Is Bulletproof Coffee OK When Fasting?

October 10, 2025 •

4 min read

According to research from Johns Hopkins University, intermittent fasting promotes a metabolic state where the body burns fat for energy, but introducing calories disrupts this process. So, is bulletproof coffee ok when fasting? The answer depends on your specific fasting goals, as the added fats technically break a traditional fast.

Quick Summary

The impact of bulletproof coffee on fasting depends on your goals, as the fats technically break a pure fast due to their caloric content. While it won't raise insulin like carbs, it does provide fuel, which can hinder some benefits like autophagy. However, it can help manage hunger and support ketosis for certain fasting variations.

Key Points

Not for Strict Fasting: Bulletproof coffee contains calories from fat, which technically breaks a traditional or "pure" fast aimed at maximizing cellular repair (autophagy).
OK for Ketogenic Fasting: For those prioritizing ketosis and a fat-burning state, bulletproof coffee is often used to suppress hunger and provide energy, as it doesn't cause a significant insulin spike like carbs.
Timing is Key: If incorporated, it's best to consume bulletproof coffee at the beginning of your eating window rather than during a true fasting period.
Moderation is Important: As a high-calorie beverage, drinking too much bulletproof coffee can provide excess energy, potentially slowing down weight loss by using dietary fat for fuel instead of stored body fat.
Listen to Your Body: Individual responses vary. Start with small amounts of MCT oil and grass-fed butter to avoid digestive issues and see how your body responds.

The Controversy: Does Bulletproof Coffee Break a Fast?

The debate over whether bulletproof coffee is acceptable during a fast is nuanced and centers on one key point: caloric intake. A traditional or "pure" fast involves consuming zero calories to put the body into a state of metabolic rest. Because bulletproof coffee—a blend of brewed coffee, MCT oil, and grass-fed butter—contains a significant number of calories (up to 200-300 per cup), it does technically end this state. However, the impact varies based on your specific fasting objectives, like weight loss, metabolic health, or cellular repair (autophagy).

The Role of Macronutrients

Not all calories are processed the same way by the body. Carbohydrates and proteins trigger an insulin response to metabolize glucose, whereas the fats in bulletproof coffee do not cause a significant insulin spike. This low insulin response is why advocates argue it doesn't disrupt the "spirit" of an intermittent fast, especially for those focused on staying in a fat-burning state. The added fats provide a readily available source of energy, and MCT oil is quickly converted into ketones by the liver, which can help sustain focus and suppress appetite during the fasting window.

Fasting Goals and the Bulletproof Coffee Question

Your personal health objectives are critical in determining if bulletproof coffee is right for your fasting routine. For those focused strictly on achieving the maximum benefits of autophagy, any caloric intake, even from fats, is considered counterproductive. For others, particularly those practicing a ketogenic diet, the benefits of staying in ketosis with sustained energy outweigh the technical break of the fast.

Here's a breakdown based on different goals:

For Strict Fasting and Autophagy: A true, or "clean," fast requires zero calories. Since bulletproof coffee contains a high number of calories, it will interfere with the body's cellular repair processes, even if it doesn't cause an insulin spike. In this case, it is not recommended.
For Ketogenic Fasting: Many in the keto community use bulletproof coffee as a tool. It provides energy, suppresses hunger, and boosts ketone production without the carbohydrates that would kick you out of ketosis. It can help extend the fasting window and make the experience more manageable.
For Weight Loss: The high fat content can be a double-edged sword. While it provides satiety and can help extend your fast by curbing hunger, the calories could slow down weight loss by providing an alternative fuel source. Your liver may burn the fat from the coffee instead of your stored body fat.

Comparison of Coffee Options During Fasting


Feature	Black Coffee	Bulletproof Coffee	Coffee with Milk/Sugar
Calories	Near zero (3-5 kcal)	High (200-300+ kcal)	Moderate to High
Insulin Response	Minimal/None	Minimal	Significant Spike
Fat Burning	Supports enhanced fat burning	May slow body fat burning	Stops fat burning
Appetite	Suppresses appetite	Strongly suppresses appetite	Can cause hunger crashes
Autophagy	Unhindered/Potentially enhanced	Hindered due to calories	Hindered
Best For	Strict or clean fasting	Fat-adapted keto fasting	Eating window only

Potential Benefits and Drawbacks

Drinking bulletproof coffee while on a modified fast has potential upsides, such as providing a steady energy source and reducing hunger, making the fasting period feel easier. The MCT oil, in particular, is lauded for its ability to boost mental clarity. However, the high-fat beverage is also low in micronutrients compared to a balanced meal, making it a poor breakfast replacement. There are also potential drawbacks, including its high saturated fat content, which can impact heart health in some individuals. For those with sensitive stomachs, the fats can cause digestive issues.

How to Incorporate Bulletproof Coffee into Your Routine

If you decide that bulletproof coffee aligns with your fasting goals, timing and moderation are essential.

Timing: Consume it at the start of your eating window if you are doing intermittent fasting (e.g., 16/8 method). For a fat fast or keto-focused approach, have it in the morning to extend your fasted state without consuming carbs. Many recommend consuming only one cup a day.
Preparation: Always blend your bulletproof coffee to ensure the fats are properly emulsified, creating a smooth, latte-like texture. Simply stirring the fats will leave an unappealing, oily layer on top.
Quantity: Start with smaller amounts of MCT oil and butter to avoid digestive discomfort, and gradually increase as your body adapts. Remember that it's a high-calorie drink, and excessive consumption can hinder weight loss.

Conclusion: Making the Right Choice for Your Fasting Goals

In the end, the question of whether bulletproof coffee is okay during fasting is not a simple yes or no, but a reflection of your specific health goals. For a pure fast focused on cellular autophagy and metabolic rest, the caloric content makes it a non-starter. However, for those using intermittent fasting to support a ketogenic lifestyle or manage hunger, bulletproof coffee can be a valuable tool. It provides sustained energy and supports ketone production without a major insulin spike. The key is to be mindful of your overall objectives, listen to your body, and understand that consuming calories, even from pure fat, will alter your fasted state to some degree.

Frequently Asked Questions

No, bulletproof coffee will not completely stop fat burning, especially for individuals on a ketogenic diet. The fats, particularly MCT oil, are converted into ketones, which the body uses for energy. However, the liver will prioritize burning these dietary fats over your stored body fat, which can temporarily pause or slow down the weight loss process.

No, adding collagen to your bulletproof coffee will break your fast. Collagen is a protein that provides calories and triggers an insulin response, disrupting the fasting state and preventing benefits like gut rest and cellular repair.

Yes, MCT oil contains calories and will technically break a pure fast, just like butter. However, since it doesn't cause an insulin spike, many people on a ketogenic diet use it to maintain energy and stay in a fat-burning state.

A "clean" fast is a traditional fast with zero caloric intake, typically allowing only water, black coffee, and herbal tea. A "dirty" fast allows for a minimal number of calories, such as those from bulletproof coffee, that don't trigger a significant insulin spike. While dirty fasting may still support some weight loss, it can interfere with other fasting benefits like autophagy.

Bulletproof coffee is not a nutritionally complete breakfast replacement. While it provides calories and can suppress appetite, it lacks essential vitamins, minerals, and fiber found in whole foods. It is a caloric shortcut designed to fuel ketosis, not a comprehensive meal.

Yes, especially when you first start drinking it. The fats, particularly MCT oil, can cause digestive upset, including stomach pain, bloating, and diarrhea. It's recommended to start with a smaller amount of MCT oil and gradually increase it as your body adjusts.

For a strict fast, the best alternative is plain black coffee, as it has minimal calories and does not disrupt the fasting state. For those who need a milder taste, unsweetened herbal tea is another excellent zero-calorie option.