Why fasting feels amazing for some, and miserable for others — and how to understand what your metabolism is really telling you.
You’ve probably heard about fasting as the secret to better focus, fat loss, and longevity. But if you’ve ever tried it and felt dizzy, anxious, or irritable, you’re not alone.
The truth is: fasting isn’t universal — it’s physiological. Some people thrive during a fast, while others feel completely drained. The difference isn’t mindset or discipline — it’s metabolic flexibility.
🔬 What Is Metabolic Flexibility?
Metabolic flexibility refers to your body’s ability to switch between using carbohydrates and fats for energy.
When you’re metabolically flexible, your body easily shifts to fat oxidation when carbs are low — so you feel steady, focused, and calm.
But when your metabolism is rigid or underdeveloped, fasting can become stressful. Cortisol rises, blood sugar drops, and your body struggles to maintain energy.
That’s when you get headaches, brain fog, and fatigue — not because fasting is bad for you, but because your metabolic system needs better balance.
🧠 The Phases of Fasting — and What They Mean for Your Body
1. The Fed State (0–4 hours after eating) Your body is primarily burning glucose (carbs) from your last meal. Insulin levels are high, promoting nutrient storage.
Tip: This phase isn’t “fasting” yet — it’s digestion. Use it to set up a balanced meal that will support the hours ahead.
2. The Early Fasting Phase (4–12 hours) As insulin drops, your body begins tapping into stored glycogen for energy.
If you feel fine here: You likely have stable blood sugar control. If you feel shaky or moody: It’s a sign of glucose instability — your metabolism may rely too heavily on carbs.
3. The Fat-Adaptive Phase (12–24 hours) Your glycogen stores run low, and fat oxidation increases. Your body begins producing ketones as an alternate energy source.
If fasting feels good here: Your body efficiently switches fuels — this is metabolic flexibility in action. If it feels draining: Your mitochondria may not be optimized for fat metabolism yet.
4. The Deep Fasting Phase (24+ hours) Ketones become your main energy source. Inflammation decreases, and cellular renewal (autophagy) activates.
However, not everyone should aim for long fasts — the ideal fasting duration depends on your metabolism, stress levels, and recovery capacity.
⚙️ How We Measure Your Metabolism
At Mancuso Clinic, we use advanced metabolic testing to evaluate how your body produces and uses energy. Through a non-invasive breathing test, we measure:
🔹 RER (Respiratory Exchange Ratio): Are you burning carbs, fats, or both? 🔹 VO₂ + VCO₂: How efficient is your oxidative metabolism — your body’s energy engine? 🔹 Fat Oxidation Rate: Can your body actually access and use stored fat as fuel?
This fat oxidation test gives us precise data on your metabolic efficiency — allowing us to personalize nutrition, training, and fasting strategies for optimal results.
🚫 Why Fasting Feels Hard for Some People
If your metabolism isn’t conditioned for fat use, fasting can trigger energy crashes and irritability. Instead of burning fat, your body raises cortisol to keep blood sugar up — leaving you tired and anxious.
Fasting isn’t “bad” — but doing it without metabolic flexibility can backfire.
🌿 How to Build a More Flexible Metabolism
Balance Your Blood Sugar: Eat meals rich in protein, fiber, and healthy fats.
Ease Into Fasting: Gradually increase your fasting window instead of jumping straight to long fasts.
Train Smart: Light activity while fasted helps improve fat oxidation.
Prioritize Sleep and Recovery: Stress and sleep deprivation impair metabolic regulation.
Test — Don’t Guess:Metabolic testing shows how your body truly works, so you can fast safely and effectively.
🔍 Discover How Your Body Produces Energy
Fasting can be a powerful tool — when it’s personalized to your physiology.
At Mancuso Clinic, our team uses VO₂ metabolic testing to understand your fat-burning efficiency, energy production, and recovery profile. Then we create a custom nutrition and fasting plan based on your data.
📊 Ready to learn how your metabolism really works? Book a FREE Health Discovery Session today and uncover your body’s unique blueprint for energy and performance.
