What breaks your fast? The definitive guide to fasting success

January 7, 2026 •

4 min read

Research indicates that intermittent fasting can potentially improve metabolic health and support weight management by shifting the body from burning sugar to fat. However, the success of any fast depends entirely on understanding and avoiding substances that trigger a metabolic response, effectively breaking the fasted state.

Quick Summary

This guide details which foods, beverages, and supplements can interrupt a fast, explaining the different rules for strict and modified fasting protocols. It outlines how calories and macronutrients affect metabolic processes like autophagy and insulin response.

Key Points

Calorie-containing substances are fast-breakers: Any food or beverage with calories, especially from carbohydrates or protein, will trigger an insulin response and break a fast.
Black coffee and unsweetened tea are generally safe: Plain, black coffee and herbal tea contain negligible calories and can help with appetite and focus without breaking a fast.
Artificial sweeteners are a gray area: While zero-calorie, artificial sweeteners in diet sodas may trigger a metabolic response in some individuals, potentially disrupting fasting benefits.
Know your vitamins and supplements: Water-soluble vitamins in capsule form (like B and C) are safe, but fat-soluble vitamins (A, D, E, K) need to be taken with food to be absorbed. Avoid sugary gummies or powders.
Accidents happen, don't give up: If you accidentally break your fast, simply return to your schedule. The long-term benefits come from consistency, not perfection.
Consult a doctor for medications: Never alter medication schedules to accommodate fasting without first consulting a healthcare professional. Injected medications may be fine, but oral ones may need adjustment.

Understanding the Fundamentals of Fasting

At its core, fasting involves abstaining from calorie intake for a specific period to trigger beneficial metabolic changes. These changes, like shifting to fat-burning (ketosis) and cellular cleanup (autophagy), offer health benefits including improved insulin sensitivity and weight management. However, the rules for what constitutes a fast-breaker can vary depending on your specific health goals.

The Golden Rule: Calories and Insulin Response

For most fasting protocols, the simplest rule is to avoid any substance that contains calories. Any caloric intake, especially from carbohydrates and proteins, triggers an insulin response. This insulin spike halts the processes your body activates during fasting, essentially ending the fasted state. While fat has a minimal impact on insulin, even small amounts of fat from sources like bulletproof coffee will technically break a fast, especially if the goal is autophagy. For weight loss, a flexible approach might allow up to 50 calories, but this is a modified approach, not a strict fast.

The Impact of Macronutrients

Carbohydrates and Sugar: These are the most significant fast-breakers. Even small amounts of sugar or carbs cause a rapid insulin spike, instantly pulling the body out of a fasted, fat-burning state. This includes obvious sources like sugary drinks, as well as hidden sugars in supplements and sauces.
Protein: Protein causes a moderate insulin response. Consuming protein, such as collagen powder, bone broth, or amino acids like BCAAs, provides calories and amino acids that the body must process, interrupting autophagy.
Fats: While fat has a minimal effect on insulin, it still contains calories. For strict fasts, this means even a small amount of oil is off-limits. However, for modified fasting focused on metabolic health, some people find adding a minimal amount of fat (like MCT oil) can help manage hunger without a significant insulin spike.

Common Suspects: Drinks, Supplements, and More

Many people wonder about the most common items consumed during a fasting window. Here is a breakdown of what to avoid and what is generally safe.

Beverages:

Black Coffee and Tea: Plain, black coffee and unsweetened tea are generally accepted as fasting-friendly because they contain negligible calories. In fact, caffeine can help suppress appetite and may even enhance fasting benefits like autophagy. Avoid adding milk, cream, sugar, or flavored syrups.
Diet Soda and Artificial Sweeteners: This is a gray area. While diet sodas are calorie-free, the artificial sweeteners (e.g., aspartame) can trigger a cephalic phase insulin response in some individuals. This metabolic confusion can undermine fasting benefits. Many purists recommend avoiding them, though for some, they may help manage cravings.
Water and Sparkling Water: Plain water is the gold standard for hydration during a fast. Sparkling water is also safe, as long as it contains no added sugars or artificial sweeteners.

Supplements and Medication:

Vitamins: Whether vitamins break a fast depends on the type. Water-soluble vitamins (B and C) are generally fine, especially if taken in capsules without added sweeteners. Fat-soluble vitamins (A, D, E, K), however, require fat for absorption and should be taken with a meal. Gummy or chewable vitamins almost always contain sugar and will break your fast.
Medication: This requires a doctor's consultation. Some medications, particularly oral ones, may break a fast. Injected medications (like insulin) and topical creams generally do not. Never change your medication schedule for fasting without medical advice.

Oral Hygiene and Other Actions:

Brushing Teeth: Brushing your teeth with toothpaste will not break a fast, as long as you are careful not to swallow it. Swallowing traces of toothpaste is typically not enough to trigger a metabolic response.
Chewing Gum: Sugar-free gum is a potential fast-breaker. The act of chewing can stimulate digestive enzymes, and artificial sweeteners could cause an insulin response. It's best to avoid it for a strict fast.
Intentionally Vomiting: Inducing vomiting intentionally breaks a fast, particularly in religious contexts. Unintentional vomiting does not.

Comparison: Fasting-Friendly vs. Fast-Breaking Items


Item	Fasting-Friendly Status	Reason
Black Coffee	✅ Yes	Contains negligible calories; appetite suppression.
Coffee with Cream/Sugar	🚫 No	Adds calories and triggers insulin response.
Plain Water	✅ Yes	Zero calories; crucial for hydration.
Diet Soda	⚠️ Gray Area	Zero calories, but artificial sweeteners may cause an insulin response in some.
Herbal Tea (Unsweetened)	✅ Yes	Zero calories and can provide flavor.
Protein/Collagen Powder	🚫 No	Contains calories and triggers insulin.
Gummy Vitamins	🚫 No	Contains sugar and calories.
Water-Soluble Vitamins (Capsule)	✅ Yes	No calories; well absorbed without food.
Fat-Soluble Vitamins (A, D, E, K)	🚫 No (Best with food)	Requires fat for proper absorption; should be taken during eating window.
Bone Broth	🚫 No (Modified fast only)	Contains calories and protein; acceptable for some less-strict fasts.

Conclusion: Consistency Over Perfection

Understanding what breaks your fast is essential for maximizing the benefits of your chosen protocol. For a strict fast aiming for autophagy, the rule is simple: consume nothing with calories. For most intermittent fasting for weight loss, a flexible approach may be more sustainable, but the safest options remain water, black coffee, and unsweetened tea. Accidents can happen, but they don't negate your overall progress. Simply get back on track with your next fasting window. The most effective strategy is the one you can adhere to consistently over the long term. For more in-depth research on the health effects of fasting, you can review findings published in publications like the New England Journal of Medicine.

Frequently Asked Questions

No, black coffee does not break a fast as it contains a negligible amount of calories. In fact, it can help suppress appetite and increase alertness during a fasting window.

This is a gray area. While calorie-free, the artificial sweeteners in diet soda may cause an insulin response in some individuals. For a strict fast, it is best to stick to water, black coffee, or unsweetened tea.

It depends on the type. Water-soluble vitamins in pure capsule form generally won't. However, fat-soluble vitamins require food for absorption and should be taken during your eating window. Any supplement with sugar or calories will break your fast.

For a strict fast, any caloric intake will break it. For those focusing on weight loss, a modified approach might allow up to 50 calories from fat, but this is less strict. For maximum benefits like autophagy, aiming for zero calories is recommended.

Accidentally eating will temporarily break your fast, but it is not a reason to give up. The key is to simply resume your fasting schedule and not be discouraged. Consistency over time is what matters most.

It is best to avoid chewing gum during a fast. The chewing motion can stimulate digestive processes, and the artificial sweeteners in sugar-free gum could potentially cause an insulin response.

Bone broth contains protein and calories, which will break a strict fast, especially one focused on autophagy. Some less-strict, modified fasting protocols may permit it to help with hydration and electrolytes.