The question of what to drink while fasting is common for people exploring time-restricted eating, such as Intermittent Fasting (IF). The goal of a fast is to keep the body in a metabolic state that promotes benefits like fat burning, improved insulin sensitivity, and cellular repair. A fast is broken not just by calories, but by any substance that signals the body to switch from a fasted state to a fed state. This guide provides clear boundaries for maintaining the desired metabolic benefits.
The Core Rule: What Defines Breaking a Fast
Breaking a fast is fundamentally about triggering a significant insulin response. Insulin moves glucose into cells, signaling that nutrients are available and halting the use of stored body fat for fuel. The goal of many fasting protocols is to keep insulin levels low and steady, allowing the body to shift into a state of fat-burning and ketosis.
A stricter definition of breaking a fast involves autophagy, a cellular cleanup mechanism. Autophagy is highly sensitive to nutrient intake, particularly protein and carbohydrates, which activate the mTOR growth pathway. Consuming even small amounts of protein or sugar can switch off this repair process, regardless of the calorie count.
A practical guideline often used for those focused on fat loss, but not strict autophagy, is the 50-calorie rule. Consuming anything under 50 calories, especially if those calories come exclusively from fat, is generally accepted as a “dirty fast” that will not substantially disrupt fat burning. However, those seeking the deepest metabolic benefits should aim for true zero-calorie intake.
Zero-Calorie Staples: The Safe List
The safest drinks contain zero calories and have no active ingredients that would provoke an insulin or cephalic (brain-based) response. These beverages are universally accepted across all fasting protocols, including those focused on maximizing autophagy, due to their lack of carbohydrates, protein, or fat.
Plain water is the ultimate safe beverage, whether tap, filtered, still, or sparkling. Water is necessary to prevent dehydration during fasting periods. Adding a squeeze of lemon or lime juice is generally acceptable, as the caloric content is negligible and unlikely to cause a metabolic shift.
Black coffee and plain tea are also staples, provided they are consumed without any additions. Coffee contains minimal calories, and the caffeine content may help stimulate fat release. Plain teas, whether green, black, or herbal, are safe, but they must be unsweetened and free of fruit pieces or added flavorings that might contain hidden sugars or calories.
The Gray Area: Beverages Requiring Scrutiny
Certain drinks fall into a gray area because they either contain minimal calories or include non-caloric ingredients that might still provoke a metabolic response. The acceptability of these beverages depends heavily on an individual’s specific fasting goals, such as weight loss versus maximizing autophagy.
Artificial Sweeteners and Diet Sodas
Artificial sweeteners and diet sodas are the most contentious items in this category. Non-nutritive sweeteners like Aspartame, Saccharin, and Erythritol contain virtually no calories and generally do not raise blood glucose or insulin levels significantly. However, some sweeteners, like Sucralose, have been linked to potential insulin spikes in sensitive individuals. The intense sweetness of these products can also trigger a cephalic response in the brain, which may increase hunger and make adherence to the fast more challenging.
Electrolytes and Mineral Water
Electrolytes and mineral water become important for longer fasts, typically those extending beyond 24 hours. Prolonged fasting can lead to a loss of essential minerals like sodium, potassium, and magnesium. To safely replenish these, unflavored, unsweetened electrolyte mixes are necessary, ensuring they contain zero calories and no artificial sweeteners.
Small Fat Additions
Small fat additions, such as a teaspoon of Medium-Chain Triglyceride (MCT) oil or ghee, are used by some fasters, particularly those following a ketogenic diet. These additions contain calories, which technically breaks a water-only fast. Since fat is the least insulinogenic macronutrient, it will not significantly raise insulin or knock the body out of ketosis, though it will halt the process of autophagy.
Bone Broth
Bone broth is often used in modified fasts because it provides electrolytes and beneficial amino acids. While it is low in calories, the protein content is enough to activate the mTOR pathway, which stops autophagy. Therefore, bone broth is an excellent choice for a modified fast focused on gut health and weight loss, but it is not appropriate for a strict fast aimed at cellular renewal.
Clear Fast Breakers: What to Absolutely Avoid
These beverages contain significant amounts of calories, sugar, or protein, which will definitively break a fast by causing a rapid and substantial insulin spike. Any drink containing ingredients that require digestion should be avoided during the fasting window.
The following drinks contain ingredients that exceed the accepted threshold and disrupt the metabolic goals of fasting:
- Fruit juices and sugary drinks: Even 100% natural fruit juices are concentrated sources of sugar (fructose and glucose). This high sugar load forces the pancreas to release a large amount of insulin, immediately ending the fasted state. Sugary sodas, energy drinks, and flavored coffees like lattes also contain massive amounts of refined sugar and calories.
- Dairy and most non-dairy alternatives: These include cow’s milk, oat milk, and soy milk, which contain a combination of carbohydrates and protein. Cow’s milk contains lactose, a sugar that spikes insulin, and the protein content activates the mTOR pathway.
- Alcoholic beverages: Alcohol is a clear fast-breaker because it contains a high number of calories, approximately seven calories per gram. The body prioritizes metabolizing alcohol over fat, effectively pausing the fat-burning state that fasting promotes.