Understanding the Risks: How long should you dirty fast?

October 6, 2025 •

5 min read

While intermittent fasting is backed by extensive research, the practice of "dirty fasting" is not supported by scientific consensus, making the question of 'how long should you dirty fast?' difficult to answer definitively. This approach, which involves consuming a small number of calories during a fasting window, lacks specific guidelines and established safety protocols.

Quick Summary

The duration of dirty fasting lacks scientific consensus and depends heavily on individual health goals and tolerance. It involves consuming minimal calories during a fasting window, but this may negate the full metabolic benefits of a true, or “clean,” fast. Consultation with a healthcare provider is essential for determining a safe, effective approach.

Key Points

No Standard Duration: Due to a lack of scientific research, there is no clinically recommended duration for dirty fasting.
Flexibility Comes with Compromise: While it may feel easier, consuming calories during a dirty fast can interrupt the metabolic state required for key benefits like ketosis and autophagy.
Minimal Calorie Allowance: The general, though unproven, rule is to keep caloric intake under 100 calories during the fasting window to minimize the metabolic disruption.
Focus on Healthy Eating: The success of dirty fasting largely depends on what you eat during your non-fasting window, with a focus on nutrient-dense, whole foods.
Consult a Doctor: Before attempting any fasting, especially if you have pre-existing health conditions, it is crucial to consult a healthcare professional.
Consider Clean Fasting First: Clean fasting, which allows zero calories during the fasting period, is the evidence-based approach with more established health benefits.

What Defines a 'Dirty' Fast?

Dirty fasting is a less restrictive variation of intermittent fasting (IF) that permits a limited intake of calories—typically less than 100—during the fasting period. This contrasts with the traditional "clean fast," which allows only zero-calorie beverages like water, black coffee, or plain tea. Proponents of dirty fasting suggest that this small calorie allowance makes the regimen more manageable and easier to stick with, especially for beginners. Common allowances might include a splash of milk in coffee, a serving of bone broth, or certain non-caloric sweeteners, although many experts believe even these minimal additions can break the fasted state.

Scientific Limitations and Terminology

It's crucial to understand that the term "dirty fasting" is not used in the medical community, and there is a significant lack of clinical research specifically investigating its effects. Most studies on fasting focus on clean fasting methods, where complete calorie restriction is the standard during fasting periods. Therefore, any claims about dirty fasting's benefits are largely anecdotal or based on broader intermittent fasting research, not on controlled, scientific evidence. The lack of standardized rules also means that what one person considers a dirty fast, another may not, further complicating any definitive health analysis.

How Long Should You Dirty Fast?: The Flexible Approach

Because there are no established clinical guidelines for dirty fasting, there is no one-size-fits-all answer to its duration. The most common approach is to integrate the "dirty" calorie intake into standard intermittent fasting windows, such as a 16:8 schedule, where you fast for 16 hours and eat within an 8-hour window. Other possibilities include alternate-day fasting, where a small number of calories are consumed on fasting days. However, extending a dirty fast for prolonged periods—such as several days or weeks—is not advisable and comes with significant risks. As Johns Hopkins neuroscientist Mark Mattson notes, going too long without eating might actually encourage your body to start storing more fat in response to starvation. Shorter, time-restricted periods are generally considered safer, particularly for those just starting out.

Clean vs. Dirty Fasting: A Comparison

To highlight the key differences, consider this comparison:


Feature	Clean Fasting	Dirty Fasting
Calorie Intake	Zero calories permitted.	A small number of calories (typically <100) are permitted.
Allowed Drinks	Water, black coffee, plain tea.	Caloric drinks like coffee with cream, bone broth, or juice.
Fasting State	Guaranteed to remain in a fasted state for metabolic benefits like autophagy and ketosis.	Potential to trigger an insulin response, disrupting the true fasted state and limiting full metabolic benefits.
Adherence	Can be more difficult for some, especially beginners, due to strictness.	More flexible, which can make it easier to maintain for some individuals.
Scientific Backing	Well-researched and has proven benefits for weight loss, insulin sensitivity, and metabolic health.	Lacks specific clinical research; benefits are based on general calorie restriction rather than specific fasting mechanisms.

Weighing the Risks and Benefits

Potential Benefits

Easier Adherence: For some, the small calorie buffer can help curb hunger pangs and make the fasting window feel more achievable than a strict clean fast.
Gradual Introduction: It can serve as a stepping stone for those new to intermittent fasting, helping them acclimate to time-restricted eating patterns before attempting a clean fast.
Potential Weight Loss: By reducing overall daily calorie intake, dirty fasting can still lead to weight loss, even if it compromises the deeper metabolic benefits of fasting.

Significant Risks

Metabolic Disruption: The primary risk is that consuming calories, even a small amount, can trigger an insulin response, effectively pulling the body out of a true fasted state and halting beneficial processes like ketosis and autophagy. This means the body may not switch to burning fat for fuel as efficiently.
Lack of Evidence: Without scientific backing, the long-term health effects are unknown. Relying on an unproven method could lead to unexpected or negative outcomes.
Overestimation of Calories: It's easy to misjudge the calorie content of small additions, leading to the consumption of more than intended and nullifying the calorie restriction effect.
Nutrient Deficiencies: If not combined with a nutrient-dense diet during eating windows, any form of restrictive eating carries the risk of nutritional gaps.

Best Practices for Dirty Fasting (If Attempted)

If you choose to experiment with dirty fasting, proceed with caution and follow these best practices:

Keep Calories Minimal: Aim for the absolute minimum, ideally under 50 calories, to reduce the chance of a significant insulin spike. Many practitioners find success sticking to just 10 calories or less.
Prioritize Nutrient-Dense Foods: During your eating window, focus on clean, whole foods rich in protein, fiber, and healthy fats to support your body's needs.
Stay Hydrated: Regardless of fasting type, ample water intake is non-negotiable to prevent dehydration and support bodily functions.
Listen to Your Body: Pay close attention to how you feel. If you experience negative side effects like headaches, fatigue, or mood swings, it's a sign to re-evaluate your approach.
Consult a Professional: Speak with a healthcare provider or a registered dietitian before starting any new fasting regimen. This is especially important for individuals with underlying health conditions, such as diabetes.

Conclusion: Prioritize Clean Fasting for Proven Benefits

While the concept of dirty fasting may seem like an easier path to intermittent fasting, the lack of scientific evidence and the risk of compromising key metabolic benefits make it a less-than-ideal strategy. There is no set answer for how long you can or should dirty fast, and longer durations without medical supervision are ill-advised. For those new to fasting, a gradual approach with a clean, time-restricted eating window (like 12- or 14-hour fasts) offers a safer, more predictable way to see benefits without introducing unnecessary risks. When it comes to reaping the full, scientifically backed rewards of fasting, such as autophagy and improved insulin sensitivity, clean is always the better way to go. For those with health concerns, always consult a professional for personalized advice.

For more detailed research on the physiology of fasting, refer to the National Center for Biotechnology Information's article, "Physiology, Fasting - StatPearls" on the NCBI Bookshelf.

Frequently Asked Questions

Clean fasting involves consuming zero calories during the fasting period, while dirty fasting allows for a small amount of calories, typically less than 100, which can come from drinks like bone broth or coffee with a splash of cream.

Yes, adding cream to your coffee introduces calories and can trigger an insulin response, which technically breaks your fast. This is why it's considered part of a 'dirty' fast.

Dirty fasting can lead to weight loss by creating a calorie deficit over time, similar to any calorie-restricted diet. However, it may be less effective for promoting specific metabolic changes compared to clean fasting because it can disrupt the fasted state.

Some people use dirty fasting as an easier entry point into time-restricted eating due to its flexibility. However, since the metabolic benefits are less certain, beginners are often better off starting with a shorter, clean fasting window to see how their body reacts.

The main risks include disrupting key metabolic processes like ketosis and autophagy, the potential for overconsuming calories unknowingly, and a general lack of scientific research on long-term effects. It is not recommended for individuals with a history of eating disorders, diabetes, or other medical conditions without medical supervision.

No specific foods are universally agreed upon for dirty fasting. The approach is defined by consuming any calories during the fasting window, often in the form of low-sugar or high-fat items like bone broth or coconut oil in coffee, but even these items can negate the full benefits of fasting.

Autophagy, the cellular cleaning process, is triggered by prolonged calorie restriction. Consuming even a small amount of calories during a dirty fast can potentially trigger an insulin response and disrupt this process, making it less effective for promoting autophagy compared to a clean fast.