Intermittent fasting (IF) is a dietary pattern that involves regularly cycling between periods of eating and periods of not eating. The goal is to extend the time the body spends in a fasted metabolic state, which is associated with benefits like weight management and metabolic health improvements. As people seek to maintain this state, the question of whether a low-calorie item like miso soup is acceptable often arises. The answer depends entirely on the specific metabolic goal of the fast.
Nutritional Profile of Miso Soup
Traditional miso soup consists primarily of dashi, a flavorful broth typically made from dried fish and kelp, and miso paste, which is a fermented paste of soybeans, salt, and koji (a fungus). A standard eight-ounce serving of basic miso soup is relatively low in calories, generally falling between 35 and 60 calories. The macronutrient distribution is modest, providing approximately 2 to 6 grams of protein and 4 to 7 grams of carbohydrates, with minimal fat.
The protein and carbohydrates come from the fermented soybean paste, which contains all nine essential amino acids. Common additions, such as small cubes of silken tofu, scallions, and wakame seaweed, contribute to the nutrient profile without drastically increasing the caloric load.
Restaurant versions or ready-made packets that include ingredients like potatoes, larger amounts of tofu, or added oils can push the calorie count significantly higher. While the fermentation process provides beneficial probiotics, the soup also contains a notable amount of sodium, which is important for those monitoring salt intake.
Defining the Parameters of a Fast
Determining if any food item “breaks” a fast requires establishing the specific purpose of the fasting period. The most common goal is standard intermittent fasting, focused on weight loss and metabolic efficiency by maintaining low insulin levels. For this goal, a widely accepted guideline suggests keeping calorie intake below 50 calories to avoid a significant metabolic shift. Technically, any calorie intake can interrupt a completely fasted state, but the 50-calorie rule is a practical threshold.
The second primary reason for fasting is to stimulate autophagy, a cellular process that clears out damaged cells and recycles components. Autophagy is highly sensitive to nutrient intake, particularly protein and amino acids.
Even a small amount of protein can trigger signaling pathways that halt this cellular recycling process. Therefore, achieving the benefits of autophagy requires strict adherence to zero or near-zero calorie and protein intake.
Miso Soup and Fasting Goals
When applying these parameters, miso soup’s impact on a fast becomes conditional. For standard intermittent fasting aimed at weight management, a small cup of basic miso soup (35 to 60 calories) may be considered a “dirty fast” compromise. Consuming a serving under 50 calories might allow the body to remain largely in the fat-burning state. The broth can also help with hydration and electrolyte balance, potentially easing hunger, but its modest carbohydrate and protein content means it is not a zero-calorie beverage.
For those specifically fasting to maximize autophagy, miso soup will likely break the fast. The miso paste, being a fermented soybean product, contains protein and all nine essential amino acids. In this strict context, the presence of these amino acids signals to the cells that nutrients are available, thereby inhibiting the autophagy process.
To maintain an autophagy-focused fast, the safest approach is to avoid all caloric intake and any source of protein. Individuals choosing to consume miso soup during a fast should check ingredient labels carefully to avoid added sugars or significant amounts of high-fat additions.
Ultimately, whether miso soup is acceptable depends on the individual’s fasting philosophy. A practical approach to weight loss may allow for a small, basic serving, while a focus on cellular repair demands a zero-calorie, zero-protein commitment.