Alternate Day Fasting: How It Works and What Trials Found

In 30 seconds

Alternate-day fasting alternates fasting days, with little or no food, and ordinary eating days. In a network meta-analysis of 99 randomized trials it reduced weight compared with usual eating, and by slightly more than daily calorie restriction in shorter trials (about 1.3 kg), but not in trials of 24 weeks or longer.

• Alternate-day fasting (ADF) alternates fasting days with ordinary eating days; many trials allow a small meal on fasting days.
• ADF gave slightly more weight loss than daily calorie restriction in shorter trials (about 1.3 kg) and no clear difference in trials of 24 weeks or longer.
• Cholesterol and blood sugar results were mixed.
• With diabetes or glucose-lowering medicines, do ADF only with your clinician's supervision.

What is alternate-day fasting?

Many trials test a modified version that allows a small amount of food on fasting days. If you would rather fast on two fixed days a week, see the 5:2 guide; a daily eating window is covered in the 16:8 guide.

In a 3-month randomized trial in 160 adults with obesity, modified alternate-day fasting meant three fasting days a week at 400–800 kcal (25–30% of energy needs) and structured normal-calorie meals on the other four days; overall, every arm of the trial was calorie-restricted.Source 22

How much weight did people lose?

Weight fell with alternate-day fasting. The extra loss in shorter trials was below the 2 kg the authors considered clinically important, and in trials of 24 weeks or longer it was no better than daily calorie restriction.

In a network meta-analysis of 99 randomized trials, alternate-day fasting led to weight loss versus usual eating and slightly more weight loss than other fasting patterns or daily calorie restriction in shorter trials (about 1–1.7 kg, below the 2 kg the authors considered clinically important). In trials of 24 weeks or longer, it was no better than daily calorie restriction.Source 11

Blood markers moved less clearly than weight, and the differences between diets were small.

Compared with time-restricted eating, alternate-day fasting lowered total cholesterol, triglycerides and non-HDL cholesterol; no fasting strategy changed HbA1c or HDL cholesterol, and declines in fasting glucose and insulin resistance compared with usual eating were trivial.Source 11

The estimates favour alternate-day fasting for weight in shorter trials, but the benefit is small and trials differ in diets, length and adherence. Most trials with a 52-week follow-up reported poor adherence in the fasting groups.Source 11

Does the time of day matter?

Alternate-day trials were not designed to compare clock times, so there is no evidence-based best hour to start or end a fasting day.

Evidence on meal timing comes from time-restricted eating, not alternate-day fasting: in a network meta-analysis, eating windows that ended earlier lowered weight and fasting insulin more than late ones. These results do not define an alternate-day schedule.Source 33

Who should get 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.

Related guides

• 16/8 Fasting: How It Works and How to Start

  16/8 fasting (also written 16:8) means about 16 hours of fasting and an 8-hour eating window. See a sample day, how to build up to it and what trials found.

• 5:2 Fasting: How It Works, With a Sample Week

  5:2 fasting means five usual eating days and two fasting days a week. See a sample week, what trials found and who should ask first.

• 20:4 Intermittent Fasting: How It Works

  20:4 intermittent fasting means about 20 hours of fasting and a 4-hour eating window. See a sample day, what trials found and when a gentler schedule wins.

• Can You Drink Apple Cider Vinegar While Fasting?

  Apple cider vinegar is a grey area on a strict fast: no fasting trial tested it. What a review found on blood sugar and weight, and who should ask a clinician first.

• Does Stevia Break a Fast? Sweeteners While Fasting

  Stevia and sucralose add little or no energy, yet no trial cited here tested them during a fast. See what the trials allowed and what WHO and NHS advise.

Next steps

• Find a gentle starting point

  Three quick steps to explore a simple routine, with safety before schedules. This is not a medical assessment.

• When does my fast end?

  See the time remaining and an educational timeline. Hours on a clock are not a measurement of your body.

• Take the fasting quiz

  A few questions about your day, then a gentle place to start.

• Intermittent fasting protocols

  Compare the common schedules side by side.

Frequently asked questions

What is alternate-day fasting?

Alternate-day fasting alternates fasting days, with little or no food, and ordinary eating days. In the modified version, fasting days allow a small meal.Source 11

Will I lose weight on alternate-day fasting?

Trials found weight loss compared with usual eating, but results vary with adherence and trial length. The extra loss over daily calorie restriction was about 1.3 kg on average, below the 2 kg the authors considered clinically important, and it was not seen in trials of 24 weeks or longer.Source 11

Is alternate-day fasting safe if I have diabetes or take medications?

If you have diabetes or take glucose-lowering medicines, alternate-day fasting is one of the harder patterns to do safely because medicines are complicated to adjust; it can be done under medical supervision, not on your own.Source 44

Sources

1. 1

   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

2. 2

   Effect of a ketogenic diet, time-restricted eating, or alternate-day fasting on weight loss in adults with obesity: a randomized clinical trialJosé Ignacio Martínez-Montoro et al. Springer Science and Business Media LLC, July 1, 2025.DOI: 10.1186/s12916-025-04182-z · PMID: 40598397Human randomized trial

3. 3

   Effects of timing and eating duration of time restricted eating on metabolic outcomes: systematic review and network meta-analysisYu-En Chen et al. BMJ, January 21, 2026.DOI: 10.1136/bmjmed-2024-001071 · PMID: 41586347Systematic review and meta-analysis

4. 4

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