Intermittent fasting and therapeutic fasting have gained widespread attention for their potential metabolic benefits. As people adopt these eating patterns, a common question arises: what specific items can be consumed without interrupting the body’s fasted state? The answer lies not just in counting calories, but in understanding the specific metabolic processes that define a fast. The permissibility of any food or drink ultimately depends on the body’s physiological response to it.
The Physiological Definition of Breaking a Fast
The defining characteristic of a fasted state is a metabolic shift where the body transitions from burning stored glucose to burning stored fat for fuel. When food is consumed, the body first utilizes circulating glucose, then turns to glycogen stores in the liver, which are typically depleted after 12 to 24 hours of fasting. Once these reserves are exhausted, the body enters a state of ketosis, breaking down fat tissue into free fatty acids and ketone bodies for energy.
The mechanism that “breaks” this fat-burning state is the release of the hormone insulin. Insulin is the primary signal for energy storage, and even small amounts of macronutrients, particularly carbohydrates and protein, can trigger its secretion. Elevated insulin levels signal to fat cells to stop releasing fatty acids, which halts the process of fat burning. Therefore, breaking a fast is fundamentally about introducing anything that causes a significant insulin response.
Zero-Calorie Consumables That Maintain the Fast
A few select substances are universally accepted during a fast because they contain zero or near-zero calories and do not stimulate an insulin response. Plain water, whether still or sparkling, is the foundation of any fast and is necessary for hydration and electrolyte balance. Unsweetened black coffee is permissible because its minimal caloric content is generally considered negligible, and the caffeine can even support fat breakdown.
Similarly, plain teas, such as green or herbal varieties, can be consumed without breaking the fast. They must be served without sugar, honey, milk, or any other additives. The integrity of the fast depends on these beverages being consumed in their unadulterated form. These items keep the insulin level low and allow the body to remain in the metabolically-shifted, fat-burning state.
Foods and Additives That Cause Confusion
Many items exist in a gray area because they are low in calories but may still provoke a metabolic reaction. Non-nutritive sweeteners, such as sucralose, stevia, and monk fruit, are highly controversial because of the Cephalic Phase Insulin Response (CPIR). The sweet taste alone can, in some individuals, signal the pancreas to release a small amount of insulin in anticipation of calories, even though none arrive. While many studies suggest these sweeteners do not cause a significant CPIR, others show conflicting results, and long-term use may affect gut microbiota.
Bone broth is another debatable item, as it contains protein in the form of amino acids and a small number of calories, typically around 10–50 calories per cup. While the low-calorie, low-carbohydrate nature of bone broth means it has a minimal impact on blood glucose and insulin, the amino acids can signal the cellular growth pathway known as mTOR. This signaling technically stops a strict fast, but it is often tolerated during longer, modified fasts for its benefits in maintaining electrolytes and gut health.
Small amounts of pure fat, such as a teaspoon of MCT oil or heavy cream added to coffee, are sometimes included in a “dirty fast.” Since fat does not typically trigger a significant insulin spike, these additions primarily impact the caloric restriction aspect of a fast. However, consuming any calories, including those from fat, will interrupt the process of cellular cleanup known as autophagy, which is a goal of therapeutic fasting. Finally, supplements in liquid or gummy forms should be avoided during a fast, as they often contain hidden sugars, artificial colors, or other caloric ingredients that can inadvertently break the fast.
Fasting Goals Dictate Permissibility
The ultimate decision about whether a food breaks a fast depends entirely on the individual’s primary health objective. For those focused on metabolic health and weight loss, the goal is primarily to keep insulin levels low to promote fat burning. This goal is more permissive, and a small splash of fat or a low-calorie, non-nutritive sweetener may be acceptable if it helps the person adhere to the overall fasting schedule. These modest additions do not significantly interrupt the fat-burning state.
A far stricter approach is required for therapeutic fasting, where the goal is to maximize cellular repair processes like autophagy. Autophagy is stimulated by a state of total nutrient deprivation. Introducing any calories, especially protein or fat, will signal the body to halt this cellular recycling. For this goal, the fast must be limited to zero-calorie, non-nutritive consumables to ensure the maximum level of cellular renewal is achieved.