What Foods Don't Break Intermittent Fasting?

November 29, 2025 •

4 min read

According to research published by Johns Hopkins, intermittent fasting triggers a 'metabolic switch' where the body moves from burning sugar to burning fat. To sustain this crucial state, it's essential to understand what foods don't break intermittent fasting by keeping your insulin levels low and your body in a fasted state.

Quick Summary

Intermittent fasting relies on consuming zero or near-zero calorie items during fasting periods to prevent an insulin response and maintain a fat-burning state. Fasting-friendly beverages include water, black coffee, and unsweetened tea, while minimal amounts of fat-based items like MCT oil or bone broth can be incorporated in some protocols. The primary goal is to avoid carbohydrates and protein, which trigger a metabolic shift away from fasting benefits.

Key Points

Zero-Calorie is Key: The most critical rule is to consume only zero or near-zero calorie items during your fasting window to avoid triggering an insulin response and breaking your fast.
Optimal Fasting Beverages: Plain water, black coffee, and unsweetened teas are the safest and most effective choices for staying hydrated without breaking your fast.
Avoid Sugars and Artificial Sweeteners: Adding sugar, cream, or milk to beverages immediately breaks a fast. Artificial sweeteners in diet sodas and gum can also trigger an insulin response, which should be avoided for a 'clean fast'.
Bone Broth for Modified Fasts: While containing some calories, bone broth can be used in some modified fasting protocols to provide electrolytes and reduce hunger, though it will technically break a 'strict' fast.
Metabolic Flexibility is the Goal: Fasting helps your body switch from burning sugar to burning fat. This process, known as metabolic switching, is a primary benefit that is only achieved by maintaining a fasted state.
What You Eat Matters Too: The success of intermittent fasting is not only about what you avoid but also about focusing on nutrient-dense foods like lean protein, healthy fats, and high-fiber vegetables during your eating window.

The Fundamentals of a Fasted State

For intermittent fasting to be effective, especially for metabolic benefits like ketosis and autophagy, you must avoid foods and drinks that trigger an insulin response. Insulin is the hormone that regulates blood sugar by moving it into your cells for energy. When you fast, insulin levels drop, and your body switches to burning stored fat for fuel. Consuming anything with a significant amount of calories, especially from carbohydrates and protein, will signal to your body that the fast is over, halting this process. Therefore, the central principle is to consume zero or near-zero calorie items during your fasting window.

Approved Liquids for Your Fasting Window

Staying hydrated is critical during any fast, and fortunately, several beverages are perfectly acceptable and can help manage hunger.

Plain Water: This is the gold standard. Still or sparkling water has no calories, sweeteners, or additives that could break your fast.
Black Coffee: Unsweetened, plain black coffee is acceptable and may even enhance fat oxidation due to the caffeine content. Avoid adding milk, cream, sugar, or artificial sweeteners, which could trigger an insulin response.
Unsweetened Tea: Herbal, green, and black teas are fine as long as they contain no sugar or cream. Like coffee, tea contains compounds that may aid in fat burning.
Bone Broth: While it contains a small number of calories, bone broth is often used in modified fasts to provide electrolytes and help curb hunger. It's high in protein, which can trigger an insulin response, so it's typically used in more relaxed fasting protocols or as a way to transition into or out of a longer fast.
Apple Cider Vinegar (diluted): A small amount of apple cider vinegar diluted in water is generally accepted during a fast and may offer metabolic benefits.

Navigating the 'Gray Area' with Caution

Some items exist in a gray area, and their effect on a fast can depend on individual sensitivity and specific fasting goals. For maximum metabolic benefits like autophagy, strict purists avoid anything but water, coffee, and tea.

Diet Soda and Artificial Sweeteners: The debate over diet drinks is contentious. While calorie-free, artificial sweeteners can trigger a cephalic phase insulin response, where the body expects sugar and releases insulin. They can also increase cravings and affect the gut microbiome. It's best to avoid them for a "clean fast".
Chewing Gum: Sugar-free gum contains minimal calories but carries similar risks to diet soda. The sweet taste can trigger an insulin response and some individuals find it increases their appetite. A single piece is unlikely to cause major issues for most, but it should be consumed with caution and in moderation.

Fasting-Friendly vs. Fast-Breaking Foods

Understanding the distinction is crucial for success.


Item	Calorie Count	Insulin Impact	Verdict	Best Practice
Water	0	None	Fast-Friendly	Drink liberally throughout the fasting period.
Black Coffee	2-5	Minimal	Fast-Friendly	Stick to plain black. Avoid all additives.
Unsweetened Tea	0-2	Minimal	Fast-Friendly	Choose herbal or black teas without added sweeteners.
Diet Soda	0	Varies	Gray Area	Best avoided for strict fasting due to artificial sweeteners.
Chewing Gum (Sugar-Free)	2-6	Varies	Gray Area	Use sparingly if needed to curb cravings; some feel it stimulates appetite.
Bone Broth	~30-50	Low	Modified Fast	A warm mug can curb hunger but contains a small number of calories.
Cream/Sugar in Coffee	Varies	High	Fast-Breaking	Reserve for your eating window as it spikes insulin.
Fruit Juice	Varies	High	Fast-Breaking	Loaded with sugar and calories; a definite fast-breaker.

The Goal of Fasting: Beyond Just Calorie Restriction

It's important to remember that intermittent fasting's benefits go beyond simple calorie restriction. The practice also aims to trigger metabolic switching, improve insulin sensitivity, and promote cellular repair processes like autophagy. These benefits are maximized when you maintain a strict fast, as even minor disruptions from artificial sweeteners or low-calorie items can potentially interfere with these deeper physiological processes. If your goal is strictly calorie control for weight loss, the effects of a diet soda might be negligible. However, if you are pursuing the full spectrum of metabolic health benefits, a cleaner fast is preferable. The sustained fat-burning state and the cellular cleaning process of autophagy are key drivers of many of the health improvements associated with fasting, and they are most effective when no metabolic activity is triggered.

What to Eat During Your Eating Window

While the focus of the fast is what to avoid, what you eat during your feeding window is equally important for long-term success. To maximize energy levels, prevent overeating, and maintain good health, focus on nutrient-dense whole foods.

Lean Proteins: Chicken, fish, eggs, and legumes help with satiety and muscle maintenance.
Healthy Fats: Avocados, nuts, seeds, and olive oil provide sustained energy and are crucial for hormone health.
Fiber-Rich Vegetables: Leafy greens, broccoli, and other non-starchy vegetables are packed with vitamins and minerals and help you feel full.
Whole Grains: Brown rice, quinoa, and oats offer complex carbohydrates and fiber for sustained energy.

When breaking a fast, especially a longer one, it's wise to start with a small, easily digestible meal like bone broth or a simple vegetable soup before moving on to larger meals to avoid shocking your digestive system.

Conclusion

For most intermittent fasters, the key to not breaking your fast is sticking to zero-calorie and additive-free beverages during your fasting period. Water, plain black coffee, and unsweetened tea are the safest choices. While items like bone broth and a minimal amount of MCT oil are sometimes accepted in modified protocols, consuming carbohydrates and protein will always break a fast. For optimal metabolic benefits and a truly 'clean' fast, it is best to avoid artificial sweeteners and chewing gum as well. By being mindful of your intake during your fasting window and focusing on nutrient-rich foods during your eating period, you can maximize the health benefits of intermittent fasting.

Here is some additional resource on the science of intermittent fasting

Frequently Asked Questions

No, adding cream, milk, or sugar to your coffee provides calories and will trigger an insulin response, which breaks your fast.

The consensus is that diet soda should be avoided during a fast. While calorie-free, the artificial sweeteners can trigger an insulin response and may increase cravings, potentially undermining the benefits of the fast.

Bone broth is often used in modified fasts to provide nutrients and curb hunger, but it does contain calories and protein, which can technically break a strict fast. It is a gray-area option for those who are less focused on a "clean fast".

Sugar-free gum is generally not recommended for strict fasting, as artificial sweeteners can stimulate an insulin response and potentially increase appetite. A single piece is unlikely to cause major metabolic issues, but it's best to avoid it for a clean fast.

As long as flavored water or herbal tea contains no sugar or artificial sweeteners, they are generally fine to consume during a fast. Check labels carefully to ensure they are truly zero-calorie.

A small squeeze of lemon juice in your water is generally acceptable during a fast. The calorie count is negligible and unlikely to trigger a significant insulin response.

A 'clean fast' means consuming only water, black coffee, and plain unsweetened tea during your fasting window. This approach is considered the most effective for maximizing metabolic benefits.