What can you drink while fasting — a drink-by-drink guide

Coffee? Tea? Diet soda? The rules are simpler than the internet makes them look — and a few clear biological thresholds tell you where each drink lands. Not medical advice.

Every new faster eventually asks some version of "wait, does coffee break a fast?" The answer depends less on the drink than on which mechanism of fasting you are trying to preserve. If you want the reasoning in full, the decision-tree article lays out the three thresholds — caloric balance, insulin suppression, and mTOR / autophagy suppression. This article is the practical companion: one drink at a time, a direct yes / no / grey-area answer, and the biology in a sentence.

A minimum-viable definition: if a drink is calorie-free (or close), sugar-free, and protein-free, it passes. Milk, juice, alcohol, and anything sweetened with sugar or protein powder fail. A handful of grey-area drinks — coffee with a splash of cream, diet soda, kombucha — depend on which goal you are optimising for. The table at the bottom of this article summarises every case; the sections in between explain why.

Does water break a fast?

No. Plain water carries no calories, produces no insulin response, and contains no amino acids. Drink it freely during any fast. Expect to drink more than usual because you miss the water that normally arrives with food — for extended fasts, 2–3 L per day is typical.

Does black coffee break a fast?

No. Black coffee has 2–5 calories per cup and does not meaningfully raise insulin in the fasted state. Caffeine may modestly support lipolysis and has been implicated in mild autophagy induction in cell models. Across all three fasting mechanisms — caloric, insulin, autophagy — black coffee is the cleanest coffee option.

Does plain tea break a fast?

No. Unsweetened green, black, white, oolong, and herbal teas are all fine. Like coffee they carry fewer than five calories per cup and do not raise insulin. Matcha is a partial exception — the whole leaf is in suspension, so it is closer to 5–10 calories per cup — still trivial for any daily fasting goal.

Does coffee with cream break a fast?

Grey. A teaspoon of heavy cream (~15 calories, ~0.1 g protein) has essentially no insulin effect and is compatible with metabolic fasting. For strict autophagy, it is not strictly zero — any amount of protein activates mTOR to a small degree. The common rule: fine for daily 16:8; not fine for extended autophagy-focused fasts.

Does diet soda break a fast?

Grey. Diet sodas have zero calories and do not raise blood glucose in most people. There is moderate evidence that artificial sweeteners produce a cephalic-phase insulin response in some individuals — a small insulin rise triggered by the sweet taste alone. The practical test: if diet soda makes you hungrier, you have your answer. For most daily fasters, a serving is fine.

Does milk — dairy or plant — break a fast?

Yes. Unsweetened cow's milk carries roughly 5–8 g of lactose per 100 ml, which triggers an insulin response; skim or whole makes little difference at this threshold. Plant milks vary. Unsweetened almond milk has almost no carbs or protein; most commercial oat, rice, and sweetened plant milks do. Read the label — if it has more than 1–2 g of carbs, it breaks the fast.

Does bone broth break a fast?

Yes. Bone broth averages 30–50 calories and 6–10 g of protein per cup, largely collagen and branched-chain amino acids. The amino acids activate mTOR (breaking autophagy) and the protein triggers a small insulin response. Bone broth is a food. It belongs in your eating window, not in your fasting window.

Does alcohol break a fast?

Yes. Alcohol carries 7 calories per gram, takes metabolic priority over fat oxidation, and most drinks carry sugar or mixers on top. A drink during a fast functionally ends the fast in both the caloric and the metabolic sense. A drink on an empty stomach also reaches peak blood alcohol faster, which is worth knowing regardless of the fasting question.

Can I have electrolytes during a fast?

Yes — and past 24 hours they are essential. Sodium loss accelerates once insulin drops (covered in the electrolytes article). Unsweetened electrolyte powders with sodium, potassium, and magnesium do not break a fast. Avoid anything with added sugar, protein, or sweeteners. Plain salt in water works too, cheaper.

Does lemon water or apple cider vinegar break a fast?

No, in normal amounts. A squeeze of lemon or 1–2 tablespoons of apple cider vinegar has negligible calories and does not trigger insulin. Apple cider vinegar before meals has some evidence of improving glucose response to the next meal, though the effect size is small.

Does juice break a fast?

Yes. Fruit juice is essentially sugar water without fibre — 20–30 g of carbohydrate per cup, an immediate insulin spike. Even "100% fruit juice" with no added sugar fails. Vegetable juices are better but still carry 10–15 g of carbs per cup; they also break an insulin-preservation fast.

Does kombucha break a fast?

Usually yes. Commercial kombucha averages 20–50 calories and 2–8 g of residual sugar per 8 oz — enough for a small insulin response. Some low-sugar varieties (under 2 g) sit at the edge of the grey area. If you can find a version below 10 calories per serving, it behaves like diet soda above.

Does chewing gum break a fast?

Grey. Most sugar-free gum has a few calories from sugar alcohols (polyols) and produces a mild cephalic insulin response from the sweet taste. Trivial for caloric balance, probably fine for daily intermittent fasting, probably worth avoiding for autophagy-focused extended fasts. One or two pieces a day matters to almost no one.

What about pre-workout supplements?

Check the label. BCAAs (branched-chain amino acids) are designed to activate mTOR and will break an autophagy fast; they also trigger some insulin. Caffeine alone is fine. Creatine is typically fine during fasting — it does not spike insulin and carries no protein. Any pre-workout listing sugar, sucralose, or BCAAs on the front fails.

The quick reference

Drink	Breaks a fast?	Why
Water	No	Zero calories, zero insulin
Black coffee	No	<5 cal/cup, no insulin response
Plain tea (any type)	No	<5 cal/cup
Coffee + tsp cream	Grey	Fine for daily IF; not for autophagy
Diet soda	Grey	No cal, possible taste-driven insulin
Unsweetened almond milk (<1 g carb)	No*	*Only if the label is clean
Milk — dairy or most plant	Yes	Lactose / added sugar + protein
Bone broth	Yes	It's a food — calories + amino acids
Juice (fruit or veg)	Yes	Sugar / insulin spike
Alcohol	Yes	Calories + metabolic priority
Kombucha	Yes (most)	Residual sugar
Electrolytes (unsweetened)	No	Essential past 24 h
Lemon water	No	Negligible calories
Apple cider vinegar	No	Negligible calories
Sugar-free gum	Grey	Trivial but cephalic
Pre-workout with BCAAs	Yes	BCAAs spike mTOR
Pre-workout, caffeine only	No	No insulin, no amino acids

The pragmatic rule

If a drink is a clear yes on this list, enjoy it. If a clear no, skip it. For the grey area — coffee with cream, diet soda, sugar-free gum — decide based on your goal. Daily metabolic 16:8 forgives most grey-area choices. Extended autophagy fasts do not. When in doubt, err clean. The discipline of the defined window is part of the benefit; protecting the edges is easier than litigating them.

Sources

1. Antoni R, Johnston KL, Collins AL, Robertson MD. "Effects of intermittent fasting on glucose and lipid metabolism." Proc Nutr Soc, 2017. doi:10.1017/S0029665116002986
2. Anton SD, Moehl K, Donahoo WT, et al. "Flipping the metabolic switch: Understanding and applying the health benefits of fasting." Obesity (Silver Spring), 2018. doi:10.1002/oby.22065
3. Cahill GF Jr. "Fuel metabolism in starvation." Annu Rev Nutr, 2006. doi:10.1146/annurev.nutr.26.061505.111258
4. Suez J, Korem T, Zeevi D, et al. "Artificial sweeteners induce glucose intolerance by altering the gut microbiota." Nature, 2014. doi:10.1038/nature13793
5. Related reading: the decision-tree framework and the electrolytes guide.