Yes. Bulletproof coffee is coffee blended with butter and MCT oil, which adds roughly 200 to 450 calories per cup, so it ends a clean, calorie-free fast. It fits in your eating window, not your fasting window. Some keto and "fat fasting" followers still drink it because it keeps carbs low, but that is a different goal than a true fast.
Key takeaways
- Bulletproof coffee breaks a clean fast. The butter and MCT oil are almost pure fat, and fat carries about 9 calories per gram, more than double the calories in carbs or protein. [3]
- A typical cup adds roughly 200 to 450 calories, depending on how much butter and oil you use. That is a small meal, not a fasting-window drink. [3]
- Fasting works by keeping the fasting window free of calories; the usual choices during a fast are water, black coffee, and plain tea. [1]
- Keto and "fat fasting" followers sometimes allow it because fat barely moves insulin, but low-insulin is not the same as calorie-free. If your goal is a clean or strict fast, it does not qualify.
- Bulletproof coffee is not a required or proven health drink. It is high in saturated fat, and too much saturated fat can raise LDL ("bad") cholesterol for some people. [2]
- If you watch your cholesterol, have heart disease, or take medication for it, be cautious with daily butter and MCT oil, and talk with a clinician. [2]
- GoFasting can help you log your fasting window, calorie intake, water intake, weight, and steps; if you drink bulletproof coffee, log it as intake. Separately, notice your own energy and hunger.
Does bulletproof coffee break a fast?
Yes. If your goal is a clean, calorie-free fasting window, bulletproof coffee breaks the fast, because the butter and MCT oil add a meaningful number of calories. [3] Intermittent fasting works by separating an eating window from a fasting window, and during the fast the usual guidance is water and zero-calorie drinks such as black coffee or plain tea. [1]
So the honest answer is not "it depends on who you ask." For a true fast, adding fat to coffee ends the fast. The more useful question is what you want the fast to do and where the coffee belongs in your day. For most people, bulletproof coffee fits inside the eating window, not the fasting window.
What bulletproof coffee is, and why it is not a fasting drink
Bulletproof coffee, popularized by Dave Asprey, blends brewed coffee with grass-fed butter and MCT (medium-chain triglyceride) oil into a high-fat, low-carb drink. It is marketed as a breakfast replacement, mostly for people following a ketogenic diet, because the fat is meant to provide steady energy and MCT oil is converted to fuel quickly.
None of that changes the calorie math. Plain black coffee is essentially calorie-free, which is why it is a standard fasting-window drink. [1] The moment you blend in a tablespoon or two of butter and a tablespoon of MCT oil, you have added a small meal's worth of calories. That is the line a clean fast is built around.
Why bulletproof coffee breaks a clean fast
A fast is defined by the absence of calories, not by the absence of carbs. Fat is the most calorie-dense nutrient: it provides about 9 calories per gram, more than double the roughly 4 calories per gram in carbohydrate or protein. [3] Butter and MCT oil are almost entirely fat.
A single tablespoon of butter or oil is already around 100 calories. A common bulletproof recipe uses one to two tablespoons of butter plus about a tablespoon of MCT oil, which lands a cup in the range of roughly 200 to 450 calories. [3] Whether you call that "breakfast" or "coffee," your body is taking in energy, so a clean fast is over.
This is why bulletproof coffee belongs on the eating side of your schedule. It is not a way to extend a fast; it is a way to start eating with a fat-heavy drink.
Bulletproof coffee and fasting: a quick decision guide
| Situation | Bulletproof coffee during fasting? | Better move | Why |
|---|---|---|---|
| Clean, calorie-free fast | No | Water, black coffee, plain tea | Butter and MCT oil add calories and end the fast. [1][3] |
| Strict fast for weight management | No | Keep the fast clean; save fat for meals | Those calories count toward your daily intake. [3] |
| Eating window, want a fatty coffee | Yes, in the window | Treat it as part of a meal and count it | It is food, so plan for the calories. [3] |
| Keto or "fat fasting" goal | Only if you accept it is not a true fast | Decide which goal you actually want | Low insulin is not the same as zero calories. |
| Watching cholesterol or on heart medication | Be cautious | Limit daily butter and MCT; ask a clinician | High saturated fat can raise LDL for some people. [2] |
"But keto followers say it won't break your fast"
You will see the claim that bulletproof coffee does not break a fast because fat barely raises insulin. This deserves an honest answer rather than a yes-or-no.
It is true that fat causes little insulin response, so if your only goal is to stay in ketosis or blunt hunger, a fatty coffee interferes less than a sugary drink would. That is the logic behind "fat fasting." But keeping insulin low is a different goal from a calorie-free fast. Bulletproof coffee still delivers 200 to 450 calories, so it does not meet the definition of a clean or strict fast, and benefits that depend on a true fasting state are reduced once you add that fuel. [3]
In short: if your goal is low-carb energy, bulletproof coffee can fit that. If your goal is an actual fast, it does not. It is fair to choose the fat-heavy coffee, but not accurate to call the window a fast afterward.
Better choices for the fasting window
If you want to keep the fast clean and still have coffee, the reliable options are simple.
- Black coffee, with nothing added, stays essentially calorie-free and is a standard fasting-window drink. [1]
- Plain tea, hot or iced, without milk, sugar, or oil. [1]
- Water, still or sparkling, which also helps with hunger between meals.
Save the butter and MCT oil for your eating window, where a fatty coffee can be part of a planned meal instead of a break in the fast.
How GoFasting can help you stay aware
The bulletproof coffee decision gets easier when your fasting boundary and your intake are both visible. GoFasting can help you log your fasting window, calorie intake, water intake, weight, and steps, so a fat-heavy coffee shows up as intake rather than slipping past unnoticed. If you drink it, log it in your eating window and let it count toward the day. Separately, pay attention to your own energy, hunger, and whether the routine still feels sustainable.
Keep the fast clean, then decide what fits
Use GoFasting to track the basics, then judge whether a fatty coffee belongs in your window or your meal.
- Fasting window — Keep bulletproof coffee in the eating window.
- Calories — Log the drink as intake, not as "free."
- Water — Track hydration during the fast.
- Weight and steps — Review trends, not a single morning.
When should you be cautious with bulletproof coffee, or ask a clinician first?
⚠️ Bulletproof coffee is high in saturated fat. Eating too much saturated fat can raise LDL ("bad") cholesterol, which raises the risk of heart disease and stroke. If you have high cholesterol or heart disease, take cholesterol or heart medication, are pregnant or breastfeeding, or have a digestive condition, do not make daily bulletproof coffee a routine without talking to a clinician first. [2]
Two points matter most here. First, this is a health choice, not a health requirement: bulletproof coffee is not a proven or necessary drink, and skipping it costs you nothing nutritionally. Second, the saturated fat adds up. The American Heart Association suggests keeping saturated fat under about 6% of daily calories, and butter and tropical-source oils are named sources of saturated fat. [2] A daily cup built on butter and MCT oil can use up much of that limit before you have eaten anything else. If you are watching your cholesterol, the safer move is to limit how often you drink it and to raise it with your clinician. [2]
FAQ
Does adding butter to coffee break a fast?
Yes, for a clean fast. Butter is almost pure fat, and fat carries about 9 calories per gram, so even a tablespoon adds enough calories to end a calorie-free fast. Keep it for your eating window. [3]
Does MCT oil in coffee break a fast?
Yes, if your goal is a strict, calorie-free fast. MCT oil is a fat and supplies calories, so it ends a clean fast even though it barely affects insulin. Some keto followers accept it, but that is a low-carb goal, not a true fast. [3]
How many calories are in bulletproof coffee?
Usually around 200 to 450 calories per cup, depending on how much butter and MCT oil you use. Fat provides about 9 calories per gram, so a fat-heavy drink adds up quickly. [3]
Is bulletproof coffee healthy?
It is a personal choice, not a proven health drink. It is high in saturated fat, and too much saturated fat can raise LDL cholesterol for some people, so it is not a "healthier" fasting option than black coffee. [2]
What can I drink during a fast instead?
Water, black coffee, and plain tea are the standard calorie-free choices that keep a fast clean. Add the fat to a coffee in your eating window if you want it. [1]
Can I have bulletproof coffee if I watch my cholesterol?
Be cautious. Because it is high in saturated fat, limit how often you drink it and check with a clinician, especially if you have high cholesterol, heart disease, or take related medication. [2]
Medical disclaimer: This article is for educational purposes only and is not medical advice. Speak with a qualified healthcare professional before making bulletproof coffee a regular habit if you have high cholesterol, heart disease, or a digestive condition, take medication, are pregnant or breastfeeding, or are unsure whether fasting is appropriate for you.
References
- Johns Hopkins Medicine. Intermittent Fasting: What Is It, And How Does It Work? Accessed July 7, 2026 https://www.hopkinsmedicine.org/health/expert-qa/intermittent-fasting-what-is-it-and-how-does-it-work
- American Heart Association. Saturated Fats. Accessed July 7, 2026 https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/saturated-fats
- MedlinePlus, U.S. National Library of Medicine. Dietary fats explained. Accessed July 7, 2026 https://medlineplus.gov/ency/patientinstructions/000104.htm