Intermittent fasting (IF) is a popular eating pattern that cycles between periods of eating and abstaining from calories. The primary goal of fasting, beyond simple calorie restriction, is to shift the body into a metabolic state where it utilizes stored body fat for fuel, a process often associated with low insulin levels and the production of ketones. Maintaining this metabolic shift is the central consideration when determining what can be consumed during the fasting window without signaling to the body that the fast has ended. The challenge lies in navigating the gray area of non-caloric or very low-caloric items that may still trigger a physiological response.
Defining the Fasting Threshold
A fast is physiologically broken when a consumable item triggers a significant insulin response. Insulin is a storage hormone that, when released, signals the body to stop burning fat and start utilizing or storing incoming energy, effectively ending the fat-burning state. Carbohydrates and proteins are the macronutrients most potent at stimulating insulin release, with carbohydrates causing the strongest spike.
For those fasting for metabolic health or weight loss, the goal is to keep this insulin response minimal. While a purist definition states that any calorie breaks a fast, a practical guideline suggests staying under a threshold of approximately 50 calories. This low caloric intake, particularly when sourced from fat, is insufficient to halt the fat-burning process. Protein, however, can also activate the mTOR pathway, a cellular growth mechanism that counters the benefits of fasting like cellular cleanup (autophagy), making protein-containing items a greater risk.
Guaranteed Safe Beverages
To ensure a fast remains unbroken, consume liquids that are completely calorie-free and do not provoke an insulin response. Water is universally safe and essential for hydration. This includes plain tap water, filtered water, and unsweetened sparkling water.
Black coffee is a safe staple, as its negligible caloric content does not trigger a metabolic shift. Similarly, plain green tea and most herbal teas are acceptable, provided they are consumed without any added sugar, cream, or milk. The natural compounds in coffee and tea, such as caffeine, may help suppress appetite and support the fat-burning process.
Low-Calorie Additions and Exceptions
Certain low-calorie additions are often used during a fast to curb hunger, though they fall into a practice sometimes called “dirty fasting.” Bone broth is a common exception, typically containing 40 to 45 calories and 8 to 9 grams of protein per cup. While the protein content technically breaks a strict fast by activating the mTOR pathway, the very low overall calorie count makes it a tolerable addition for many seeking hunger relief.
Pure fats, which are the least insulinogenic of the macronutrients, are another common exception. Adding a small amount of pure fat, such as a teaspoon of MCT oil or butter, to black coffee is practiced by some to maintain ketosis and energy levels. A single teaspoon of MCT oil contains approximately 35 to 40 calories. The total caloric intake from these additions must be carefully monitored to stay below the practical 50-calorie threshold.
Natural flavorings can also be used in moderation to make plain water more palatable. A small squeeze of lemon or lime juice, which is often 1 calorie or less per teaspoon, is acceptable. Non-caloric spices, such as a dash of cinnamon on black coffee or a pinch of salt in water for electrolytes, also will not break a fast.
Addressing Sweeteners and Supplements
Non-nutritive sweeteners, which contain no or very few calories, represent a contentious gray area during fasting. Synthetic sweeteners like sucralose and aspartame, or natural ones like stevia and monk fruit, do not contribute calories but may still trigger a metabolic response in certain individuals. The intense sweet taste can activate cephalic phase insulin release, where the body prepares for a sugar load that never arrives, although the magnitude of this effect is highly debated.
Sugar alcohols, with the exception of erythritol, should be avoided, as they contain a small amount of digestible carbohydrates that can raise blood sugar and insulin levels. For most fasters, the safest non-nutritive options are those that have demonstrated minimal impact on blood sugar, such as pure erythritol or allulose.
Supplements also require scrutiny, as many contain hidden caloric fillers. Multivitamins and mineral capsules are typically safe, provided they are not in a gummy form, which often includes sugar or corn syrup. Branched-chain amino acids (BCAAs), despite being marketed as a workout aid, will explicitly break a fast because they are amino acids. BCAAs stimulate insulin release and activate the mTOR pathway, signaling to the body that protein is available for synthesis, which halts the fasting state.