Intermittent Fasting: The Complete Guide

Intermittent fasting is not a diet, it's an eating schedule. You cycle between periods of eating and fasting, allowing your body to shift from glucose metabolism to fat burning and ketone production. Unlike calorie restriction, IF focuses on when you eat, not just what you eat.

Dave Asprey popularized 'Bulletproof Intermittent Fasting', extending morning fast with fat-fueled coffee to stay in ketosis while gaining cognitive benefits. The broader IF movement spans 16:8, 18:6, OMAD (one meal a day), and multi-day fasts.

After 12–16 hours without food, liver glycogen depletes and your body ramps up lipolysis, breaking down fat for fuel. Ketones (beta-hydroxybutyrate) become available as an alternative brain fuel, often described as 'cleaner' energy than glucose.

Autophagy, the cellular recycling process where damaged proteins and organelles are broken down and reused, increases during extended fasts. Yoshinori Ohsumi won the 2016 Nobel Prize for autophagy research. Animal studies suggest 16–24+ hour fasts activate meaningful autophagy; human data is still emerging.

Insulin sensitivity improves with consistent IF. Fasting lowers insulin, allowing fat cells to release stored energy. Mark Mattson's research at Johns Hopkins shows IF may protect against neurodegenerative disease, though long-term human trials are ongoing.

16:8 Beginner: Stop eating by 8pm, resume at noon. Eight-hour window (noon–8pm) fits social eating. Black coffee, tea, and water only during the fast.

Bulletproof IF: 16+ hour fast with Bulletproof Coffee (coffee + MCT oil + grass-fed butter/ghee) in the morning. Fat calories don't spike insulin significantly, maintaining ketosis while providing energy. See our Bulletproof Coffee guide.

18:6 / 20:4: Tighter windows for experienced fasters. OMAD (one meal a day): single 1–2 hour eating window, advanced, not for beginners.

5:2: Eat normally 5 days, restrict to 500–600 calories 2 non-consecutive days. Different mechanism than daily IF but similar metabolic benefits.

Days 1–3: Hunger pangs, irritability, difficulty concentrating, normal adaptation. Electrolytes (sodium, potassium, magnesium) help.

Week 1–2: Hunger typically decreases as ghrelin rhythms adjust. Many report morning mental clarity once fat-adapted.

Week 3+: Fasting becomes habitual. Energy stabilizes. Breaking fasts with high-carb meals can cause blood sugar spikes, ease in with protein and fat.

Women: Some experience menstrual irregularity or increased cortisol on aggressive IF. Consider 14:10 or cycled fasting (fast 3–4 days/week).

Avoid or consult a physician if: pregnant, breastfeeding, underweight, history of eating disorders, type 1 diabetes, or on medications requiring food (metformin timing matters for type 2).

Over-restriction: IF is not permission to eat junk in your window. Nutrient density still matters, see our foundation supplement stack.

Cortisol elevation: Aggressive fasting + high stress + poor sleep = counterproductive. Fix sleep first.

r/intermittentfasting and r/Biohackers agree: 16:8 is the sustainable default. OMAD is for experienced fasters only. Dave Asprey's fat-fueled morning coffee is the most-cited 'make fasting easier' hack.

Peter Attia recommends IF primarily for metabolic health in insulin-resistant individuals, not as a universal longevity intervention. Huberman suggests aligning eating windows with circadian rhythm, eat during daylight hours.