Intermittent fasting (IF) involves cycling between periods of eating and voluntary abstention from food. The primary goal during the fasting window is to maintain the body’s metabolic state, shifting from relying on incoming glucose to burning stored fat. Understanding what can be consumed without interrupting this metabolic shift is necessary for maximizing the benefits. This guide clarifies the science behind a “broken fast” and provides a practical list of safe and borderline consumables.
The Metabolic Threshold Defining a Broken Fast
Breaking a fast is fundamentally about triggering the body’s digestive and storage processes, which are largely controlled by the hormone insulin. When you eat, particularly carbohydrates or protein, the pancreas releases insulin to manage the incoming nutrients. This rise in insulin signals the body to switch from its fasted state back to its fed state, halting the fat-burning process.
The shift to using stored body fat and producing ketones for fuel is known as the metabolic switch, which occurs after the body has depleted its glucose reserves, typically after 12 hours or more without food. For those fasting for weight loss, the goal is to keep insulin levels low enough to sustain this fat-burning state. This is why the type of calories consumed is often more important than the exact quantity.
Experts generally suggest a rough threshold of 50 calories or less for maintaining the metabolic benefits of fasting, though this is not a firm rule. Consuming calories, especially those from fat sources like medium-chain triglycerides (MCT) oil, may not entirely stop ketosis. However, for those seeking the cellular cleanup process known as autophagy, even a minimal calorie intake is thought to interrupt the process, making a zero-calorie approach necessary.
Core Zero-Calorie Essentials
The safest path to maintaining a fast is consuming only items that contain zero calories and do not stimulate an insulin response. Plain water is the foundation of any fast, as it provides hydration without any metabolic interference. This includes still water, sparkling water, and purified water.
Black coffee is widely accepted because it contains virtually no calories and its caffeine content can help suppress appetite. It is important that the coffee remains plain, meaning no added cream, milk, sugar, or artificial sweeteners, which could potentially trigger an insulin response.
Unsweetened teas, like herbal, green, or black tea, also contain negligible calories and are acceptable, provided they are consumed without any caloric additions. Adding a pinch of salt to water can also be helpful for electrolyte balance during longer fasts, as sodium does not contain calories.
Navigating the Gray Zone Minimal Impact Consumables
Many common consumables fall into a “gray zone” because they contain minimal calories or potentially stimulate a metabolic response. Unsweetened, sugar-free electrolytes are often used during longer fasts to replace minerals lost through increased urination. These are generally considered acceptable as long as they contain no sugar, but they should be monitored for hidden caloric ingredients or flavorings derived from carbohydrates.
The use of artificial sweeteners, such as sucralose, aspartame, stevia, or monk fruit, is a frequent point of debate. While they contain zero or near-zero calories, some research suggests they may trigger a cephalic phase insulin response (CPIR) in certain individuals. The CPIR is an anticipatory release of insulin stimulated by the taste of sweetness. Those aiming for a “clean” fast often avoid all forms of sweetness to eliminate this potential metabolic trigger.
The concept of “dirty fasting” allows for small amounts of fat, such as a teaspoon of heavy cream or MCT oil, primarily to aid satiety and support ketosis. Since fat has a minimal impact on insulin compared to carbohydrates and protein, consuming a small amount of pure fat may allow the body to remain in a fat-burning state. However, this practice breaks a “clean” fast and will interrupt autophagy, as any caloric intake signals the body to halt cellular cleanup.
Specific supplements, particularly branch-chain amino acids (BCAAs) and protein powders, should be avoided during a fast. BCAAs, a form of protein, will trigger a clear insulin response and activate the mTOR pathway, which is directly opposite to the goals of fasting.
Common Mistakes and Guaranteed Fast-Breakers
Several seemingly harmless items are guaranteed to break a fast due to their caloric or protein content, or their ability to trigger an insulin response.
Guaranteed Fast-Breakers
Chewing gum, even the sugar-free varieties, is a common fast-breaker because the sweet taste and the act of chewing can stimulate the cephalic phase insulin response. The digestive system is signaled to prepare for food.
Flavored waters and vitamin-enhanced drinks are problematic, as they often contain hidden sugars, fruit juices, or caloric flavorings. Reading the nutrition label is necessary, as even a few grams of carbohydrates can be enough to raise insulin levels and end the fast. Mints, cough drops, and lozenges also almost always contain sugar or other caloric sweeteners.
Bone broth, despite its popularity, contains both calories and protein, typically 40 to 50 calories per cup. This protein activates the digestive system and triggers an insulin response, breaking a strict fast. While it can be a gentle way to transition out of a fast, it should not be consumed during the fasting window itself.
Adding any amount of milk, creamer, or half-and-half to coffee or tea will break the fast because these products contain both fat and milk proteins. Even a small splash introduces calories and protein, which directly oppose the goal of keeping the body in a low-insulin state.