What breaks a fast (and what doesn't)
Almost every intermittent fasting question is the same question wearing a different outfit: “does this break my fast?” The short answer is that anything with enough calories to raise your insulin breaks it. Water, black coffee, tea and unsweetened herbal teas don't. Milk, juice, beer, wine and sweetened drinks do.
This guide covers it in order: what to drink during your fasting window, the cases people ask about most (coffee, alcohol, sweeteners), how to head off dizziness with electrolytes and, when the time comes, how to break your fast without undoing the work.
The essentials in 5 points
- Water, sparkling water, black coffee, tea and unsweetened herbal teas: none of these break your fast.
- Coffee with milk, juice, “light” drinks with sugar in them, broths with fat: these do break it.
- Alcohol always breaks a fast: a 12 oz beer runs about 140 calories and stalls fat burning while your body processes it.
- On fasts longer than 16-18 hours, a pinch of salt or electrolytes keeps headaches and lightheadedness away without breaking the fast.
- To break it well: protein and fiber first, sugar and ultra-processed food later (or never).
Reading path
The articles on this topic in the order that makes sense. Each one answers a specific question.
During intermittent fasting you can have water (still or sparkling), black coffee, and tea or herbal teas with no sugar or milk: they bring almost no calories and do not interrupt the fast. Juice, sugary soda, milk, shakes and alcohol break it. Zero-calorie sweeteners are a gray zone: in some people they alter the insulin response.
Read · 5 min →Black coffee does not break your fast: it has almost no calories, and no trials show it cancels out the effects of fasting. Coffee with milk or sugar does break it, because it adds calories and stimulates insulin. Caffeine slightly raises energy expenditure and fat oxidation, and can reduce appetite short term. Direct evidence on coffee and fasting is scarce.
Read · 5 min →Yes: beer, wine and any alcoholic drink break your fast, because ethanol brings calories and many drinks add sugar. On top of that, while your body metabolizes alcohol, fat oxidation drops sharply and appetite goes up. If you drink, do it inside your eating window and in moderation.
Read · 5 min →Fasting lowers insulin and your kidneys flush out more sodium and water (the natriuresis of fasting), which explains much of the headache, dizziness and fatigue of the first hours and days. Sodium, potassium and magnesium without sugar have no calories and do not break your fast; on fasts over 18-24 hours, or with heavy sweating, a pinch of salt in your water is usually enough.
Read · 5 min →To break a fast, go for a moderate meal that starts with protein, vegetables and healthy fat, leaving carbs for last: that order lowers the glucose and insulin spike that follows. After long fasts (more than 24-48 hours), reintroduce food gradually to avoid digestive discomfort and refeeding syndrome.
Read · 6 min →What to eat in your 8-hour window to stay full, cover your protein, and not break your fast early. A real example.
Read · 6 min →What the science says
- Sucralose taken before a glucose load raised the insulin area by 20% and the glucose peak in 17 people with obesity (Pepino 2013). [1]
- 100 mg of caffeine raised resting metabolism 3-4%; repeated doses over 12 h raised daily expenditure by 150 kcal in lean people (Dulloo 1989). [2]
- After 24 g of alcohol, whole-body fat oxidation fell 73% and fatty acid release from fat tissue fell 53% in healthy men (Siler 1999). [3]
- Insulin drives sodium reabsorption in the kidney; its fall explains the natriuresis of fasting and sodium retention on refeeding (DeFronzo 1981). [4]
- Eating carbs last cut the glucose peak by 54% and the area under the curve by 53% versus eating them first (Shukla 2017). [5]
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Frequently asked questions
Does a splash of milk in my coffee break my fast?
Do sweeteners break a fast?
What happens if I break my fast with something sweet?
Can I drink broth while fasting?
Sources cited on this page (5)
- Sucralose Affects Glycemic and Hormonal Responses to an Oral Glucose Load. Diabetes Care, 2013.
- Normal caffeine consumption: influence on thermogenesis and daily energy expenditure in lean and postobese human volunteers. The American Journal of Clinical Nutrition, 1989.
- De novo lipogenesis, lipid kinetics, and whole-body lipid balances in humans after acute alcohol consumption. The American Journal of Clinical Nutrition, 1999.
- The effect of insulin on renal sodium metabolism. Diabetologia, 1981.
- Carbohydrate-last meal pattern lowers postprandial glucose and insulin excursions in type 2 diabetes. BMJ Open Diabetes Research & Care, 2017.