How Many Calories Break a Fast? It Depends

In 30 seconds

It depends on the kind of fast. The time-restricted eating trials cited here allowed no calories during the fast, only water and energy-free drinks such as black coffee, tea and, in one trial, up to two diet sodas a day. Day-based formats differ: the network meta-analysis describes 5:2 as two 24-hour fasting days a week with unrestricted eating on the other five, and one modified alternate-day trial allowed 400–800 kcal on fast days.

• None of the sources cited here sets a number of calories below which a fast stays intact.
• Modified alternate-day fasting plans some energy into fast days (400–800 kcal in one trial), so the answer depends on the protocol.
• The popular 50-calorie rule is not supported by any trial cited here.

What did the trials allow?

Start with the fast you are actually doing. In a daily eating window, the trials cited here allowed no energy outside the window. Modified alternate-day fasting is a different rule: the fast day includes a planned amount of food.

In the 12-week TREAT trial, the time-restricted group was told to eat from noon to 8 p.m. and to completely abstain from caloric intake for the rest of the day.Source 11

A 12-month time-restricted eating trial allowed water and energy-free drinks such as black tea, coffee and diet sodas, with a limit of two diet sodas a day, during the 16-hour fast.Source 22

Is there one calorie limit?

People often say a fast survives anything under 50 calories, or under 10. No trial cited here tested an allowance like that, so those numbers are popular rules, not findings. Under the strict rule, any calories end the fast; in modified alternate-day fasting the fast day includes a planned amount of food, and the review describes 5:2 fast days simply as 24-hour fasting periods.

The network meta-analysis describes whole-day fasting such as 5:2 as two 24-hour fasting periods a week with unrestricted eating on the other five days; it does not state how many calories, if any, fast days allowed.Source 33

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 44

With a daily eating window, match the trials and stick to energy-free drinks. A splash of milk is a modified fast: decide that in advance and keep it the same each day.

See what you can drink while fasting, clean vs dirty fasting, and the guides to coffee, diet soda and sweeteners. Day-based formats are explained in 5:2 and alternate-day fasting.

What do the trials show?

Each trial cited here set its own rule for the fast. None compared a small calorie allowance with a strict fast, so these sources cannot say whether 5, 10 or 50 calories change the results.Source 11Source 22Source 33Source 44

When should you get advice?

If tracking every calorie during a fast starts to feel stressful, pause fasting and talk to a GP.

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

• What Can You Drink While Fasting?

  Water, black coffee and plain tea fit a zero-calorie fast; diet soda adds little or no energy. See what you can drink while fasting and what ends a fast.

• Dirty Fasting vs Clean Fasting: What's the Difference?

  Dirty fasting adds milk or broth; clean fasting means energy-free drinks. See what trials tested and why they set no calorie cutoff for a strict fast.

• Can You Drink Coffee While Fasting?

  Yes, if it's black: a 12-month trial allowed black coffee while fasting. See which additions end a fast, how much caffeine is too much and when to stop.

• Can You Drink Diet Soda While Fasting?

  Yes: diet soda has little or no energy, and a 12-month trial allowed up to two a day. See what sweeteners, caffeine and acidity add while fasting.

• Does Intermittent Fasting Cause Muscle Loss?

  Intermittent fasting isn't proven to cause or prevent muscle loss. What trials found on lean mass, why strength training matters and who should get advice first.

Frequently asked questions

How many calories break a fast?

It depends on the fast; no universal number is established. Under the strict rule of the time-restricted eating trials cited here, any calories break the fast, while water and energy-free drinks do not.Source 11Source 22

Do 50 calories break a fast?

Under a strict no-calorie rule, yes. The 50-calorie rule is a popular rule of thumb that no trial cited here tested, so it is a convention, not a finding.Source 11Source 22

Do zero-calorie drinks break a fast?

Not under the rules of the trials cited here: one 12-month trial allowed energy-free drinks, including up to two diet sodas a day. Their effect on insulin or other fasting markers was not tested.Source 22

How many calories can you have while fasting?

It depends on the format: none outside the window in time-restricted eating trials, two 24-hour fasting days a week in 5:2 as the review describes it, and 400–800 kcal on fast days in one modified alternate-day trial.Source 11Source 33Source 44

Sources

1. 1

   Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical TrialDylan A. Lowe et al. American Medical Association (AMA), November 1, 2020.DOI: 10.1001/jamainternmed.2020.4153 · PMID: 32986097Human randomized trial

2. 2

   Time-Restricted Eating Without Calorie Counting for Weight Loss in a Racially Diverse Population: A Randomized Controlled TrialShuhao Lin et al. American College of Physicians, July 1, 2023.DOI: 10.7326/M23-0052 · PMID: 37364268Human randomized trial

3. 3

   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

4. 4

   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

5. 5

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

6. 6

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