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

What intermittent fasting is and why it is everywhere

Intermittent fasting is not a diet but an eating pattern that alternates periods of eating with periods of fasting. Where a diet tells you what to eat, intermittent fasting tells you when. 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 noon and 8 pm, sleeping through most of the fast. A 2021 review in Annual Review of Nutrition (Varady et al.) concluded that the various forms of intermittent fasting produce weight losses of 1-8% from baseline, comparable to continuous calorie restriction, and improve markers such as blood pressure and insulin resistance.

The benefits of fasting that the science actually supports

Intermittent fasting is not only a strategy for eating less. When you fast, your body activates biological mechanisms that stay dormant when you eat constantly:

• Insulin sensitivity: after 12-14 hours without food, insulin levels drop substantially, which lets your body reach stored fat as fuel more efficiently. The study everybody cites here (Sutton, Cell Metab 2018) improved insulin sensitivity in prediabetic men — but the small print matters: it was not 16/8 but a six-hour window with dinner before 3 pm, and participants were fed enough NOT to lose weight, precisely so the effect of timing could be isolated
• Autophagy: after 16-18 hours of fasting, cells step up a recycling process called autophagy, from the Greek for "self-eating". Cells clear out damaged proteins, faulty organelles and aged components. Japanese biologist Yoshinori Ohsumi won the 2016 Nobel Prize in Medicine for uncovering its mechanisms
• Inflammation: a 2018 study in Cell Stem Cell found that 24 hours of fasting improved intestinal stem cell function in mice, both young and old, by switching them towards fatty acid oxidation
• A simpler day: removing one meal cuts the mental load of deciding, cooking, eating and cleaning up. That sounds minor, but decision fatigue accumulated across a day measurably erodes the quality of your food choices by the evening
• It makes the deficit easier: with fewer hours available to eat, most people naturally take in 200-500 kcal less per day without consciously counting anything

The different protocols (16/8 is not the only one)

16/8 is the most popular, but there are several options. The best one is whichever fits your life:

• 14/10 (for beginners): fast 14 hours, eat within 10. For example, eat from 9 am to 7 pm. In practice this is simply dropping the late-night snack. It is the easiest place to start
• 16/8 (the standard): fast 16 hours, eat within 8. For example, eat from noon to 8 pm. The most studied and most recommended. You skip either breakfast or dinner
• 18/6 (advanced): fast 18 hours, eat within 6. For example, eat from 1 pm to 7 pm. Two main meals, no snacks. Faster results but it needs adaptation
• 20/4 (OMAD, one meal a day): fast 20 hours, eat within 4. One or two very large meals. Not recommended for beginners or anyone with a tendency towards disordered eating
• 5:2 (alternating): eat normally five days a week and cut to 500-600 kcal on two non-consecutive days. Popularised by Dr Michael Mosley. Psychologically it can be easier than restricting every single day

A practical guide: reaching 16/8 in three weeks

Do not go from eating every three hours to a 16-hour fast overnight. Your body needs to adapt gradually. Ghrelin, the hunger hormone, is secreted at the times your body expects food, and it takes one to two weeks to recalibrate.

• Week 1 — close the kitchen after dinner: stop snacking from 9 pm. If you eat dinner at 8 pm and breakfast at 8 am, you are already doing a 12-hour fast without noticing
• Week 2 — push breakfast later: move it to 10 or 11 am. Drink water, black coffee or unsweetened tea in the morning to blunt hunger. Black coffee does not break the fast (under 5 kcal)
• Week 3 — reach 16/8: have your first meal at noon. Your eating window is noon to 8 pm. Fit two main meals and one snack into it
• Maintenance: once adapted, morning hunger disappears for most people. Many report better mental clarity and steadier energy in the morning without breakfast

What you can and cannot have while fasting

The general rule is that anything with meaningful calories breaks the fast. But there are nuances:

• Yes: water (still or sparkling), black coffee with no sugar or milk, green or black tea unsweetened, water with a squeeze of lemon, salt if you feel light-headed
• Grey zone: a tiny splash of milk in coffee (10-20 kcal probably does not meaningfully affect autophagy, though purists will disagree), sugar-free gum (chewing can stimulate gastric juices)
• Breaks the fast: any solid food, shakes, juices (including the green ones), plant milk in any quantity, supplements with calories (protein, sweetened BCAAs), soft drinks — even diet ones, since the insulin response to sweet taste is still debated

The six mistakes that ruin intermittent fasting

• Compensating inside the window: skipping breakfast is not a licence to eat 3,000 kcal across lunch and dinner. Intermittent fasting works because it reduces total intake, not through metabolic magic. Eat the same amount in less time and you will not lose weight
• Not drinking enough water: while fasting you miss the water you would normally get from food, which supplies 20-30% of your daily hydration. Compensate with an extra 500 ml in the morning
• Obsessing over the exact window: if one day you eat at 11:30 instead of noon, nothing happens. Flexibility is what makes adherence possible. A fast is not "broken" by 30 minutes
• Training hard while fasted without experience: lifting fasted works for some adapted people, but for a beginner it can mean dizziness, nausea and poor performance. If you train in the morning, eat something light first or shift your window to include the session
• Ignoring food quality: fasting is not an excuse to eat anything during the window. Nutritional quality still matters — enough protein, vegetables, healthy fats and complex carbohydrates
• Believing fasting "speeds up your metabolism": it does not. Fasting has no magic metabolic properties. Its main benefit is simplification: it is easier to hold a calorie deficit eating twice a day than five times

Who intermittent fasting is NOT for

For all its benefits, intermittent fasting is not for everyone. These groups should avoid it or speak to a doctor before starting: people who are pregnant or breastfeeding, anyone with a history of eating disorders (anorexia, bulimia, binge eating disorder), people with type 1 or type 2 diabetes on insulin medication (risk of hypoglycaemia), people who are underweight (BMI under 18.5), under-18s, and anyone taking medication that must be taken with food at specific times.