl-CoA, and why does its position in the pathways matter for the "no switch" argument?

A9. What is the respiratory quotient at 0.7, 0.85, and 1.0? Where does a resting human sit?

A10. What is gluconeogenesis, what three substrates feed it, and which substrate conspicuously cannot?

A11. Distinguish nutritional ketosis from diabetic ketoacidosis on four axes.

A12. What do mTOR and AMPK each do, and what activates each?

A13. What is protein turnover, roughly how much protein turns over daily, and why does that make protein a daily requirement in a way fat and carbohydrate are not?

Part B — Applied Analysis ⭐⭐

B1. A runner has 18 kg of body fat (>150,000 kcal) and hits the wall at mile 20. Explain, in terms of both rate and liver glycogen, why the fat can't rescue them.

B2. Work the fat-burning-zone arithmetic yourself. A 30-minute walk burns 150 kcal at 60% from fat; a 30-minute run burns 400 kcal at 35% from fat. Calculate fat oxidized in each. Then explain why even this calculation isn't the reason the zone concept fails.

B3. Someone eats zero carbohydrate for four days and their morning blood glucose stays normal. Trace where the glucose comes from on day 1 versus day 4, and state precisely what day 4 costs them.

B4. A ketone meter reads 2.1 mmol/L and the person hasn't lost weight in three weeks. Explain both facts without contradiction.

B5. Explain the L-carnitine claim's structure — "X is required for Y, therefore more X increases Y" — and then find two other supplement claims in this book so far that share the identical structure.

B6. Why does fat provide 9 kcal/g and carbohydrate 4? Answer using the hydroelectric dam image rather than by asserting it.

B7. A product is marketed as "activating AMPK for longevity." Using §6.9, explain why permanently activating AMPK would be a bad idea, and what the marketing is quietly assuming.

B8. Someone taking an SGLT2 inhibitor wants to start a ketogenic diet. What is the specific risk, why is it easy to miss, and what should they do?

Part C — Skills & Practice ⭐⭐–⭐⭐⭐

C1. Do the fuel-mix reflection. The Project Checkpoint. Map one ordinary day onto the fuel systems — four or five moments, with the demand and the rough mix for each. Then write the paragraph: what did you previously believe about fat burning, and what do you believe now?

C2. Add a late entry to your Belief Inventory. Whatever §6.6 dislodged — the fat-burning zone, "fasted cardio burns more fat," "you go catabolic after four hours," "ketosis means fat burning" — write it down, note where you got it, and add it to the Chapter 1 sheet before resealing.

C3. Explain it to someone. Explain "there is no fuel switch" to a friend in under two minutes, without using the words glycolysis, beta-oxidation, or acetyl-CoA. If you can't, your model is thinner than it feels — go back to §6.6.

C4. Find the treadmill chart. Next time you're in a gym, photograph or note the zone chart. Then write the honest version of it — what would that chart say if it were accurate and still fit in the same space?

C5. Trace one supplement's mechanism. Pick any supplement claiming to affect fat metabolism. Identify precisely which step in §6.1–§6.9 it claims to act on. Then ask the §6.5 question: is that step rate-limiting in a healthy person? Look for evidence either way.

C6. Convert a marketing sentence. Find a piece of content using the words "metabolic switching," "fat-adapted," "primed to burn fat," or "unlocks fat stores." Rewrite each phrase in terms of what is actually happening to the sliders. Notice how much less exciting it becomes and whether anything of substance is lost.

Part D — Synthesis & Critical Thinking ⭐⭐⭐

D1. The chapter says carbohydrate is not biochemically essential, and that not eating it raises your protein requirement. Does that make carbohydrate "essential" in a practical sense? Argue both, and say what turns on the distinction.

D2. Autophagy research won a Nobel Prize; the human fasting claims got an ⚗️ Untested verdict. Explain how both can be appropriate. Then say what evidence would move that verdict, and estimate how hard it would be to obtain.

D3. The chapter argues that the fat-burning zone chart has "quietly discouraged people from working hard for thirty years." Is that fair? Consider that low-intensity exercise is more sustainable for many people, and that adherence beats optimization. Does the chart cause net harm?

D4. Mice have a metabolic rate roughly seven times higher than humans, and much of the fasting and longevity literature is rodent work. Construct a general principle for how much to discount an animal finding — then test your principle against a case where animal work translated well.

D5. §6.2 notes that using protein for energy means "dismantling functional tissue," and §6.7 notes that gluconeogenesis runs on amino acids. Combine these into an argument about who should be most cautious with prolonged fasting. Who's on your list, and why?

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

M1. (With Chapter 5.) Chapter 5 introduced RQ as a measurement technique; this chapter used it as evidence for the threshold concept. Explain how a measurement method became an argument.

M2. (With Chapters 3 and 5.) Trace one gram of dietary fat from the plate to ATP. Name every stage: digestion (Ch 3), absorption route (Ch 3), transport, mitochondrial entry, beta-oxidation, citric acid cycle, electron transport. Where does the carbon end up, and how does it leave your body?

M3. (With Chapter 4.) Chapter 4 said fat leaves the body "mostly through the lungs as CO₂." Which specific stage in this chapter produces that CO₂?

M4. (With Chapter 2.) Run the "fasting activates autophagy for longevity" claim through the full six-question Claim Filter. Which question does it fail hardest?

M5. (With Chapters 4 and 5.) Someone argues that a ketogenic diet has a "metabolic advantage" of 300 kcal/day. Using Chapter 4's error bars and Chapter 5's ±10%, explain why this claim is difficult to establish even if true — and what study design would be needed.

M6. (With Chapter 3.) Why does the liver's position downstream of the portal vein (Ch 3 §3.5) matter for its role in gluconeogenesis and ketogenesis (§6.7–§6.8)?

M7. (Revisit at Chapter 10.) Predict now, before Chapter 10: in controlled trials matching calories and protein, how much fat-loss difference do you expect between a ketogenic and a higher-carbohydrate diet? Write a number. Seal it.

Part E — Research & Extension ⭐⭐⭐⭐

E1. Read about the crossover concept (Brooks and Mercier) and the concept of Fatmax. At what intensity does absolute fat oxidation peak, and how does training status shift it? Then consider: does knowing your personal Fatmax have any practical use for fat loss? Defend your answer.

E2. Investigate the Cori cycle and lactate shuttling. How did lactate go from being taught as a waste product to being understood as a fuel? Find when the shift happened and why it took so long to reach the popular understanding — this is a good case study in how long a corrected idea takes to propagate.

E3. Look into the human evidence on autophagy measurement. How is it actually assessed in living humans, what are the limitations of the available markers, and what would a definitive human study require? Then reassess the confident hour-thresholds circulating online.

E4. Read about euglycemic diabetic ketoacidosis associated with SGLT2 inhibitors. Why is it particularly dangerous, how does it present, and what should clinicians and patients know before combining these medications with carbohydrate restriction or prolonged fasting? (This is a genuine patient-safety issue that most nutrition content never mentions.)

E5. Examine the evidence on exogenous ketone supplements. What are the legitimate research applications? What does the weight-loss marketing claim? Write a paragraph distinguishing the two, and identify the specific confusion the marketing depends on.