The Holy Grail of human health and performance isn’t just about speed or strength. It’s about being a fat-burning machine.
It’s about Metabolic Flexibility.
In my view, this is a defining characteristic of what makes us human.
Think of your metabolism like a hybrid car with two fuel tanks:
⛽ The Gas Tank (Glycogen/Sugar): Small, limited, burns hot and fast. For emergencies (sprinting away from a lion).
🛢️ The Diesel Tank (Fat): Massive, unlimited storage. Burns slow and steady (hunting down an antelope).
Metabolic Flexibility is the ability to seamlessly switch tanks depending on demand. Sitting at your desk? You should be burning diesel. Sprint for the bus? Switch to gas, then back immediately.
The Problem: Most modern humans are stuck in “sugar-burning” mode, riding the insulin rollercoaster all day and forgetting how to access the diesel tank.
Why It Matters (Cellular Signalling)
We need a balance between two opposing systems.
AMPK (The Catabolic Signal): Triggered when energy is low. It cleans house, repairs cells, and burns fat (think autophagy and ketones).
mTOR (The Anabolic Signal): Triggered by food. It builds tissue and grows muscles.
You need both destruction/repair and building/efficiency. If you eat high-carb 6 times a day, you constantly trigger mTOR and never let AMPK do its cleanup job. That is a recipe for metabolic disaster.
The Evolutionary Advantage
To fuel our huge, energy-expensive brains during periods without food, we evolved as the ultimate fat burners. It isn’t a “biohack”—it is your factory setting.
Who is this for?
The Endurance Athlete: Avoid “bonking” when sugar runs out. Tap into an infinite fuel supply.
The CEO/Parent: Stable energy. No 3 PM slump or “hangry” outbursts.
Everyone: Metabolic inflexibility is a root driver of most modern chronic diseases (Type 2 diabetes, Alzheimer’s, heart disease).
How do we measure it?
The Lab Test (Gold Standard):
We measure your RER (Respiratory Exchange Ratio).
0.7 = Pure Fat Burning
1.0 = Pure Sugar Burning
The Goal: Be closer to 0.7 at rest and during low-intensity movement. In the lab, I see too many “sugar burners” who burn mostly carbs even at rest due to high-carb diets and frequent meals.
The Field Test (The “Hangry” Test):
Can you skip lunch without killing your colleagues? If you can go 4-6 hours fasted and feel mentally sharp, you are likely flexible. If you get shaky and irritable, you’re stuck on sugar. (Can you also do easy exercise for two hours fasted with no problems?)
Are you already metabolically flexible? How did you do it?
