Intermittent fasting (IF) involves cycling between periods of eating and voluntary abstention from food. This practice is widely adopted for potential health benefits, including weight management and metabolic improvements. A central point of confusion for many people starting IF is navigating the consumption of liquids during the fasting window. Understanding which beverages are acceptable and which ones can unintentionally halt the body’s shift to a fasted state is important for maximizing the benefits. The primary goal of fasting is to trigger a metabolic state change, allowing the body to transition from burning glucose for fuel to burning stored fat.
The Mechanism: What Constitutes Breaking a Fast
The fundamental rule of fasting revolves around controlling the body’s insulin response, a hormone released by the pancreas that regulates blood sugar. When you consume food or any caloric substance, insulin levels rise to move glucose into cells for energy or storage. A rise in insulin signals the “fed state,” which stops the metabolic switch to fat-burning and cellular repair processes.
To maintain the fasted state, the goal is to keep insulin levels as low and stable as possible. Zero calories is the ideal standard for what is known as a “clean fast.” Many practitioners follow a guideline of staying under a minimal caloric threshold, often cited as 50 calories, though this is a practical estimate. Even small amounts of carbohydrates or protein can trigger an insulin spike, halting the beneficial effects of fasting.
The source of calories matters significantly, as macronutrients elicit different insulin responses. Carbohydrates cause the most rapid and substantial insulin release, followed by protein, which causes a moderate response. Fat has the least impact on insulin levels. However, consuming any significant amount of calories will still disrupt processes like autophagy, a cellular clean-up mechanism highly sensitive to nutrient intake. Therefore, for those seeking maximum metabolic and cellular benefits, strict adherence to zero calories is recommended.
Permissible Liquids That Do Not Break a Fast
For a clean fast, the safest liquids contain no calories and do not trigger a metabolic response. Plain water, whether tap, filtered, sparkling, or mineral, is the most recommended liquid and should be consumed freely to maintain hydration. Since water is non-caloric and has no effect on blood glucose or insulin levels, it is universally permissible.
Black coffee is widely accepted because it contains a negligible number of calories and may support the fasting state by boosting metabolism. It must be consumed without cream, milk, sugar, or artificial sweeteners to avoid an insulin response. Plain teas, including green, black, or most herbal varieties, are also safe options. Ensure the tea contains no added fruit pieces, honey, or flavorings that could add hidden sugars or calories.
Electrolyte supplements are sometimes needed for longer fasts to help maintain mineral balance. They must be zero-calorie and unflavored, and any product containing sugar, dextrose, or fruit juice should be avoided completely. These permissible liquids help manage hunger and maintain hydration without interfering with the body’s shift into the fat-burning state.
Common Liquids That May Break a Fast
Several popular liquids and additives cause confusion because they appear low-calorie but contain compounds that can break a fast. Bone broth, a liquid rich in minerals, contains protein and fat, carrying a caloric load typically between 15 and 50 calories per cup. While technically breaking a clean fast due to its protein content, it is sometimes used during longer fasts to provide sodium and minerals.
Artificial sweeteners and diet sodas are controversial because they are zero-calorie but may still pose an issue. Some research suggests the sweet taste alone can trigger a cephalic phase insulin response, preparing the body for incoming sugar. Furthermore, the sweetness can increase cravings, making adherence to the fast more challenging.
Low-Calorie Additives
Common additives like lemon juice, lime juice, and apple cider vinegar (ACV) are technically very low in calories. A teaspoon of lemon juice contains only about one calorie. Small amounts, such as a splash of ACV or lemon in water, are generally tolerated by many fasters and are unlikely to cause a significant insulin response.
Supplements and Dairy
Supplements like Branched-Chain Amino Acids (BCAAs) contain amino acids, which are the building blocks of protein. Consuming BCAAs will definitively break a fast and inhibit processes like autophagy because they trigger a protein-induced insulin response.
Adding any form of dairy or non-dairy milk, such as almond, oat, or creamer, to coffee or tea is not compatible with fasting. Even a small splash contains protein and sugar that will elevate insulin levels and signal the end of the fasted state.