The TL;DR
Metabolism is the sum of every chemical reaction in your body. It is not just about burning calories for weight loss; it is about how efficiently your body converts fuel (food) into energy (ATP). The holy grail of longevity is Metabolic Flexibility—the ability to switch seamlessly between burning sugar (glucose) and burning fat (ketones).
Accessibility Level
Level 1 (Foundation): You monitor this via subjective energy levels and hunger. Advanced testing (Level 2) includes VO2 Max and lactate threshold.
The Science of Flexibility
The Hybrid Engine
Imagine a hybrid car.
- Glucose: Is high-octane gas. Great for speed (sprinting), but the tank is small (glycogen).
- Fat: Is diesel. Great for long distances (living), and the tank is huge (body fat).
Metabolic Health is the ability to run on diesel (fat) most of the day, sparing the gas for emergencies. Metabolic Dysfunction (Insulin Resistance) is being stuck running on gas 24/7. When you run out of sugar, you “crash” (hangry) because you cannot access your fat stores (high insulin blocks fat burning).
Metabolic Rate
- BMR (Basal Metabolic Rate): Calories burned in a coma (~60-70% of total). Determined largely by muscle mass.
- TEF (Thermic Effect of Food): Calories burned digesting. Protein has the highest TEF (25-30%).
- NEAT (Non-Exercise Activity): Fidgeting, walking, standing. The biggest variable in daily burn.
Evidence Matrix
| Source | Verdict | Notes |
|---|---|---|
| Iñigo San Millán | Expert | Defines metabolic health as mitochondrial flexibility (Zone 2). |
| Herman Pontzer | Evolution | Showed that hunter-gatherers burn similar calories to us; diet drives obesity, not just “slow metabolism.” |
| Clinical Data | Strong | Metabolic inflexibility is the root cause of Type 2 Diabetes. |
How to Optimize
- Zone 2 Cardio: Trains the mitochondria to burn fat. Ideally 3-4 hours/week.
- Fasting: Forces the body to switch fuel sources when glycogen runs low.
- Build Muscle: Increases BMR. Muscle is metabolically expensive tissue.
- Walk (NEAT): Sitting is the new smoking. Aim for 8k-10k steps to keep the metabolic fires simmering.
References
Goodpaster, B. H., & Sparks, L. M. (2017). Metabolic Flexibility in Health and Disease. Cell Metabolism, 25(5), 1027-1036.
Pontzer, H., et al. (2012). Hunter-gatherer energetics and human obesity. PLOS ONE.
San-Millán, I., & Brooks, G. A. (2018). Assessment of Metabolic Flexibility by Gas Exchange Measurements in Diabetes. Sports Medicine.