First, decide which fast you are running
Almost every argument about whether coffee or sweeteners break a fast comes from people using the word to mean different things. There are three common goals, and they have genuinely different thresholds.
- Weight management. What matters is calories. If it has essentially none, it does not break your fast in any way you care about.
- Blood-sugar and insulin management. What matters is the insulin response. Some zero-calorie things still provoke one; protein does so without much of a glucose rise.
- A medical fast before a test or procedure. What matters is what your clinician told you. Follow those instructions exactly and ignore everything on this page.
For the overwhelming majority of people, the first definition is the relevant one. If you are fasting to manage weight or to simplify your eating, "does it have meaningful calories?" answers nearly every question below.
The common questions, answered
Black coffee
Does not break a fast on any practical definition. A cup has around two calories. It may even blunt appetite for a while, which is why it is the near-universal companion to a morning fast. Decaf is equally fine.
Tea, plain
Same answer. Black, green, white, oolong, herbal — all effectively calorie-free without additions.
A splash of milk
This is the one people most want a loophole on. A tablespoon of milk is roughly nine calories and contains a little sugar and protein. Strictly, yes, it breaks a fast. Practically, if a small splash of milk in one morning coffee is the difference between doing this and not doing it, take the milk. The compliance you buy is worth more than nine calories.
A latte is not a splash of milk. That is a small meal, and it does break the fast on any definition.
Artificial sweeteners
Calorie-free ones — sucralose, aspartame, stevia, monk fruit — do not break a fast in the weight sense. There is ongoing and genuinely unsettled research about whether some produce a small insulin response or affect appetite and gut bacteria. The evidence is mixed and the effects, where found, are small.
Reasonable position: fine if they help you get through the morning, worth experimenting without if you find sweet tastes make you hungrier. Some people clearly experience that; many don't.
Diet fizzy drinks
Same reasoning as sweeteners. No meaningful calories. Watch for the carbonation bothering an empty stomach.
Chewing gum
Sugar-free gum is two to five calories a piece. Negligible. Some people find it suppresses hunger; others find chewing makes them hungrier. Test it on yourself.
Bone broth
Yes, this breaks a fast — it contains real protein and calories. It is often recommended during longer fasts precisely because it provides something. That is a deliberate compromise, not a technicality.
Apple cider vinegar
A tablespoon is about three calories, so it does not break a fast. It is also not doing the things it is marketed as doing. The evidence for meaningful metabolic benefit is thin. Harmless if you like it; not worth building a routine around, and hard on tooth enamel undiluted.
Supplements and vitamins
Most tablets and capsules contain trivial calories and do not break a fast. Three practical notes: fat-soluble vitamins (A, D, E, K) are absorbed considerably better with food, so take them inside your window; gummy vitamins contain real sugar; and several supplements — iron and magnesium especially — cause nausea on an empty stomach.
Medication
Take your medication as prescribed. Never delay or skip a dose to protect a fasting window. If a medication must be taken with food and that conflicts with your schedule, that is a conversation with your pharmacist or doctor about adjusting the schedule — the fast is the flexible part, not the prescription.
Toothpaste and mouthwash
Not a fast-breaker. You swallow a negligible amount. Brush your teeth.
Sparkling and mineral water
Fine. Flavoured sparkling water is fine too provided it has no added sugar — check, because some "flavoured waters" contain a surprising amount.
A rough threshold
If you want one rule of thumb rather than a list: under about ten to fifteen calories, and it is not going to matter for weight-focused fasting. Above roughly fifty, you have eaten something.
The bigger risk is not any single item on this list. It is the accumulation — a splash of milk in four coffees, a few pieces of gum, a spoonful of something while cooking. Individually negligible, collectively a small meal.
If you are fasting before blood work or a procedure, follow your clinician's instructions precisely rather than anything here. Some tests require water only; some prohibit even black coffee, because it can affect certain results. Ask, and be specific about what you intend to drink.
What you are protecting
It is worth knowing why the window is worth defending at all, because it makes every decision on this page obvious.
The switch you are waiting for
Once the food stops arriving, the liver works through its stored glycogen and then starts converting fat into ketones — described in the New England Journal of Medicine as a metabolic switch from glucose-based to ketone-based energy.1 That is the event your fasting hours are buying. Anything with real calories in it restarts the clock and pushes the switch further away.
Intermittent fasting can trigger a “metabolic switch from glucose-based to ketone-based energy.”
de Cabo & Mattson, New England Journal of Medicine, 20191
By eighteen hours, the cells are cleaning house
Hold the window and it goes further than fuel. In an 8am–2pm eating pattern, researchers measured raised morning ketones together with increased expression of LC3A, a gene involved in autophagy — the process by which cells break down and recycle their own damaged components — and SIRT1, associated with stress resistance and ageing.2
That is the thing a milky coffee costs you. Not nine calories — an hour or two of the part that makes fasting different from dieting.
And it is not just about eating less
The cleanest proof that the window itself matters comes from a trial that fed men enough to hold their body weight perfectly constant. No weight was lost, and the fasting schedule still improved insulin sensitivity, beta-cell responsiveness, blood pressure and oxidative stress.3 Keep the window clean and you get that whether or not the scale moves.
On sweeteners, briefly
A 2022 trial gave 120 adults saccharin, sucralose, aspartame or stevia for two weeks. All four shifted the gut microbiome, and saccharin and sucralose measurably impaired glucose tolerance — though the effect tracked each person's own microbiome rather than applying to everyone.4 If you are going to the trouble of holding a sixteen-hour window, the simplest move is black coffee, plain tea and water. That is what the trials above used.
References
- de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541–2551. doi:10.1056/NEJMra1905136
- Jamshed H, Beyl RA, Della Manna DL, Yang ES, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients. 2019;11(6):1234. doi:10.3390/nu11061234
- Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212–1221.e3. doi:10.1016/j.cmet.2018.04.010
- Suez J, Cohen Y, Valdés-Mas R, et al. Personalized microbiome-driven effects of non-nutritive sweeteners on human glucose tolerance. Cell. 2022;185(18):3307–3328.e19. doi:10.1016/j.cell.2022.07.016