Does sparkling water break a fast?
Plain sparkling water does not break a fast. Carbonated water is water with dissolved carbon dioxide and contains no calories, carbohydrate or protein. Flavoured sparkling waters are usually also fine, but "flavoured" and "sweetened" are different words and the label is the only way to tell them apart.
| Fasting for | Verdict | Why |
|---|---|---|
| Weight loss | Doesn't break it | Zero calories in plain carbonated water. |
| Autophagy | Doesn't break it | No protein, no energy, no nutrient signal. |
| Blood work | Depends | Many clinics say plain water specifically. Ask rather than assume — the cost of being wrong is a repeated test. |
| Ketosis | Doesn't break it | Plain carbonated water contains no carbohydrate and does not interrupt ketosis. Tonic water is not sparkling water and carries roughly 32 g of sugar per 330 ml. |
| Gut rest | Depends | Carbonation increases gastric fullness and can worsen bloating and reflux. It genuinely is not the same as still water here. |
Why
Dissolved CO₂ adds no energy, so plain sparkling water is metabolically identical to still water. It is not physiologically identical, though, and the gut-rest verdict reflects measured effects rather than caution. Carbonated water increases sensations of gastric fullness, and controlled work in young women found measurable effects on gastric activity and fullness after drinking it. A separate trial in people with functional dyspepsia and constipation reported symptom improvement, so the direction of the effect depends on who is drinking it. Either way it is doing something to the stomach, which is the definition of not being digestive rest.
0 kcal for plain carbonated water.
The exceptions
This is usually where the real question lives.
- Tonic water is not sparkling water. It contains roughly 32 g of sugar per 330 ml and breaks a fast.
- Naturally flavoured seltzers are typically zero-calorie, but some brands add juice or sugar.
- "Sparkling mineral water" with added sodium is fine for fasting and may even help on a long one.
- Hard seltzer is alcohol. See that entry.
Sources
- Effects of carbonated water on functional dyspepsia and constipation — Eur J Gastroenterol Hepatol, 2002PMID 12352219
- The effects of carbonated water upon gastric and cardiac activities and fullness in healthy young women — J Nutr Sci Vitaminol (Tokyo), 2012PMID 23327968
No clinician reviews this site. Sources on this entry were verified by the author, who is not a doctor or a dietitian — what that means.
Last updated 2026-08-15.
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