Chapter 6 — Key Takeaways
One page. The reference card for every claim that depends on a switch.
The Seven Ideas
1. ATP is the only currency. Cash in your pocket. Glycogen is the current account. Body fat is the pension. All of metabolism is converting the second and third into the first.
2. The stores are wildly unequal.
| Store | Amount | Energy |
|---|---|---|
| ATP + creatine phosphate | trace | seconds |
| Muscle glycogen | ~300–500 g | ~1,200–2,000 kcal (local use only) |
| Liver glycogen | ~80–120 g | ~320–480 kcal (the only source of blood glucose) |
| Body fat | 10–30+ kg | ~90,000–270,000+ kcal |
| Protein | — | Not a storage form |
You cannot run out of fat. Muscle glycogen can't leave the muscle. And using protein for energy means dismantling functional tissue — selling furniture, not spending savings.
3. Glycolysis is fast and needs no oxygen. Lactate is a fuel, not a waste product — recycled by heart, muscle, and liver (the Cori cycle). It doesn't cause next-day soreness; that's mechanical damage and repair.
4. Mitochondria are a hydroelectric dam. The citric acid cycle pumps the reservoir; the electron transport chain drains it through a turbine to make ATP; oxygen is the drainage channel at the bottom. Fat yields 9 kcal/g because it carries more electrons to pump with. More mitochondria = more aerobic fat-burning capacity — which is what endurance training builds.
5. Beta-oxidation converges with glycolysis at acetyl-CoA. Same cycle, same dam. Fat is high-capacity/low-rate; carbohydrate is limited-capacity/high-rate. Neither is better — they're for different jobs. The brain can't use fatty acids, which is why §6.7 and §6.8 exist.
6. 🚪 There is no switch.
| RQ | Fuel mix |
|---|---|
| 0.70 | ~100% fat |
| 0.80 | ~67% fat — a resting human |
| 0.85 | ~50/50 |
| 1.00 | ~100% carbohydrate |
Sliders move toward fat with: low intensity · fasted state · endurance training · low carbohydrate intake · depleted glycogen · long duration. Toward carbohydrate with the opposites. Continuously.
7. Protein turnover never stops. You break down and rebuild 200–300 g/day of your own protein. The amino acid pool is small and nitrogen loss is one-way — which is why protein is a daily requirement in a way carbohydrate and fat are not.
Gluconeogenesis and Ketosis, in four lines
- Gluconeogenesis makes glucose from lactate, glycerol, and amino acids — not from fatty acids. So not eating carbohydrate is biochemically survivable and the cost is paid in protein.
- Ketone bodies cross the blood–brain barrier where fatty acids can't, letting the brain run largely on fat-derived fuel and sparing protein. A survival adaptation, not a hack.
| Nutritional ketosis | Diabetic ketoacidosis | |
|---|---|---|
| Ketones | ~0.5–3 mmol/L | often >10–15 |
| Insulin | Low but present — the brake works | Effectively absent |
| pH | Normal | Acidotic |
| Status | Normal physiology | Medical emergency |
The Pattern to Extract
"X is required for Y, therefore more X increases Y."
Almost always wrong. Pathways have a rate-limiting step, and it's rarely the ingredient being sold. A road being required to reach a city doesn't mean widening it helps if the jam is at the bridge.
Seen here in L-carnitine and BCAAs; you'll meet it again in most of Chapters 13 and 16.
What to Actually Do
- Ignore the zone chart. Exercise at an intensity you'll sustain and enjoy. Higher intensity often oxidizes more total fat, and either way the books balance over days.
- If you restrict carbohydrate, raise protein — gluconeogenesis runs on amino acids and there's no protein store.
- Don't buy a ketone meter to track fat loss. It measures a slider, not the account.
- Endurance training builds mitochondria — that's a real, earned improvement in fat-oxidation capacity, and no supplement substitutes.
Common Mistakes (and the fix)
| Mistake | Fix |
|---|---|
| Believing in a fat-burning zone / mode / switch | RQ ~0.80 at rest. It's a mixing board. |
| Treating a ketone reading as progress | It reports fuel mix, not energy balance |
| "Fat has all the energy I need for this run" | Rate, not quantity, is the ceiling at high intensity |
| Thinking lactate causes soreness | It's a fuel; soreness is mechanical damage |
| "X is required, so more X helps" | Find the rate-limiting step first |
| Quoting fasting hour-thresholds | Those numbers come from rodents with ~7× our metabolic rate |
| Assuming ketosis and ketoacidosis are on one scale | Different situations. Insulin is the difference. |
Verdict Summary
| Claim | Verdict | Why |
|---|---|---|
| L-carnitine increases fat burning / aids weight loss | ❌ Not supported | Mechanism real; not rate-limiting in healthy people |
| Train in the fat-burning zone to lose fat | 🟠 Probably false | True proportion, wrong variable; energy balance decides |
| Carbohydrate is essential — the brain runs on glucose | 🟠 Probably false | Gluconeogenesis + ketones cover it; the cost is protein |
| Ketosis is "fat-burning mode," so keto burns more body fat | 🟠 Probably false | Oxidizing fat ≠ losing fat; controlled trials find little advantage |
| Fasting activates autophagy, slowing ageing | ⚗️ Untested in humans | Real mechanism; human outcome data absent; hour-figures are rodent-derived |
| Exogenous ketones make you burn body fat | ❌ Not supported | Backwards — supplying ketones reduces the demand on your own fat |
⚠️ When to See a Professional
Do not begin ketogenic or extended-fasting protocols without medical supervision if you: have type 1 diabetes · take an SGLT2 inhibitor (risk of euglycemic DKA — ketoacidosis with normal blood glucose, easily missed) · are pregnant or breastfeeding · have kidney or liver disease · have a history of eating disorders · or have had pancreatitis.
One Thing to Remember
Your metabolism is a mixing board, not a gearbox.
And the corollary, which does most of the work: a measurement of which fuel you're using is not a measurement of whether your energy stores are shrinking. The ketone meter, the RQ reading, and the treadmill chart all report a slider position. None reports the balance of the account.