Fasting Guides / Getting Started
What Actually Breaks a Fast?
The definitive guide to what you can and cannot consume during a fast — based on what actually triggers an insulin response.
The Core Principle
A fast is broken when something causes a meaningful rise in insulin. Insulin signals the body to stop burning stored fat and start processing incoming fuel. Anything that significantly raises insulin — primarily carbohydrates and protein — breaks the metabolic state of fasting.
Definitely Fine
Plain water is always fine. Black coffee and plain tea (without milk, sugar, or cream) contain negligible calories and do not meaningfully raise insulin for most people. Many studies show coffee may actually enhance fat oxidation and autophagy. Sparkling water without any flavorings or sweeteners is also fine.
Grey Areas
A small amount of heavy cream (1–2 tablespoons) in coffee raises insulin minimally for most people — many strict fasters avoid it, while others accept the trade-off. The same applies to MCT oil and butter in coffee (popularized by Bulletproof coffee). These fat-only additions stay close to zero carbohydrates and protein, so the insulin response is small. Whether this technically breaks your fast depends on your goals.
What Definitely Breaks It
Any food breaks a fast. This includes small snacks, protein shakes, gum with calories, juice, milk, sports drinks, and any beverage with sugar or significant calories. Even diet soda, while containing zero calories, is controversial — the sweet taste may trigger a cephalic insulin response in some people and could increase cravings.
A Practical Framework
If your goal is weight loss and metabolic health, plain water, black coffee, and plain tea are your safest options. If you are strictly pursuing autophagy, some researchers suggest even small amounts of amino acids (from protein) may blunt autophagy, so water-only fasting is preferred for those goals. Most people doing IF for general health and weight management will see good results with black coffee and tea included.