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

In 30 seconds

Clean fasting keeps the fasting window free of energy: water, black coffee and plain tea. Dirty fasting is an informal name for allowing additions such as milk or broth, so it is a modified fast. The trials cited here used an energy-free rule and set no calorie amount that keeps a strict fast intact.

• Clean fasting keeps the fasting window free of energy: water, black coffee and plain tea.
• Dirty fasting is an informal term, not a research protocol, for allowing small amounts of energy.
• The trials cited here defined the fast by energy; one allowed up to two diet sodas a day.
• None of the sources cited here sets a calorie number below which a fast stays intact.
• Whichever rule you choose, keeping it the same each day makes your records comparable.

What counts as clean fasting?

During a clean fast, stick to drinks without energy: water, black coffee and tea brewed with water. The trials cited here used this energy-based rule, even though they did not use the word clean.

In the 12-week TREAT trial, the time-restricted group was told to eat from 12:00 to 8:00 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

What counts as dirty fasting?

Dirty fasting has no agreed definition. People use it for small additions during the fasting window, such as milk or cream in coffee, butter or oil in a drink, or broth. These add energy, so this is a modified fast rather than the strict fast used in the cited trials.

NHS guidance notes that the fat content of dairy products varies widely and that cream is high in fat.Source 33

Which fasting rule should you use?

If you want milk in your coffee, you can choose a modified rule. Decide before the fast, record what you actually had and use the same rule when comparing your own records. None of the trials cited here sets a calorie allowance that makes an energy-containing addition part of a strict fast.

For the full list of energy-free drinks, see what you can drink while fasting. Two common additions have their own guides: milk and cream in coffee and bone broth. To compare fasting schedules rather than fasting rules, see types of intermittent fasting.

What did the trials actually test?

Neither trial compared clean and dirty fasting or set a calorie threshold for additions. TREAT asked for no caloric intake outside its noon-to-8 p.m. window, and a 12-month trial allowed only water and energy-free drinks. These sources cannot tell us how a splash of milk or cup of broth changes outcomes.Source 11Source 22

When do fasting rules need care?

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.

• How Many Calories Break a Fast? It Depends

  How many calories break a fast depends on the protocol: none in a strict eating window, and 400–800 kcal on fast days in one alternate-day trial.

• 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.

• Does Milk or Creamer in Coffee Break a Fast?

  Yes, on a strict fast: milk, cream and creamers all add energy to coffee. See your options if you want to keep a coffee ritual during a fast.

• Does Intermittent Fasting Affect Sleep?

  Studies of intermittent fasting and sleep find small or no effects in controlled trials. How meal timing, caffeine and habits still shape your night.

Frequently asked questions

What is dirty fasting?

It is an informal name for allowing small amounts of energy during a fasting window, such as milk in coffee or broth. The cited trials used a stricter rule: no caloric intake, with water and energy-free drinks allowed.Source 11Source 22

Does a splash of milk ruin a clean fast?

By the energy rule, it ends a clean fast: milk contains fat, protein and milk sugar, so even a splash adds energy. Whether that matters to you is your choice; if you allow it, do it consistently.Source 33

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

   Dairy and alternatives in your dietNHS. NHS, December 11, 2023.Official guidance

4. 4

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

5. 5

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