What is intermittent fasting? How it works

In 30 seconds

Intermittent fasting is about when you eat, not what: you eat during planned periods and have little or no calories in between, either every day (like 16:8) or on set days (like 5:2). In trials, common schedules led to modest, broadly similar changes in weight and cardiometabolic markers.

• It's about timing: an eating window and a fasting stretch, daily (like 16:8) or on set days (like 5:2).
• In trials, common schedules had modest, broadly similar average effects, so the one you can keep matters most.
• Check with a clinician first if you have diabetes, are pregnant or have had an eating disorder.

What does intermittent fasting mean?

It’s an umbrella term for eating patterns that switch between planned eating periods and longer stretches with little or no calories. It describes timing, not a menu or one fixed plan.

Intermittent fasting is an eating pattern that alternates periods of eating with longer periods of little or no calorie intake.Source 22Source 11Source 33

Common types include time-restricted eating (a daily eating window), alternate-day fasting and periodic days of reduced calorie intake, such as 5:2.Source 22Source 11

Try a schedule

Example times. Nothing is saved.

Schedule

12:1214:1016:818:620:4

Last meal

8:00 PM

Fasting

16 h

Eating window

8 h

Fast ends

12:00 PM

Eating window: 12:00 PM to 8:00 PM. Hours on a clock are not a measurement of your body.

Open the fasting calculator

How does intermittent fasting work?

You choose when your eating window opens and closes; the rest of the day is your fast. With 16:8, if dinner ends at 8 pm, you fast until noon the next day. Much of the fast falls overnight.

Why might timing matter? Reviews describe a “metabolic switch”: as energy from recent meals runs low, the body shifts which fuel it uses. Most of that picture comes from animal and lab studies.

Animal and mechanistic studies describe a “metabolic switch” and cellular stress responses during fasting, but when these changes happen in people, and how much they matter, remains uncertain.Source 11Source 33

Does intermittent fasting work?

Modestly. The strongest human evidence comes from randomized trials, pooled in a 2025 network meta-analysis of daily eating windows, alternate-day fasting and 5:2-style patterns.

In randomized trials, some intermittent fasting strategies led to modest weight loss and cardiometabolic changes similar to continuous calorie restriction over typical trial lengths; results varied with trial definitions, adherence and duration.Source 22

Common schedules had broadly similar average effects, and the comparisons depend on how trials defined each diet and how long people followed it. No schedule is proven best for everyone.Source 22

Is intermittent fasting healthy?

Not automatically. In trials, fasting schedules led to changes similar to cutting calories every day rather than extra health gains, and long-term effects are still unknown. Whether it suits you depends on your health, your medicines and whether you can keep it up without strain.

Long-term safety and benefit remain uncertain, and the trials summarized by the National Institute on Aging mostly didn't include children or adults older than 60.Source 33

Who should check with a clinician first?

Talk to a qualified health professional before fasting if you are pregnant or breastfeeding, under 18, living with or recovering from an eating disorder, have diabetes, take insulin or a medicine affected by meal timing, are an older adult, or are acutely ill. Do not change the dose or timing of any medicine without your clinician. See Blimbi’s safety guide.

How to try it gently

Start with the gentlest schedule that fits your life: 12:12 is a common first step, and 14:10 a step up. Move one boundary at a time and check in after a week; if it’s hard, shorten or pause. The beginner guide covers a first week, and the schedule comparison puts the options side by side.

Related guides

• Fasting Timeline: What Happens and Why Timing Varies

  This fasting timeline explains how the body moves from meal glucose to liver glycogen, then to fat and ketones, and why the timing varies.

• What Are the Benefits of Intermittent Fasting?

  Benefits of intermittent fasting: in randomized trials and reviews, mostly modest weight loss, similar to daily calorie restriction.

• How Do You Read Intermittent Fasting Research?

  Read intermittent fasting research with four checks: who took part, the comparison, the size of the difference and how long it lasted. What it shows.

• Dry Fasting: What It Is and Why It Isn't Safe

  Dry fasting means no food and no fluids, not even water. Why the dehydration risk makes it unsafe, the warning signs to know and what fasting trials allowed instead.

Frequently asked questions

What is intermittent fasting?

An eating pattern that switches between planned eating periods and longer stretches with little or no calories, daily (like 16:8) or on set days (like 5:2). It's about when you eat, not what.Source 22Source 11Source 33

How does intermittent fasting work?

You plan when your eating window opens and closes; the time in between is your fast. Reviews describe a “metabolic switch” in fuel use, mostly from animal studies. In trials, results depended on the schedule and how closely people followed it.Source 11Source 22

Will intermittent fasting make me lose weight?

Trials found modest average weight loss with some schedules, similar to cutting calories every day. Results varied with the schedule, adherence and trial length, so yours may differ.Source 22

When does the metabolic switch happen?

There's no precise hour. The idea comes mostly from animal and lab studies, and the main review was corrected, including some reported human timing. A timer counts hours; it doesn't measure ketones, autophagy, glucose or insulin, and can't tell when your body switches fuel.Source 11Source 33

Is intermittent fasting safe?

Not for everyone. Long-term safety is uncertain, and trials have typically recruited adults aged 60 or younger. Talk with a clinician first if you have diabetes or take glucose-lowering medicine, are pregnant or breastfeeding, are under 18 or have had an eating disorder.Source 33Source 44Source 55Source 66

Is intermittent fasting safe for older adults?

The evidence is limited: many trials didn't include people over about 60, and long-term safety is uncertain. Talk with a clinician before starting, especially with health conditions or regular medicines.Source 33

Sources

1. 1

   Effects of Intermittent Fasting on Health, Aging, and DiseaseRafael de Cabo, Mark P. Mattson. Massachusetts Medical Society, December 26, 2019.DOI: 10.1056/NEJMra1905136 · PMID: 31881139Narrative or mechanistic review

2. 2

   Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trialsZhila Semnani-Azad et al. BMJ, June 18, 2025.DOI: 10.1136/bmj-2024-082007 · PMID: 40533200Systematic review and meta-analysis

3. 3

   Calorie restriction and fasting diets: What do we know?National Institute on Aging. National Institute on Aging, August 14, 2018.Official guidance

4. 4

   Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance

5. 5

   Weight gain in pregnancyNHS. NHS, July 6, 2022.Official guidance

6. 6

   Overview – Eating disordersNHS. NHS, January 23, 2024.Official guidance