When practicing intermittent fasting, the central metabolic goal is to maintain a state where the body is not actively processing food, keeping insulin levels low. This low-insulin environment encourages the body to switch from burning glucose to burning stored body fat, a process that can lead to ketosis. Restricting nutrient intake also promotes cellular cleanup, known as autophagy, where the body recycles damaged cell components. To sustain this metabolic state, liquids consumed during the fasting window must be carefully considered to avoid signaling that food has been introduced.
Safe Liquids: Maintaining the Fast
The safest drinks during a fasting window contain zero calories and zero macronutrients, ensuring the metabolic state remains undisturbed. Plain water, whether still or sparkling, is always acceptable and serves the important function of maintaining hydration. These liquids do not trigger a digestive or insulin response, which is the primary concern for breaking a fast.
Black coffee is generally considered safe because a standard cup contains only about 3 to 5 calories and negligible amounts of fat, protein, or sugar. This minimal caloric load does not stimulate an insulin spike or interrupt the shift into fat-burning. Similarly, unflavored and unsweetened teas, including green, black, and herbal varieties, are safe options.
Immediate Fast Breakers
Any beverage containing a significant caloric load or macronutrients, especially carbohydrates and proteins, will immediately halt the fast by triggering a metabolic response. The presence of sugar (a carbohydrate) or protein causes the pancreas to release insulin. This insulin spike signals that the feeding period has begun, effectively stopping the fat-burning and autophagy processes.
Sweetened sodas, fruit juices—even fresh-squeezed varieties—and sports drinks are immediate fast breakers due to their high sugar content. Milk, whether dairy or plant-based alternatives like oat or soy milk, contains natural sugars or protein, making them unsuitable for the fasting window. Likewise, any coffee creamer, caloric coffee syrup, or alcoholic beverage will provide the body with calories and macronutrients that break the fast.
Protein powders and meal replacement shakes are designed to deliver protein and carbohydrates, which are strong insulin triggers. Even seemingly healthy options like smoothies, which blend fruits and vegetables, contain enough natural sugar to stimulate a full digestive response. Consuming any of these drinks shifts the body out of the low-insulin, fasted state and back into a fed state.
The Question of Artificial Sweeteners
Drinks containing non-nutritive sweeteners (NNS), such as diet sodas or flavored waters with sucralose, aspartame, stevia, or monk fruit, represent a “gray area” in fasting protocols. Since these substances provide sweetness with zero calories, they often do not affect blood glucose levels. The conventional view is that a zero-calorie drink is fast-safe because it does not provide the fuel needed to end the fat-burning state.
However, tasting something sweet, even without calories, can potentially activate the cephalic phase insulin response (CPIR) in some individuals. This preparatory reflex causes the body to release a small amount of insulin in anticipation of an incoming carbohydrate load. While research on the CPIR from NNS is mixed, this response could theoretically counteract the goal of maintaining minimal insulin levels.
Furthermore, some studies suggest that non-nutritive sweeteners may alter the composition of the gut microbiota, which can indirectly impact metabolic health. For the most stringent fasters, or those whose primary goal is to maximize autophagy, it is recommended to avoid all sources of sweetness, even zero-calorie ones, to eliminate any potential metabolic signal. Individuals should consider personal experimentation; if an artificially sweetened drink does not increase hunger or cravings, it may be acceptable, but the safest approach remains sticking to plain liquids.