Intermittent fasting (IF) is an eating pattern that alternates between defined windows of eating and voluntary fasting. Rather than restricting which foods you eat, it restricts when you eat — common schedules include 16:8 (16 hours fasting, 8 hours eating), 5:2, and alternate-day fasting.
Why Timing, Not Just Food Choice, Matters
Intermittent fasting is unusual among diet strategies because it says nothing about what you eat — only when. That simplicity is part of its appeal: there's no macro counting or food-elimination required, just a defined eating window each day (or week). During the fasting period, insulin levels drop and the body increasingly relies on stored fat for energy, which is the mechanism many proponents point to for its metabolic effects.
It's important to be clear-eyed about what fasting actually does, though. It doesn't create a metabolic "hack" that burns calories out of nowhere — weight loss still depends on eating fewer calories than you burn over time. What fasting often does well is make that calorie deficit easier to achieve for some people, since compressing eating into a shorter window naturally leads many people to eat less without consciously tracking every bite.
Worked Example
Consider someone whose TDEE is 2,400 kcal/day, following 16:8 with an eating window of noon to 8pm.
| Component | Detail | Value |
|---|---|---|
| Fasting window | 8pm – noon next day | 16 hours |
| Eating window | Noon – 8pm | 8 hours |
| Daily calorie target for -0.5 kg/week | TDEE − 500 | 1,900 kcal |
| Meals typically fit in window | Lunch + dinner + snack | 2-3 meals |
To lose weight, this person still needs to hit roughly 1,900 kcal within their 8-hour window — the fasting schedule doesn't override the calorie math, it just changes when those calories are consumed.
Common Fasting Schedules Compared
- 16:8 (Leangains): Fast 16 hours daily, eat within an 8-hour window. The most beginner-friendly and sustainable option.
- 5:2 Diet: Eat normally 5 days a week, restrict to ~500-600 calories on 2 non-consecutive days.
- Alternate-Day Fasting: Alternate between normal eating days and very-low-calorie or zero-calorie fasting days.
- OMAD (One Meal a Day): An extreme version — roughly a 23:1 split, eating all daily calories in a single sitting.