What actually breaks a fast?

The honest answer is: it depends on why you’re fasting. For a clean rule of thumb, anything with calories breaks a fast — but the edge cases (black coffee, sweeteners, electrolytes) are where everyone gets stuck.

The simple rule

Zero calories = doesn’t break your fast. Water, black coffee, plain tea, and calorie-free electrolytes keep you fasted. The moment something delivers energy — sugar, protein, or fat — your body starts digesting and the fasted state pauses.

What’s safe

• Water — still or sparkling, plain.
• Black coffee — no milk, no sugar. The caffeine may even nudge fat metabolism slightly.
• Plain tea — green, black, or herbal, unsweetened.
• Electrolytes — sodium, potassium, magnesium with no calories (see electrolytes while fasting).

What breaks it

• Anything with sugar or cream — a splash of milk, a sugar cube, “just a little” juice.
• Protein and BCAAs — they trigger digestion and an insulin response.
• Gummy vitamins and chewable supplements — usually carry sugar.
• Most gum and mints — small carbs add up if you work through a pack.

The grey area: sweeteners

Artificial sweeteners are debated. They have roughly zero calories, but some may provoke an insulin response in some people. If your goal is weight loss, a diet soda is probably fine; if you’re chasing a clean metabolic phase or autophagy, skip them and stick to water.

It depends on your goal

• Weight loss / time-restricted eating — the calorie rule is what matters. Black coffee, fine.
• Resting blood sugar — avoid anything that spikes insulin, including some sweeteners.
• Autophagy — be strictest; even small protein hits may matter, and human timing is unsettled.

FastHQ doesn’t auto-detect any of this. You start and stop the fast yourself; the 16:8 timer just keeps honest time.

This article is informational and not medical advice. If you are diabetic, on blood-sugar medication, pregnant, or have a history of disordered eating, talk to a doctor before fasting.