Plus, why your melatonin is backfiring.
Robin’s Short Version
- Muscle is medicine: Skeletal muscle absorbs ~80% of post-meal glucose, making strength training 2–3x per week a direct fix for insulin resistance.
- Sleep wrecks insulin: Under 5 hours of sleep cuts insulin sensitivity by 24%, putting poor sleep on equal metabolic footing with a bad diet.
- Fat-burning is trainable: Zone 2 cardio at conversational pace for 150–300 minutes per week rebuilds mitochondrial density and restores your ability to burn fat.
You don’t need a faster metabolism. You need a more flexible one.
Many of my patients in their 40s and 50s tell me the same thing: they’re gaining weight, workouts feel harder, and they’re just…tired. They assume their metabolism is slowing down.
But what’s really happening is a loss of metabolic flexibility: your body’s ability to switch between burning carbs and fat for fuel.
🚦Here’s how it should work:
- At rest, sleeping, or light movement: You burn fat (a slow, steady fuel source).
- During moderate activity: You burn fat + carbs for a balanced energy mix.
- In intense exercise: You burn mostly carbs for quick power.
When you’re metabolically inflexible—often from insulin resistance or overfeeding—your body gets “stuck” in carb-burning mode. You lose the ability to tap into stored fat.
Here’s how to know if you’re stuck and how to get more flexible.
What to know: Metabolic flexibility is the engine of longevity.
Metabolic inflexibility drives nearly every chronic condition we see today:
🧠 Alzheimer’s
🫀 Heart disease
🍩 Type 2 diabetes
🍷 Fatty liver
🌀 PCOS + hormone dysfunction
🎯 Cancer
Over 1 in 3 Americans already have prediabetes—many without knowing it. (This is one of the many reasons I recommend functional labs 2x per year starting at age 25!)
Signs your metabolism is stuck
If these feel familiar, your body may be struggling to switch fuels (even if your labs look “normal”):
- You crash after meals or need caffeine/sugar to get through the afternoon
- You feel dizzy or irritable if you skip a meal
- You gain weight easily—despite eating “clean”
- You’re always hungry (especially for carbs)
- Your endurance is tanking
- Skin flare-ups (acne, rosacea, eczema) won’t quit
How to measure metabolic flexibility
🧪The gold standard is Indirect Calorimetry, which measures oxygen and CO2 exchange to show your current fuel mix.
A flexible metabolism has:
- Low RER at rest (burning fat)
- High RER during effort (burning carbs)
You can get this done at sports performance labs. Some wearables like Lumen offer consumer versions.
🧬 Easier-to-track clues:
- Fasting insulin + glucose → Elevated levels suggest insulin resistance
- CGM or post-meal glucose → >140 mg/dL after eating = early dysfunction
- Triglycerides >100 or low HDL (<50 mg/dL for women, <40 mg/dL for men) → Poor fat metabolism
- Waist circumference >35 (for women or >40 for men) → Strong predictor of metabolic disease
- Low VO2 max → Sign of poor mitochondrial function poor fat oxidation
What to do: Muscle building, restorative sleep, magnesium supplements and hormone-therapy reverse metabolic rigidity.
The beauty of metabolic flexibility? It’s trainable.
With the right mix of movement, food, and recovery, you can rewire it in months.
1. Build muscle
Muscle is your largest glucose sink; it soaks up ~80% of post-meal glucose (Comprehensive Physiology, 2021).
