What Breaks a Fast According to Dr. Jason Fung?

January 13, 2026 •

5 min read

According to Dr. Jason Fung, a primary driver of metabolic health issues is consistently high insulin, not just excess calories. For this reason, what breaks a fast Dr. Jason Fung and his principles emphasize avoiding anything that triggers an insulin response during the fasting window to achieve therapeutic benefits like weight loss and improved insulin sensitivity.

Quick Summary

This article explains which foods and beverages cause an insulin response and disrupt the key metabolic processes of fasting, based on the principles of Dr. Jason Fung. It clarifies the rules for clean fasting, addressing common questions about items like coffee, sweeteners, and bone broth, and discusses the different goals people might have for their fasting practice.

Key Points

Insulin is key: Anything that raises insulin levels, not just calories, breaks a fast according to Dr. Fung.
Clean fast for autophagy: For maximum cellular repair (autophagy), a strict clean fast with only water, black coffee, and unsweetened tea is required.
Avoid artificial sweeteners: Even zero-calorie sweeteners like stevia and sucralose should be avoided, as they can potentially trigger an insulin response.
Bone broth breaks autophagy: While a small amount of broth is tolerated by some for weight loss, its protein content will turn off the autophagy process.
Goal-dependent flexibility: Your fasting rules can be adapted based on your specific health goals, with weight loss allowing more flexibility than deep metabolic healing.
Hydration is vital: Plain water is essential for staying hydrated and preventing common fasting side effects like headaches and fatigue.
Assess your progress: If weight loss or other health goals stall, re-evaluating and eliminating potential 'dirty fasting' items is necessary.
Diet soda is out: Diet drinks are generally not recommended during a fast due to the potential for an insulin spike, even if they are zero-calorie.

The Insulin and Autophagy Perspective

Dr. Jason Fung’s approach to fasting is rooted in hormonal regulation, specifically controlling insulin levels. Unlike simple caloric restriction, which can slow metabolism, intermittent fasting (IF) works by giving the body periods of rest from food intake, allowing insulin levels to fall. Low insulin levels are key to unlocking several benefits, including encouraging the body to burn stored fat for energy and initiating a cellular repair process called autophagy.

To achieve these therapeutic effects, the definition of what breaks a fast is stricter than simply avoiding solid food. Any substance that triggers an insulin spike will halt these processes. This means focusing on 'clean fasting,' consuming only items that do not provoke an insulin response. For people new to fasting, a slightly modified approach might still offer weight loss benefits, but for those seeking deeper metabolic healing and autophagy, strict adherence is crucial.

What to Avoid During a Fast

Any food or drink that contains calories, protein, or carbohydrates will typically break a fast. The main offenders include:

Sugary drinks and juices: Sodas, fruit juices, and any sweetened beverages cause a rapid insulin spike, immediately ending the fasted state.
Artificial sweeteners: Even though they are zero-calorie, Dr. Fung advises caution with artificial sweeteners like aspartame, sucralose, and stevia. Some research indicates that the sweet taste can still trigger an insulin release, though results can vary. Fung suggests that if you aren’t seeing results, they should be the first thing to eliminate.
Milk and cream: Adding milk, cream, or other dairy products to coffee or tea introduces protein and fat, which trigger an insulin response.
Broth with protein: While some fasting methods allow for bone broth, Dr. Fung clarifies that the protein and calories in it will turn off autophagy, the cellular recycling process. For metabolic weight loss, a small amount might be permissible, but for maximum autophagy, it breaks the fast.
Supplements with additives: Many vitamins and supplements contain sugars, fillers, and other caloric ingredients. Check labels carefully and stick to plain, unflavored supplements where possible.
Protein powders: Shakes and powders, even if low in carbs, will elicit an insulin response due to the protein content.

The Grey Area: A Matter of Goals

Dr. Fung acknowledges that a person's fasting goal dictates the level of strictness required. A person fasting for weight loss might tolerate a 'dirty fast' with a splash of cream or small calories and still see results. However, someone aiming to maximize autophagy or reverse insulin resistance needs to be much stricter with a 'clean fast.'

Clean vs. Dirty Fasting: A Comparison Table


Item	Clean Fast (Maximizes Autophagy & Metabolic Healing)	Dirty Fast (Focuses on Weight Loss)
Water	Plain, mineral, or sparkling water is perfectly acceptable.	Same as clean fast. Hydration is key for everyone.
Coffee	Black coffee only. No additives. Does not contain calories, protein, or carbs.	Small amount of cream, coconut oil, or MCT oil might be tolerated for weight loss.
Tea	Plain herbal tea or black/green tea without any sweeteners or milk.	Small amounts of stevia or monk fruit could be used if it doesn't hinder progress.
Bone Broth	Not recommended, as the protein will stop autophagy.	Can be used to curb hunger during longer fasts, but will break the autophagy process.
Artificial Sweeteners	Best to avoid entirely to prevent potential insulin spikes.	Trial and error is recommended; stop if weight loss stalls.

What is Allowed While Fasting?

For a clean fast, the list of approved items is short and simple:

Water: Plain water, mineral water, and sparkling water are all fine and essential for hydration.
Black Coffee: Unsweetened and without any added cream, milk, or flavors. Can be helpful for suppressing appetite and providing a gentle energy boost.
Unsweetened Tea: Herbal tea, green tea, and black tea are acceptable, as long as they are free of any sweeteners.
High-Quality Salt: Adding a pinch of salt to water can help with electrolyte balance, especially during longer fasts, preventing headaches and fatigue.

Fasting for Different Goals

Dr. Fung acknowledges that everyone's journey is different. He highlights that fasting is a flexible tool that can be adapted based on your specific health objectives, not a rigid set of rules that applies equally to all people at all times. For example, a beginner doing intermittent fasting for weight loss might start with a 16/8 schedule and be a bit more lenient, as long as they are still seeing progress. A person battling Type 2 diabetes or using fasting for deeper cellular benefits like autophagy will need a stricter, 'clean' approach to see the best results. The ultimate measure of success, according to Fung, is whether you are getting the health benefits you desire.

Conclusion: Focus on Insulin, Not Calories

The core principle behind what breaks a fast, according to Dr. Jason Fung, is any food or drink that significantly raises insulin levels. While consuming zero-calorie beverages is generally acceptable, items that contain protein, carbohydrates, or even trick the body with a sweet taste can disrupt the hormonal balance and metabolic processes that make fasting so effective. For maximum therapeutic benefit, especially for autophagy and insulin resistance, sticking to a clean fast of only water, black coffee, and unsweetened tea is the most reliable strategy. However, for those primarily focused on weight loss, a more flexible, modified fast can still yield positive results, as long as progress is being made.

Choosing the Right Fasting Approach

When determining what to consume during a fast, the first step is to clarify your goals. If you are seeking deep metabolic and cellular repair, a strict clean fast is the path to follow. If your primary goal is weight management, you have more flexibility. By focusing on the hormonal response rather than just a calorie count, you can make informed choices that align with the principles of one of the world's leading fasting experts. Remember, fasting is a powerful tool, but its application should be tailored to your individual needs.

Start with your objective: What is the primary reason you are fasting? Is it weight loss, improved insulin sensitivity, or autophagy?
Assess your tolerance: Can you handle black coffee or unsweetened tea? If not, try a modified 'dirty fast' and observe the results.
Monitor your progress: If your progress stalls, re-evaluate what you are consuming. The most likely culprit is something that is spiking your insulin, even if it seems benign.
Hydrate adequately: Regardless of your fasting type, staying hydrated with plenty of water is essential for your well-being and for managing hunger.
Break your fast carefully: When your eating window arrives, break your fast gently with nutrient-dense foods to avoid a sudden insulin surge and digestive upset.

By following these principles, you can navigate your fasting journey with confidence, using Dr. Fung's wisdom to make choices that support your long-term health and wellness goals.

Source: The Complete Guide to Fasting by Dr. Jason Fung - bagerbach.com

Frequently Asked Questions

No, black coffee does not break a fast. Dr. Fung explicitly states that black coffee is acceptable, as it contains virtually no calories and will not trigger an insulin response.

For those seeking the deep metabolic benefits of fasting, such as autophagy, Dr. Fung states that bone broth will break the fast due to its protein content. However, some might use a small amount for weight loss purposes, but it's not a clean fast.

Dr. Fung advises caution with artificial sweeteners. Although they have no calories, the sweet taste can potentially trigger an insulin release, which defeats the hormonal purpose of fasting. If weight loss stalls, they should be eliminated.

A 'clean' fast involves only consuming water, black coffee, and unsweetened tea to avoid any insulin response and maximize benefits like autophagy. A 'dirty' fast allows for small amounts of low-calorie items like cream or bone broth, and might still yield weight loss but won't provide the same deep metabolic benefits.

Yes, even a small splash of milk or cream contains protein and fat that will trigger an insulin response, breaking the clean fast. For strict fasting, it is best to drink coffee black.

According to Dr. Fung, hunger comes in waves and will eventually pass. Staying busy, drinking water, or having a cup of black coffee or tea can help manage hunger. Adding a pinch of salt to water can also help with electrolyte balance and cravings.

The only beverages recommended for a strict clean fast are plain water, sparkling water, black coffee, and unsweetened tea. These options contain no calories, protein, or carbohydrates that would provoke an insulin response.

Dr. Fung advises against diet soda during a fast. Even without calories, the artificial sweeteners can lead to an insulin spike and may not be beneficial for weight loss in the long run.