You’ve probably seen both bulletproof coffee and intermittent fasting pop up on wellness blogs and podcasts, but what about putting them together?
Will a butter-infused coffee during your fasting window undo the benefits?
If you’ve Googled this before, you’ve probably found that advice is all over the place. That’s because breaking a fast isn’t as black-and-white as it sounds. It all hinges on why you’re fasting.
In this article, we’ll take you through the science behind bulletproof coffee’s ingredients, how they interact with different fasting goals, and how to decide if it fits into your fasting routines.
What Is Bulletproof Coffee?
Bulletproof coffee is a rich blend of black coffee, MCT oil, and unsalted grass-fed butter or ghee.
Created as a high-fat, carb-free energy drink, it can help boost focus and fuel the body without spiking your blood sugar.
It’s creamy, calorie-dense, and a go-to drink for keto enthusiasts looking for something to keep hunger at bay.
What Is Intermittent Fasting?
Though it’s often confused with a diet, intermittent fasting is a timing strategy. The idea is to cycle between periods of eating and not eating.
Some people fast for 16 hours and eat within an 8-hour window; others go longer, like 24-hour fasts once or twice a week.
The goals are also varied. Some people fast to push the body into ketosis (burning fat for energy instead of carbs), while others are trying to trigger autophagy (cleaning out old, damaged cells for new, healthy ones), improve insulin sensitivity, or just simplify their eating habits.
Does Bulletproof Coffee Break a Fast: The Technical Answer
Technically, yes. Any caloric intake breaks a fast. Traditional fasting means no protein, carbs, or fat. Water, black coffee, and plain tea are the only true zero-cal options.
Bulletproof coffee is loaded with fuel. Depending on how much butter and MCT oil you add, you’re looking at 200–500 calories almost entirely from fat. That means your fast is officially over.
However, fat has a minimal impact on blood sugar and insulin. Your body can often stay in a fat-burning (ketogenic) state, and you may still experience benefits like focus and reduced hunger.
So while Bulletproof coffee breaks a fast on paper, it can preserve some fasting benefits in practice.
Bulletproof Coffee's Impact on Different Fasting Goals
Not everyone fasts for the same results. Here’s how bulletproof coffee interacts with various goals:
- Ketosis — Supports it. MCT oil boosts ketones without spiking blood sugar.
- Autophagy — Disrupts it. Even fat can dial down cellular cleanup.
- Gut rest and digestive health — Disrupts it. Fat triggers digestion, so your gut stays active.
- Weight loss — Mixed. Curbs hunger but adds calories — plan around it.
- Insulin control — Supports it. Pure fat has minimal impact on insulin.
Scientific Evidence on Bulletproof Coffee and Fasting
While bulletproof coffee is widely discussed in wellness circles, scientific research on its effects during fasting is still emerging. To understand where it may or may not fit, it helps to break down what we do know about its ingredients and how the body responds.
Fat Consumption During Fasting Periods
We know that eating any calories breaks a fast, but fat has a much smaller effect on the body than carbs or protein.
For example, a systematic review of intermittent fasting and weight loss research highlighted that limited fat intake during fasting may not fully shut down the body’s shift into fat-burning mode [1].
This process, called “metabolic switching,” is what helps the body start using stored fat for fuel. Because fat doesn’t raise blood sugar or trigger much of an insulin response, it might allow some of the fasting benefits to continue, especially if your goal is to stay in ketosis.
However, fat isn’t completely invisible to your system. A 2012 study found that even pure fat causes a slight rise in insulin levels [2].
It’s not dramatic, especially compared to what happens after eating carbs, but it’s still a signal to the body that some form of fuel has arrived.
While fat-based drinks like bulletproof coffee don’t completely derail a fast, they do make a difference. But that difference only matters depending on your goal.
MCT Oil & Ketone Production
When added during a fast, MCT oil can raise ketone levels, even if you’re not cutting out carbs completely. In one study, healthy adults saw a clear increase in ketones after taking small amounts of MCT oil, and the more they took, the higher those levels went [3].
Other research suggests MCT oil helps with fat burning and modest weight loss, though results vary quite a bit from person to person [4].
Some people responded better than others, and the benefits often depended on diet, activity level, and dosage.
Expert Opinions and Anecdotal Evidence: What Can We Trust?
Among fasting experts, there’s also no firm consensus. They argue the same things: adding calories can interrupt certain processes, which is negative.
Or the impact fat can have is so minimal, you’ll still get certain fasting benefits like more energy and mental clarity.
At the same time, plenty of people online have reported success with adding bulletproof coffee, not so much in their intermittent fasting routine, but more so as a keto diet.
However, some have said the benefits they feel — energy boosts or appetite suppressant — they've gotten from plain black coffee.
Anecdotes should complement the research you find. They can help guide your experimentation and hone things to cut or add based on your specific goals, body type, diet, and so on. But make sure you lean more on data where it exists for your health decisions.
Alternatives & Modifications for Fasting-Friendly Coffee
If you’re trying to get more of the physiological benefits of fasting, like autophagy, gut rest, or insulin sensitivity, there are plenty of ways you can keep your morning coffee ritual without breaking your fast too early.
Here are some options:
Black Coffee
Plain black coffee is the most fasting-friendly choice. It has almost no amount of calories, won’t stimulate insulin production, and can help improve some fasting effects.
Minimal Impact Additives
If plain black coffee isn’t your thing, a few additions may be safe, depending on your goals. A splash of unsweetened almond milk or a dash of cinnamon won’t derail most metabolic benefits. Small amounts of salt or electrolytes can also help with energy and hydration. Just stay away from any type of sweetener.
Timing Bulletproof Coffee Wisely
One approach is to save your bulletproof coffee for the start of your eating window. That way, you get the mental clarity and appetite control without interrupting a fast. It's especially useful for the 16:8 schedule, and you want to keep the early hours low in calories.
Modified Bulletproof Coffee
To keep the benefits while minimizing the impact, try scaling back your fat ratios. Half a tablespoon of MCT oil instead of two, or swapping butter for a smaller amount of ghee, can significantly reduce the calorie load while supporting energy and mental sharpness.
FAQs: Bulletproof Coffee & Fasting
The answers below can break down the most common questions about bulletproof coffee and fasting.
1. Can You Do Bulletproof Coffee While Intermittent Fasting?
Yes, but it depends on your goals. It fits better with modified fasting approaches like 16:8, where you consume calories during your eating window or use fats to extend fasting benefits.
2. Does Bulletproof Coffee Put You in Ketosis?
It can help, as the MCT oil boosts ketone production, helping you get into ketosis faster. However, this effect doesn’t mean you’re in full nutritional ketosis unless you’re also restricting carbs.
3. Does Bulletproof Coffee Trigger Insulin?
Yes, but only slightly. Fat has a minimal insulin response compared to other energy types. For most, the small rise won’t disrupt metabolic goals, though people with insulin sensitivity issues should be careful.
4. Can You Lose Weight with Bulletproof Coffee?
It depends on a number of factors! It may help by reducing hunger and preventing snacking, which can support a calorie deficit. However, it still adds 200 to 500 calories, so you’ll have to adjust your diet so you can stay under your goal number.
5. What Is the Best Time to Drink Bulletproof Coffee?
Morning is the ideal time, especially during a fast, for energy and appetite control. Avoid it late in the day so the caffeine doesn’t mess with your sleep. Some prefer it as a pre-exercise drink for sustained fuel.
6. Does MCT Oil Stop Autophagy?
Probably, but it's not fully clear. Calories, especially from fat, are believed to reduce autophagy, but the human data are limited. If autophagy is your priority, it's safer to avoid MCTs during fasting periods.
7. Is It OK to Drink Bulletproof Coffee Every Day?
It can be, but monitor your health markers. Some people see increased LDL cholesterol from regular saturated fat intake. Others handle it well. If used daily, check your lipid panels and adjust based on the results.
8. What Are the Side Effects of MCT Oil?
Digestive issues like cramping, nausea, or diarrhea are common when starting. Begin with a small amount, around 1 tsp, and increase it gradually. You’ll also want to buy high-grade oils.
9. Do You Drink Bulletproof Coffee on an Empty Stomach?
Yes, many do. It’s often the first thing people drink in the morning during a fast. Some people with sensitive stomachs may feel discomfort, but adjusting the fat content or timing it can help.
10. Who Shouldn’t Drink Bulletproof Coffee?
Anyone with gallbladder disease, high cholesterol, or fat metabolism issues should avoid it. Pregnant or nursing women should check with a doctor before drinking it.
References
- Welton S, Minty R, O'Driscoll T, Willms H, Poirier D, Madden S, Kelly L. Intermittent fasting and weight loss: Systematic review. Can Fam Physician. 2020 Feb;66(2):117-125. PMID: 32060194; PMCID: PMC7021351.
- Klempel MC, Kroeger CM, Varady KA. Alternate-day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet. Metabolism. 2013 Jan;62(1):137-43. doi: 10.1016/j.metabol.2012.07.002. Epub 2012 Aug 11. PMID: 22889512.
- Nakamura, K., Hagihara, K., Nagai, N., Egashira, R., Takeuchi, M., Nakano, M., Saito, H., Moriguchi, M., Tonari, S., Fujii, H., Miyake, A., Omae, Y., & Ashida, K. (2023). Ketogenic effects of medium‑chain triglycerides containing formula and its correlation to breath acetone in healthy volunteers: A randomized, double‑blinded, placebo‑controlled, single dose‑response study. Frontiers in Nutrition, 10, Article 1224740.
- Fernández-Verdejo, R., Mey, J. T., & Ravussin, E. (2023). Effects of ketone bodies on energy expenditure, substrate utilization, and energy intake in humans. Journal of Lipid Research, 64, 100442.
