The honest answer is that it depends what you are fasting for, and almost nobody says that before listing foods.
If your goal is stable insulin and fat metabolism, the threshold is fairly forgiving. If your goal is autophagy, the threshold is stricter and the evidence is thinner. If you are fasting for a blood test, ask the clinic — their rules are the only ones that count.
What follows assumes the common case: a 16:8 or 18:6 fast for metabolic reasons.
The short list
| Item | Breaks the fast? |
|---|---|
| Water, still or sparkling | No |
| Black coffee | No |
| Plain tea | No |
| Coffee with a splash of milk | Technically yes, practically minor |
| Coffee with sugar or syrup | Yes |
| Diet soda | Probably not, but see below |
| Chewing gum (sugar-free) | Almost certainly not |
| Electrolytes, unflavoured | No |
| Bone broth | Yes |
| Toothpaste | No |
| Most medication | No — and take it anyway |
The ones people argue about
Coffee with a splash of milk. A tablespoon of milk is roughly ten calories. It will not meaningfully move insulin and it will not stop fat oxidation. It is not zero, so if you are chasing autophagy at hour eighteen, it is a compromise. For everyone else, if it is the difference between fasting and not fasting, have the splash.
Diet soda. No calories, so metabolically it should be neutral, and in most measurements it is. The complication is that some people get a real appetite response to sweetness with no food behind it. If diet soda makes hour fifteen harder, it broke your fast whatever the calorie count says.
Sugar-free gum. Two or three calories of polyol. Not a metabolic event. Same caveat as above about triggering appetite.
Bone broth. This one gets waved through constantly and it should not be. It contains protein — that is the point of it. Protein raises insulin. Bone broth is a small meal, not a fasting aid.
Apple cider vinegar. Negligible calories, does not break a fast. Also does not do most of the things claimed for it.
Medication
Take your medication. A fasting schedule is a lifestyle preference; your prescription is not. If a medication must be taken with food, that requirement wins and you should build your window around it — or accept that fasting is not compatible with your treatment right now. Ask your prescriber rather than an app.
The rule that actually helps
If you are unsure about something, ask what it is doing there. Water, salt and caffeine are there to make the fast survivable. Cream, honey and broth are there because you want to eat. That distinction gets you the right answer more often than any calorie threshold.
A note on precision
You will find articles giving exact numbers — fifty calories breaks a fast, autophagy stops at exactly this dose. Those numbers are far more confident than the research behind them. The metabolic transitions here are gradients, not switches, and they vary between people and between days.
Track what you did, notice how the fast went, and let your own record decide. That is what the journal in Fastrackie is for.