Intermittent fasting 16/8: what it is, how to start, mistakes to avoid

What is intermittent fasting and why is it everywhere

Intermittent fasting (IF) is not a diet but an eating pattern that alternates between periods of eating and fasting. Unlike a diet that tells you what to eat, IF tells you when to eat. The 16/8 protocol is the most widespread: you fast for 16 consecutive hours and eat within an 8-hour window. In practice, most people simply skip breakfast and eat between 12:00 and 20:00, sleeping through most of the fast. A study from the Annual Review of Nutrition in 2022 analyzed 130 clinical trials on intermittent fasting and concluded that it produces weight loss comparable to continuous caloric restriction, with additional benefits in metabolic markers.

The science-backed benefits of fasting

Intermittent fasting is not just a strategy to eat less. When you fast, your body activates biological mechanisms that do not activate when you eat constantly:

• Insulin sensitivity: after 12-14 hours of fasting, insulin levels drop significantly. A 2018 University of Alabama study showed that 16/8 fasting improved insulin sensitivity by 36% in prediabetic men
• Autophagy: after 16-18 hours of fasting, your cells activate a recycling process called autophagy (from Greek "self-eating"). Cells eliminate damaged proteins and defective organelles. Japanese biologist Yoshinori Ohsumi won the 2016 Nobel Prize in Medicine for discovering autophagy mechanisms
• Inflammation reduction: a 2019 study published in Cell Stem Cell found that 24 hours of fasting reduced blood inflammatory markers and improved intestinal stem cell regeneration
• Day simplification: eliminating one meal reduces the mental load of deciding what to eat, cooking, eating, and cleaning. Decision fatigue accumulated throughout the day significantly affects your ability to make good food choices at night
• Easier caloric deficit: with fewer hours to eat, most people naturally consume 200-500 fewer kcal per day without actively counting calories

The different fasting protocols (not just 16/8)

• 14/10 (for beginners): fast 14 hours, eat in 10. Example: eat from 9:00 to 19:00. Essentially just skipping the evening snack. The easiest to start
• 16/8 (the standard): fast 16 hours, eat in 8. Example: eat from 12:00 to 20:00. The most studied and recommended. Skip breakfast or dinner
• 18/6 (advanced): fast 18 hours, eat in 6. Example: eat from 13:00 to 19:00. Two main meals without snacks
• 20/4 (OMAD): fast 20 hours, eat in 4. One or two very large meals. Not recommended for beginners
• 5:2 (alternate): eat normally 5 days a week and 2 non-consecutive days reduce to 500-600 kcal. Popularized by Dr. Michael Mosley

Practical 3-week guide to start 16/8

• Week 1 — Close the kitchen after dinner: stop snacking after 9:00 PM. If you eat dinner at 8:00 PM and breakfast at 8:00 AM, you are already doing a 12-hour fast effortlessly
• Week 2 — Delay breakfast: move breakfast to 10:00 or 11:00 AM. Drink water, black coffee, or unsweetened tea in the morning. Black coffee does not break the fast (less than 5 kcal)
• Week 3 — Reach 16/8: have your first meal at 12:00. Your eating window is 12:00-20:00. Have 2 main meals and 1 snack in that window
• Maintenance: once adapted, morning hunger disappears for most people. Many report greater mental clarity and energy in the mornings without breakfast

What can and cannot you have during the fast

• Yes: water (still or sparkling), black coffee without sugar or milk, green or black tea without sugar, water with lemon (a few drops), salt (if you feel weak)
• Gray zone: a tiny splash of milk in coffee (10-20 kcal probably does not significantly affect autophagy), sugar-free gum (chewing may stimulate gastric juices)
• Breaks the fast: any solid food, smoothies, juices (even "green" ones), plant milk in quantity, calorie-containing supplements (protein, sweetened BCAAs)

The 6 mistakes that ruin your intermittent fast

• Compensating during the window: skipping breakfast does not give you license to eat 3,000 kcal at lunch and dinner. IF works because it reduces total intake, not through metabolic magic
• Not drinking enough water: during the fast, you do not get the water that normally comes from food (which provides 20-30% of your daily hydration). Compensate by drinking 500ml extra in the morning
• Obsessing over the exact window: if you eat at 11:30 instead of 12:00 one day, nothing happens. Flexibility is the key to adherence
• Intense fasted exercise without experience: fasted strength training can work for adapted individuals, but for a beginner it can cause dizziness, nausea, and poor performance
• Ignoring food quality: IF is not an excuse to eat anything during the window. Nutritional quality still matters
• Thinking fasting accelerates metabolism: it does not. Its main benefit is simplification: it is easier to maintain a deficit eating 2 times a day than 5

Who should NOT do intermittent fasting

Despite its benefits, IF is not for everyone. These populations should avoid it or consult a doctor first: pregnant or breastfeeding women, people with history of eating disorders (anorexia, bulimia, binge eating), type 1 or type 2 diabetics on insulin medication (risk of hypoglycemia), underweight individuals (BMI < 18.5), those under 18, and people taking medication that must be taken with food at specific times.