Intermittent fasting is a popular eating pattern that cycles between periods of eating and abstaining from calories. During the fasting window, individuals often consume liquids to maintain hydration and manage hunger. Broths provide nutrients and a savory flavor, leading to the question of whether they compromise the metabolic benefits of the fast. The answer is nuanced, depending on the specific ingredients and the individual’s fasting goals.
Defining the Fasted Metabolic State
To understand if broth breaks a fast, one must first define the fasted metabolic state. Fasting causes a shift from using glucose as the primary fuel source to burning stored body fat, a process known as ketosis. This metabolic state is generally maintained as long as insulin levels remain low. Many experts suggest that consuming a minimal amount of calories, often fewer than 50, may not completely disrupt this process.
This caloric threshold is based on the idea that a tiny amount of food may not trigger a significant insulin response that halts fat burning. However, fasting for cellular repair, known as autophagy, is far more sensitive to nutrient intake. Autophagy is a cellular cleansing process strongly suppressed by the presence of amino acids and insulin. For this sensitive goal, consuming any calories or protein, even below the 50-calorie limit, is considered a metabolic interruption.
Nutritional Components of Broth
The nutritional profile of broth varies significantly depending on its preparation. Simple vegetable broths, made by simmering vegetables and herbs, are low in calories and fat but contain minimal trace carbohydrates. The caloric contribution from these is usually less than 10 calories per cup.
Bone broth, in contrast, is made by simmering bones and connective tissue for an extended time. This process extracts proteins, primarily collagen and gelatin, making the protein content the main concern for fasting. A single cup of bone broth can contain between 6 and 9 grams of protein and range from 39 to 90 calories, depending on the richness and the amount of fat present. This higher protein load is most likely to influence the fasted state.
The Threshold: Broth and Metabolic Interruption
The answer to whether broth breaks a fast depends entirely on the individual’s goal. If the fast is primarily for weight management and maintaining a low-insulin, fat-burning state, a small amount of simple, low-calorie broth is often permissible. This practice is sometimes referred to as “dirty fasting,” where an intake below 50 calories is tolerated to help adherence. The low-fat and low-carbohydrate nature of a clear broth means the insulin response will be minimal, allowing ketosis to continue largely uninterrupted.
Bone broth is more marginal because its protein content can stimulate an insulin response, even if a mild one. The presence of amino acids is a direct signal to the body that nutrients are available. This signaling is particularly relevant for the goal of autophagy, which is highly sensitive to amino acid and protein intake. Therefore, bone broth is likely to interfere with the delicate process of cellular cleaning, even if it does not fully stop ketosis. A strictly water-only fast is required for maximizing autophagy benefits.
Selecting a Fasting-Friendly Broth
For those who choose to incorporate broth during their fasting window, careful selection is necessary to minimize metabolic impact. The most fasting-friendly broths are those that are clear, strained, and nearly calorie-free. Vegetable stock is preferable, provided it is strained to remove any solid vegetable matter that contains carbohydrates and fiber.
It is important to avoid broths that include added ingredients that can trigger an insulin response. These include vegetable pieces, yeast extracts, flavor enhancers, or any form of added sugar or sweetener. The goal is to consume a liquid that provides electrolytes and hydration with the lowest possible calorie and protein count. Look for products that list zero or near-zero calories per serving, or make a simple homemade broth from strained, clear liquid.