Can You Drink Diet Soda While Fasting?

In 30 seconds

Yes. Diet and zero-sugar sodas add little or no energy, so they don’t break a fast defined by calories, and a 12-month trial allowed up to two a day during the fast. The catches lie elsewhere: WHO advises against relying on sweeteners for weight control, colas contain caffeine and fizzy drinks can erode teeth.

• A 12-month fasting trial allowed up to two diet sodas a day during the fasting window.
• Regular soda contains sugar and ends the fast; plain or unsweetened sparkling water doesn't.
• WHO advises against using non-sugar sweeteners as a way to control weight.
• A can of cola has about 40 mg of caffeine, and the acidity of fizzy drinks can erode teeth.

Does diet soda break a fast?

No, not by the definition the fasting trials behind this guide use. The fast means no caloric intake, and diet or zero-sugar sodas provide little or no energy. At least one long trial allowed them outright, with a cap.

In a 12-month randomized trial of eating only between noon and 8 p.m., participants could drink energy-free drinks such as black tea, coffee and diet sodas during the 16-hour fast, with a limit of two diet sodas a day.Source 11

Is diet soda a good daily habit?

That’s a separate question. Fitting your fast is about energy; being a good daily habit is about sweeteners, caffeine and your teeth.

A 2023 WHO guideline advises against using non-sugar sweeteners as a means of weight control or of reducing the risk of noncommunicable diseases; it is not a toxicological safety assessment.Source 44

NHS guidance states that approved sweeteners are considered safe within acceptable daily intakes, describes the research on sweeteners and health as mixed, and notes that carbonated drinks are associated with dental erosion whether or not they contain sugar.Source 33

NHS estimates about 40 mg of caffeine in a can of cola and about 80 mg in a 250 ml can of energy drink.Source 66

A long fasting trial treated diet soda as an energy-free drink, capped at two a day, while WHO advises against relying on non-sugar sweeteners for weight control and the NHS calls the wider health evidence mixed. In short: a diet soda won't end your fast, but water, black coffee and tea make a better default.Source 11Source 44Source 33

Who should be careful?

Frequently asked questions

For caffeine totals across coffee, tea and cola, see coffee during intermittent fasting; for plain and flavoured water, see water and lemon water.

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.

• 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 Water (and Lemon Water) While Fasting?

  Plain and sparkling water never break a fast, and a long fasting trial encouraged it. See where lemon, juice and coconut water fit and how much to drink.

• 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.

• Fasting-Mimicking Diet: What a 3-Month Trial Found

  The fasting-mimicking diet means 5 low-calorie days a month. A 3-month trial with commercial ties found lower weight and blood pressure. Its limits and who should avoid it.

Frequently asked questions

Does Coke Zero or Diet Coke break a fast?

No, not by the energy-based definition used in fasting trials. Diet and zero-sugar colas add little or no energy, and one 12-month trial allowed up to two diet sodas a day during the 16-hour fast. They still contain caffeine, about 40 mg per can.Source 11

Is sparkling water OK while fasting?

Yes. Plain sparkling water has no energy, and NHS guidance suggests it for people who find still water hard to drink. Check flavoured versions for sugar or juice.Source 55

Do sweeteners help or hurt weight loss while fasting?

WHO advises against using non-sugar sweeteners as a weight-control tool, and NHS guidance describes the wider evidence as mixed. They can help replace sugary drinks, but they are not a fasting aid.Source 44Source 33

Sources

1. 1

   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

2. 2

   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

3. 3

   The truth about sweetenersNHS. NHS, February 20, 2023.Official guidance

4. 4

   Use of non-sugar sweeteners: WHO guidelineWorld Health Organization. World Health Organization, May 15, 2023.Official guidance

5. 5

   Water, drinks and hydrationNHS. NHS, May 17, 2023.Official guidance

6. 6

   Foods to avoid in pregnancyNHS. NHS, June 15, 2026.Official guidance

7. 7

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