The pragmatic definition: a fast is broken by anything with meaningful calories or insulin response. Black coffee, water, plain tea, and zero-cal sparkling water fit under that bar. Everything else is a judgment call.
Fine during a fast
- Water — of course. Sparkling water too.
- Black coffee — 2–5 calories per cup. Doesn't meaningfully affect insulin or fat oxidation.
- Plain tea (black, green, herbal) — negligible calories.
- Electrolyte powder without sugar — unflavored, sugar-free electrolytes (plain salt, potassium, magnesium) don't break a fast.
Breaks a fast
- Cream or milk in coffee — even a tablespoon. 50+ calories, fat triggers digestion.
- MCT oil or butter in coffee — calories are calories. The "bulletproof" fast is a different protocol, not a true fast.
- Bone broth — protein and calories. Fine for fasting-mimicking diets; breaks a true fast.
- BCAAs or protein-containing supplements — amino acids trigger mTOR and insulin.
- Gum with sugar — small amounts of sugar and sweeteners can trigger response.
The gray zone
- Sugar-free gum / zero-cal sweeteners — technically 0 calories, but some studies suggest sucralose and saccharin can provoke minor insulin responses. For strict metabolic fasts, skip. For calorie-deficit-focused IF, fine.
- Diet soda — same logic as sweeteners.
- Lemon water — a slice of lemon is negligible. A tablespoon of juice is more ambiguous.
- Apple cider vinegar — small amounts (1 tsp) are generally considered OK.
The practical test
Ask: does this have meaningful calories, protein, or fat? If yes, it breaks a fast. Black coffee passes because the calories are truly negligible and it doesn't trigger digestion. Cream fails because it does both.
// TRY THE TOOL
PLAN YOUR WINDOW.
Pick 16:8, 18:6, 20:4, or OMAD. Visualize fasting vs feeding hours on a 24-hour timeline.
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