Caffeine

Caffeine does not break a fast. On its own, and in black coffee or plain tea, caffeine contains essentially no calories and does not provoke a meaningful insulin response, so it leaves the fasted state intact. What breaks a fast is what often accompanies caffeinated beverages: milk, cream, sugar, or sweetened syrups, which all bring calories and end a fast.

Black coffee may even align with some of the goals of fasting rather than working against them. In mice, both caffeinated and decaffeinated coffee have been shown to trigger autophagy, the cell's recycling process, by lowering mTOR activity, an effect similar to what fasting itself produces. This has not been confirmed in fasting humans, but it is part of why plain coffee is generally treated as fasting-friendly rather than something to avoid.

This makes plain coffee and tea staples of the fasting window for many people, useful for energy and appetite control during a fast. The dividing line stays simple: black and unsweetened keeps the fast, while anything added for flavor or richness, even a splash of milk, moves you into broken-fast territory. The dependable zero-calorie options during a fast remain water, black coffee, plain tea, electrolytes, and certain non-caloric supplements.

Source: Pietrocola et al., Coffee Induces Autophagy in Vivo, Cell Cycle (2014)