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Calories break a fast. Almost everything else people argue about comes down to how much, and what you are fasting for.
The clear answers first, then the five drinks that actually cause the arguments.
BREAKS A FAST
Food of any kind
Sugary drinks and juice
Milk, in coffee or anywhere else
Protein shakes
Smoothies
These carry enough energy to move you out of the fasted state. There is no version of this that is up for debate.
DOES NOT BREAK A FAST
Water, still or sparkling
Plain black coffee
Plain tea: green, black, herbal
Salt
Electrolytes without calories
Black coffee and green tea also make a long fast noticeably easier, which is why they are standard on longer fasts.
The judgment calls
A splash of milk is not a meal, and a stick of gum is not lunch. Whether either matters depends on why you are fasting.
Milk has calories, so a latte ends the fast. A single splash in black coffee is a smaller thing, but it is no longer a clean fast. Drink it black if the answer matters to you.
No calories, so by the strict definition it does not break a fast. Whether sweeteners affect appetite or insulin is genuinely contested. Sparkling water does the same job without the argument.
A piece of sugar-free gum carries a couple of calories at most. For a 16 hour fast that is nothing. For a fast you are running specifically for autophagy, keep it out.
Amino acids are the clearest signal you can send that food has arrived. If you want them around a workout, put them inside your eating window.
Bulletproof coffee is a few hundred calories. It may keep you in ketosis, but it is not a fast. Two different goals, often confused for one.
If you are fasting for weight and routine, small non-caloric additions are not worth the anxiety. If you are fasting long for the deeper stages, keep it to water, salt, black coffee and plain tea.
Why it depends
What matters is total energy across the day and whether you can keep the pattern going. Ten calories of gum will not decide anything. Quitting in week two will.
Past 24 hours the point is an uninterrupted stretch without food. Here the strict definition is the useful one, because anything you add resets the thing you were waiting for.
Coffee, tea, and sparkling water during a fast sits in Burn's library alongside fourteen other articles, all free.
More guides →
That calories end a fasted state is well established. The effect of non-caloric sweeteners on appetite and insulin is an active research area, and the claims above are drawn from peer-reviewed work and traced in our evidence packet.
Burn times the fast you tell it you are running and names the stage the clock has reached. It does not detect what you drank, and it does not measure ketosis, autophagy, or metabolic state.
Burn times the fast, names every stage you reach, and keeps the history so you can see which weeks actually held.
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