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# Metabolic Flexibility
The ability to switch efficiently between fuel sources (carbohydrate, fat, protein) depending on intensity and state (rest vs. exercise, fed vs. fasted). Per Andy Galpin, this is trainable and highly individual — not a fixed metabolic "type" someone is stuck with.
6–10 kcal/day per pound of muscle. A smaller lever on weight than commonly assumed; don't oversell "build muscle to boost metabolism" as a fat-loss strategy on its own.
context, not applied as a universal rule (e.g. blanket keto/low-carb advice).
(Source: Dr. Andy Galpin: How to Build Physical Endurance & Lose Fat — Huberman Lab)
Galpin separates "fat adapted" into three non-equivalent outcomes that are often conflated: maximizing fat oxidation, maximizing fat as exercise fuel, and maximizing fat loss over time. The physiological ceiling for fat oxidation is ~70% — never 100% — with the remainder always carbohydrate (tracked via RER/RQ: ~0.6 at rest, ~0.8 walking, up to ~1.1 near VO2max, as the "crossover concept" shifts fuel mix toward carbohydrate with rising intensity).
Optimizing fat-oxidation capacity necessarily downregulates carbohydrate-oxidation enzymes — metabolic flexibility has a real trade-off, not a free upgrade, so sport specialists shouldn't try to maximize both fat- and carb-oxidation capacity simultaneously; train fuel preference toward what the event demands.
Fasted training is debunked as a fat-loss enhancer: muscle/liver glycogen and blood glucose are sufficient for most exercise even fasted. See Physiological Adaptation Testing Battery for self-diagnostic tests that identify which fuel-utilization system (fat vs. carbohydrate) is actually weaker.
Which fuel is burned during a given workout does not determine net fat loss — see Carbon-In/Carbon-Out Fat Loss Model.