What is Intermittent Fasting?
Short for IF
Also: IF · 16:8 · 20:4 · time-restricted eating
Eating only inside a fixed daily window (commonly 8 hours) — not a diet so much as a clock.
Intermittent fasting (IF) is an eating pattern, not a diet — you eat only inside a defined window each day. The most common version is 16:8 (fast 16 hours, eat in an 8-hour window). 20:4 fasts 20, eats in 4. Alternate-day and 5:2 protocols exist too. IF doesn't tell you what to eat. It tells you when.
Quick definition
Most people who do IF run 16:8 — skip breakfast, eat from noon to 8 p.m. The fast is just hours; nothing magical happens at the 12-hour mark or the 16-hour mark that doesn't happen at 10 hours.
How it actually works
The published evidence (de Cabo & Mattson, 2019, NEJM) shows IF produces weight loss comparable to standard calorie restriction — when calories match. The mechanism is mostly just "fewer eating hours equals fewer chances to eat too much." A 2020 randomized trial (Lowe et al., JAMA Internal Medicine) found 16:8 produced about 1.7 percent body weight loss over 12 weeks, similar to controls who ate three meals freely.
What IF does well: simplifies food choices, naturally controls calories, and improves insulin sensitivity in some studies. After roughly 12 to 16 hours, glycogen depletes and the body shifts toward fat oxidation. That's lipolysis in action — useful, but not the magic-bullet mechanism Instagram makes it sound like.
What IF doesn't do: melt fat overnight, suspend the calorie equation, or work for everyone. People with a history of disordered eating, pregnant or breastfeeding women, and anyone on diabetes medication should talk to a doctor first.
Why it matters for weight loss
IF works for many people because it's simple. One fewer decision per day (no breakfast) often equals a couple hundred fewer calories. Read our week-by-week IF timeline for what to expect realistically.
Common misconceptions
The biggest myth is that IF works regardless of what you eat. It doesn't. You can fast 16 hours and still eat 3,000 calories in your 8-hour window. The pattern only produces fat loss if it produces a deficit.
The second myth: IF "boosts metabolism." Multiple studies show no meaningful BMR change versus regular eating. The autophagy claims at the 16-hour mark are extrapolated from mouse studies that aren't yet replicated in humans at relevant dose.
Related terms
- Calorie Deficit Eating fewer calories than you burn — the only mechanism that produces fat loss.
- Ketosis A metabolic state where the body burns fat for fuel and produces ketones — entered via fasting or very low carb intake.
- Insulin Sensitivity How responsive your cells are to insulin. High sensitivity = good. Low sensitivity (resistance) = trouble.
- Ghrelin The 'hunger hormone' produced in the stomach. Rises before meals. The opposite of leptin.
- Ketogenic Diet A very-low-carb, high-fat pattern that pushes the body into ketosis. Works short-term, hard to sustain.
Read next on Real Easy Diet
Sources
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[01]
Intermittent fasting — Harvard T.H. Chan Harvard Chan School
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[02]
de Cabo & Mattson — NEJM fasting review New England Journal of Medicine
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[03]
Intermittent fasting overview — Johns Hopkins Johns Hopkins Medicine
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