Intermittent fasting (IF) involves alternating periods of eating with voluntary abstinence from food, often used for metabolic health, weight management, and cellular function. The core challenge for those engaging in fasting is understanding precisely what can be consumed during the fasting window without negating the intended biological benefits. Permissible foods and drinks depend on the metabolic goals of the fast. This insight helps practitioners avoid accidental consumption that can trigger the body’s digestive and hormonal responses, effectively resetting the fasted state.
Defining the Metabolic Threshold for Fasting
The primary goal of intermittent fasting is to shift metabolism from using glucose toward burning stored body fat. This shift, known as the metabolic switch, occurs once liver glycogen stores are depleted, typically after 12 hours without food. The fasted state is characterized by low insulin levels, which facilitates the breakdown of fat into fatty acids and ketone bodies for energy.
A secondary benefit of extended fasting is the stimulation of autophagy, a cellular process involving the removal and recycling of damaged components. Autophagy is highly sensitive to nutrient availability and requires suppressed insulin levels. Technically, any calories break a fast, but the focus is on preventing a significant insulin response.
A widely accepted guideline suggests that consuming less than 50 calories will not substantially disrupt the metabolic state of fat-burning, especially for weight loss. However, even small caloric intake, particularly from protein or carbohydrates, can inhibit the more sensitive process of autophagy. Thus, defining “breaking a fast” depends heavily on the individual’s specific health objectives.
Essential Zero-Calorie Liquids and Beverages
The safest liquids to consume during a fast contain zero calories and do not stimulate an insulin response. Plain water, still or sparkling, is the foundation of any fasting regimen and is essential for hydration, especially since the body is not receiving water content from food.
Black coffee is generally considered safe and is often consumed during fasting periods. Coffee contains almost no calories and may help suppress appetite while providing a modest energy boost. Similarly, plain teas, including green, black, and most herbal varieties, are permissible choices.
These beverages must be consumed without caloric additions like sugar, milk, cream, or caloric creamers. Ensure that flavored or fruit-infused teas do not contain hidden sugars or fruit concentrates. Sticking to these pure, zero-calorie liquids maintains the metabolic goals of the fast.
Minimal Caloric Additives and the 50-Calorie Rule
Those practicing a less strict approach, often called “dirty fasting,” sometimes permit small, low-insulin-spiking additions to aid compliance. The 50-calorie rule serves as the practical limit for these additions, although it is not based on rigid scientific study. These additions are intended to maintain the metabolic state of ketosis, which is less sensitive to minor caloric shifts than autophagy.
Small amounts of healthy fats are common additions because they minimally impact insulin levels compared to carbohydrates or proteins. For example, a teaspoon of heavy cream or a half-teaspoon of medium-chain triglyceride (MCT) oil added to black coffee usually stays below the 50-calorie threshold.
Bone broth is another popular additive. While it contains protein and calories that technically break a fast, its low caloric density and high electrolyte content can help manage hunger and mineral balance.
Zero-calorie sweeteners like pure Stevia and Monk Fruit extract are generally considered safe as they minimally impact blood sugar and insulin levels. Erythritol is also often tolerated since it is not fully metabolized. However, avoiding all forms of sweetness is recommended for those aiming for the deepest cellular benefits, as the cephalic phase insulin response triggered by the taste of sweetness varies by individual.
Hidden Ingredients and Unexpected Fast-Breakers
Many products that appear fast-friendly contain hidden ingredients that inadvertently disrupt the metabolic state. Flavored sparkling waters, for instance, often include natural flavorings or fruit juice concentrates that add small amounts of sugar and calories. Similarly, certain gums and mints, even if sugar-free, may contain ingredients that trigger an insulin response in sensitive individuals.
Supplements are another common source of accidental fast-breaking, especially those in gummy form, which almost always contain sugar or syrups. Branched-chain amino acids (BCAAs) are frequently consumed during fasted workouts but are metabolized as protein. Since protein triggers a metabolic response and stimulates insulin, BCAAs technically break a fast and may oppose autophagy, though some people accept this trade-off for muscle preservation.
Scrutinize the ingredient lists of all supplements, as many capsules or tablets use caloric fillers, binders, or coatings like maltodextrin, gelatin, or vegetable oils. Even some zero-calorie diet sodas contain artificial sweeteners like sucralose or aspartame that can stimulate insulin release or increase appetite in some people, potentially undermining the hormonal goals of fasting.