Types of Intermittent Fasting and How They Differ
The main types of intermittent fasting: daily eating windows like 16:8, alternate-day fasting and 5:2. How each one works and what trials found.
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The main types of intermittent fasting are time-restricted eating (a daily eating window, such as 16:8), alternate-day fasting and periodic calorie restriction (restricted intake on two days a week, such as 5:2). In trials, several types led to modest weight loss, depending on the method and on sticking with it.
- Time-restricted eating sets a daily eating window: 12:12, 14:10, 16:8, 18:6 or 20:4.
- Alternate-day fasting and 5:2 work on whole days rather than hours.
- In trials, many types led to modest weight loss; no type works the same for everyone.
- Animal studies suggest how fasting might work, but can't prove long-term benefits in people.
What are the main types?
Fasting patterns differ in two ways: how often you fast and for how long.
Common fasting patterns include time-restricted eating (daily eating windows), alternate-day fasting (fasting every other day), periodic calorie restriction such as 5:2 (restricted calorie intake on two days a week) and prolonged fasts lasting several days.Source 3
| Type | How it works |
|---|---|
| Time-restricted eating | A daily eating window, for example 6–8 hours, as in 16:8 |
| Alternate-day fasting | Fasting days alternate with eating days |
| Periodic calorie restriction | Restricted calorie intake on two days a week, as in 5:2 |
| Prolonged fasting | Fasts that last several days |
How do the daily schedules compare?
Time-restricted eating comes in several sizes. The numbers name the fasting hours first, then the eating hours.
Time-restricted eating limits daily eating to a consistent window, for example 6–8 hours, and fasts for the rest of each 24-hour day.Source 3
| Schedule | Timing | What sets it apart |
|---|---|---|
| 12:12 | 12 hours to eat, 12 to fast | Gentler |
| 14:10 | 10-hour eating window | A longer daily fast |
| 16:8 | 8-hour eating window | A common daily pattern |
| 18:6 | 6-hour eating window | Narrower |
| 20:4 | 4-hour eating window | Much narrower |
| 5:2 | Two days of restricted calorie intake a week | Weekly, not daily |
Each has its own guide: 12:12, 14:10, 16:8, 18:6, 20:4 and 5:2. The early vs late eating window guide covers timing within the day.
How do alternate-day fasting and 5:2 work?
Instead of a daily window, these types work on whole days. See the alternate-day fasting guide for how trials set up fasting days.
Alternate-day fasting alternates fasting days and eating days; 5:2 means eating without restriction for 5 straight days each week, followed by 2 days of restricted caloric intake.Source 3
Which type works best?
None has been shown to be best for everyone.
Randomized trials and a network meta-analysis show many intermittent fasting types can lead to modest weight loss and changes in cardiometabolic risk factors; comparisons depend on trial definitions, adherence and duration.Source 2
Animal and lab studies help explain how fasting might work, but not what it does over years in people.
Animal and mechanistic studies identify biological processes fasting may affect, but on their own they don't prove long-term benefits in people or set a timeline for these effects.Source 1
Mixed evidence
Research gaps
Public overviews note gaps in long-term safety and benefit data, and trial findings largely don't cover children or adults over about 60.Source 3
Who should get medical advice 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.
Frequently asked questions
What are the types of intermittent fasting?
Time-restricted eating (a daily eating window, for example 6–8 hours), alternate-day fasting, periodic calorie restriction such as 5:2, and prolonged fasts of several days. Trial designs vary between types.Source 3Source 2
Which type of intermittent fasting works best for weight loss?
None has been shown to be best. In trials, many types led to modest weight loss and cardiometabolic changes, but results varied with the method, adherence and study length, and indirect comparisons add uncertainty.Source 2
Is fasting safe for older adults?
Trial data for older adults are limited and long-term safety and benefit are uncertain, so older adults should talk with a clinician before starting, especially with chronic conditions or regular medicines.Source 3
Sources
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
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
Calorie restriction and fasting diets: What do we know?National Institute on Aging. National Institute on Aging, August 14, 2018.Official guidance
Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance
Weight gain in pregnancyNHS. NHS, July 6, 2022.Official guidance