Intermittent Fasting: A Complete Beginner's Guide to 16:8, 18:6, and More
What Is Intermittent Fasting
IF is a timing pattern for eating, not a diet. You cycle between periods of eating and fasting. Unlike calorie restriction alone, IF taps into metabolic shifts that occur when insulin levels drop — increasing fat burning, autophagy, and metabolic flexibility.
The Major IF Protocols Compared
16:8: Fast 16h, eat within 8h window. Most sustainable for beginners. 18:6: Tighter window, more fat burning but harder to fit social eating. 20:4 (Warrior): One large meal + small snacks. Aggressive. 5:2: Eat normally 5 days, restrict to 500–600 kcal 2 non-consecutive days. OMAD (One Meal a Day): Extreme — only for experienced fasters.
What Happens in Your Body During a Fast
0–4h: Digestion, insulin elevated. 4–8h: Insulin drops, body begins glycogen depletion. 8–12h: Glycogen nearly depleted, fat burning increases. 12–16h: Ketosis begins. Autophagy (cellular cleanup) ramps up. 16–24h: Deep ketosis, maximum fat oxidation, growth hormone spikes.
How to Break a Fast Correctly
Avoid breaking fast with sugar/refined carbs (sharp insulin spike after prolonged fast). Start with protein + fat or vegetables. A common mistake: breaking 16-hour fast with juice or sweet coffee — this immediately halts fat burning.
Who Should Avoid IF
Pregnant or breastfeeding women. People with history of eating disorders. Type 1 diabetics (risk of hypoglycemia). Anyone on medications requiring food. Underweight individuals (BMI < 18.5).
IF and Exercise
Fasted training (working out before eating) enhances fat oxidation but may impair high-intensity performance. Morning workouts in a fasted state work well for light cardio. Strength training is better in the fed window for optimal muscle protein synthesis.