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Evidence: fasting or calories?
Randomised trials and meta-analyses from 2020–2024 agree: intermittent fasting produces the same weight loss as daily calorie restriction when calories are matched. It does not speed up metabolism and there is no special "fat-burning mode". Where it helps is behaviour: with a narrower eating window most people spontaneously eat 200–500 kcal less, late-night snacking disappears and the number of food decisions drops. Fasting is a tool, not a mechanism.
Methods
| Method | How | Who |
|---|---|---|
| 16:8 | 8-hour eating window (e.g. 12:00–20:00), 16-hour fast | The most common; for people who do not mind skipping breakfast |
| 14:10 | 10-hour window | Beginners, women, people who train |
| 5:2 | 2 days a week at 500–600 kcal, 5 days normal | Those who dislike daily restriction |
| OMAD (one meal a day) | 1–2 hour window | Not recommended: protein and micronutrients are hard to cover |
Pick the window around your life: social dinners → 12–20, morning training → 10–18. Water, plain tea and black coffee do not break the fast.
Who it suits, who it doesn't
Suits: people with no breakfast appetite, night snackers, anyone who wants fewer food decisions and simple rules. Does not suit: pregnant and breastfeeding women, type 1 diabetes and insulin users (never without a doctor), a history of disordered eating, adolescents, hard trainers with protein targets above 150 g a day (hard to fit in 8 hours), and people with low blood pressure or dizziness. In women, long fasts (20+ hours) have been linked to cycle irregularities; 14:10 is the safer start.
Combining it with training
Training fasted lowers performance and does not increase fat loss. Best arrangement: put the session inside the window or just before it, and make the first meal after training carry 40 g of protein within 1–2 hours. If muscle is the goal, widen the window to 10 hours; a 1.8–2.0 g/kg protein target wants 0.4 g/kg per meal and three meals rarely fit in 8 hours. Details in the protein guide.
Common mistakes
- Compensating in the window. Fasting 16 hours then eating 2,500 kcal in 8 does not lose weight; fasting does not guarantee a deficit. Know your number: calculator.
- Neglecting protein. Fitting 140 g into two meals takes planning; if it does not fit, muscle loss follows.
- Milk and sugar in coffee. Three lattes are 300 kcal; the fasting window is water, plain tea and black coffee only.
- Wrecking sleep. A late window (14–22) means a heavy meal before bed; broken sleep raises appetite.
- Rigidity. A family breakfast or a morning meeting: shift the window that day. Life is not broken for a rule.
Frequently asked questions
How much weight can I lose a month with intermittent fasting?
The same as any calorie deficit: 0.5–1 kg a week, 2–4 kg a month. No deficit, no loss.
What can I drink during a 16-hour fast?
Water, sparkling water, plain tea, black coffee, unsweetened herbal tea. Milk, sugar, juice and even 'diet' sweetened drinks are debated; water is cleanest.
Is skipping breakfast unhealthy?
Not for healthy adults; the claim that breakfast speeds metabolism was never proven. If you have a morning appetite, move the window earlier (8–16); skipping is not mandatory.
Does intermittent fasting cause muscle loss?
Not if protein and training are maintained; if the daily protein target becomes hard to hit in two meals, widen the window.
Related tools and pages
Tell FitCi your eating window
Shift your meal times; FitCi tracks calories, protein and training days around your window.