How many calories can I drink during fasting? A guide to navigating the rules of caloric intake

October 23, 2025 •

4 min read

According to nutrition experts, any amount of calories can technically break a fast because ingested energy will be provided to your cells. So, for those asking how many calories can I drink during fasting?, the most accurate answer is zero, though some practitioners adhere to more flexible rules depending on their goals.

Quick Summary

The number of calories one can drink while fasting depends on the type of fast and personal health goals. Strict 'clean' fasting mandates zero calories, relying on water, black coffee, and plain tea. In contrast, 'dirty' fasting allows a minimal caloric intake, usually under 50, to sustain the fast longer, though it may diminish some benefits.

Key Points

Zero for a Clean Fast: For a strict, 'clean' fast aimed at cellular repair and maximum metabolic benefits, the correct number of calories is zero.
The 50-Calorie Guideline: In 'dirty' fasting, a common, though unofficial, practice is to stay under 50 calories, often from pure fats, to help manage hunger.
Macronutrient Impact: Carbohydrates are the most likely to break a fast due to their strong insulin-spiking effect, while fats have the least impact.
Prioritize Zero-Calorie Beverages: Your safest and most reliable options during a fast are water, black coffee, and unsweetened tea, which contain negligible calories.
Avoid Sugars and Artificial Sweeteners: All sugar-containing drinks and, for best results, even zero-calorie artificial sweeteners, should be avoided during a fast to prevent insulin responses and cravings.

The Core Principle: Zero Calories for a 'Clean' Fast

For a true or 'clean' fast, the simple and absolute rule is to consume zero calories. This approach ensures that your body remains in a fully fasted state, maximizing benefits such as metabolic switching, where the body shifts from burning sugar to burning fat, and promoting cellular repair, known as autophagy. Any caloric intake, no matter how small, triggers a metabolic response that can interrupt these processes. For those with specific goals like inducing autophagy, a strict, calorie-free fast is the only way to guarantee maximum effectiveness.

Why a 'Clean' Fast Matters

When you ingest calories, your body's insulin levels rise. Insulin is a storage hormone that halts the fat-burning process. Carbohydrates, in particular, cause a significant insulin spike, while protein has a moderate effect. Even a small number of calories can be enough to signal the body that the fast is over. For this reason, purists of intermittent fasting (IF) stick exclusively to beverages that contain no calories.

The Rise of 'Dirty' Fasting: A More Flexible Approach

While the concept of a strict, zero-calorie fast is clear-cut, many people find it difficult to maintain, especially over longer fasting windows. This has given rise to the practice of 'dirty fasting'. 'Dirty' fasters allow a very small amount of calories, often cited as under 50, to make the fast more sustainable. The reasoning is that a minimal intake, particularly from non-insulin-spiking sources like fat, is unlikely to disrupt major metabolic pathways, such as ketosis, which is the state of burning fat for fuel. It is important to note that the 50-calorie rule is an anecdotal guideline, not a scientific one, and consuming any calories will technically end a true fast.

The Impact of Macronutrients

Not all calories are created equal when it comes to breaking a fast. Understanding how your body processes macronutrients can inform your choices if you opt for a 'dirty' fast.

Carbohydrates: This is the most disruptive macronutrient. Even a small amount of sugar will cause an insulin spike, ending your fast and halting fat burning. This is why sugary drinks, fruit juices, and regular soda are strictly off-limits.
Protein: Protein also causes an insulin response, though less dramatically than carbohydrates. For this reason, high-protein liquids like bone broth, which contain amino acids, are considered fast-breakers, though they may still be used in modified fasts for their nutrient content.
Fats: Pure fats have the least impact on insulin levels. This is why options like bulletproof coffee (coffee with added butter and MCT oil) are popular in the ketogenic and 'dirty' fasting communities, as they can provide energy and curb hunger without a significant insulin spike. However, even fats contain calories, so they technically break a strict fast.

Fasting-Friendly Drink Options

To stay hydrated and minimize hunger pangs during a fast, the following zero-calorie beverages are your safest options:

Plain Water: Still or sparkling, water is the foundation of any fast. It helps with hydration and can create a feeling of fullness. Adding electrolytes without sweeteners can help replenish minerals, especially during longer fasts.
Black Coffee: A classic choice for many fasters. It contains negligible calories (about 3-5 per cup) and can suppress appetite and boost metabolism. Ensure it is plain, with no sugar, milk, or cream.
Unsweetened Tea: Green, black, or herbal teas are excellent alternatives. They are rich in antioxidants and naturally calorie-free. Like coffee, they should be consumed plain.
Lemon Water: A small squeeze of lemon in water adds minimal calories and can add flavor without breaking the fast. However, be mindful of tooth enamel erosion from acidity.
Apple Cider Vinegar: Diluted in water, a small amount is often used for its potential health benefits. It contains a few calories but is generally considered acceptable in flexible fasts.

The Problem with Artificial Sweeteners

The use of zero-calorie sweeteners in diet sodas and other beverages is a topic of intense debate within the fasting community. While they don't contain calories, some research suggests that the sweet taste alone can trigger an insulin response or increase cravings. The metabolic effects can vary between individuals, but for those seeking maximal benefits, it is safest to avoid them entirely.

Comparison of Fasting Approaches


Feature	Clean Fasting	Dirty Fasting	Modified Fasting (e.g., 5:2)
Calorie Intake	Zero during fasting window	< 50 calories during fasting window	500-800 calories on fasting days
Accepted Drinks	Water, black coffee, unsweetened tea	Zero-calorie drinks + small amounts of fat (e.g., in coffee)	Any beverages, within the caloric limit
Primary Goal	Autophagy, cellular repair, pure metabolic reset	Flexibility, hunger management, weight loss	Sustainable calorie restriction, weight loss
Strictness	Very strict	Lenient	Moderately strict
Benefit Maximization	High	Moderate, may compromise some benefits	Moderate, allows for more consistency

Conclusion

The question of how many calories can I drink during fasting? depends entirely on your specific health goals and the type of fast you are observing. For the purest and most complete metabolic reset, a 'clean' fast with zero caloric intake is the gold standard. However, for those focused primarily on weight management and making fasting more sustainable, a flexible 'dirty' fast that permits a few low-carb calories may still provide significant benefits without completely derailing your efforts. Always listen to your body and, if in doubt, stick to plain water, black coffee, or unsweetened tea. For personalized advice, it is always best to consult a healthcare professional. For more information on the benefits of intermittent fasting, a good resource is Johns Hopkins Medicine.(https://www.hopkinsmedicine.org/health/wellness-and-prevention/intermittent-fasting-what-is-it-and-how-does-it-work)

Frequently Asked Questions

While diet sodas are calorie-free, the artificial sweeteners can trigger a cephalic-phase insulin response in some individuals and increase cravings, which can diminish the benefits of a fast. For a purist's fast, it's best to avoid them.

Yes, even a small splash of milk or cream contains calories and lactose (a sugar), which can trigger an insulin response and technically break a strict fast. It is recommended to drink coffee black during your fasting window.

A small squeeze of lemon or a tablespoon of apple cider vinegar in water adds a minimal number of calories (4 calories for two lemon slices). For most pragmatic fasters, this is unlikely to significantly disrupt the fast, but for a strict fast, any calories are avoided.

A 'clean' fast involves consuming zero calories and only drinking water, black coffee, or plain tea. A 'dirty' fast is a more flexible approach that allows for a small number of calories, often under 50, usually from fat, to help curb hunger.

Consuming calories, especially carbohydrates, can kick your body out of ketosis by causing an insulin spike. However, some types of fat-based calories, like MCT oil, are less likely to significantly disrupt ketosis, though they still technically break a fast.

Bone broth contains amino acids from protein, which can cause an insulin response. For a strict fast, it is not recommended. However, some who follow a 'dirty' or modified fast may use it for its electrolyte content, especially during longer fasts.

This is a debated topic. While zero-calorie gum doesn't contain energy, the act of chewing can stimulate the digestive system and potentially lead to an insulin response. Most strict fasters avoid it, though many in the 'dirty' fasting camp use it without issue.