Yes, MCT oil and bulletproof coffee break a fast in the strict sense, because both have calories. A tablespoon of MCT oil has about 100 to 130 calories, and a typical bulletproof coffee has 200 to 450. Whether that matters depends on why you fast: they barely raise insulin and can keep you in ketosis, but they count toward your calories and end a strict, water-only fast.
MCT oil is popular with people who fast and follow keto because it’s a fat that turns into ketones quickly. Bulletproof coffee is coffee blended with MCT oil and butter. Here is what each does to your fast, by goal.
Does MCT Oil Break a Fast? It Depends on Your Goal
| Your reason for fasting | MCT oil | Bulletproof coffee |
|---|---|---|
| Weight loss | Counts toward your calories. A teaspoon is a small cost; tablespoons add up. | Counts as a small meal (200 to 450 calories). |
| Insulin and blood sugar | Pure fat raises insulin very little. | Very little effect on insulin, as long as it has no sugar or milk. |
| Ketosis | Keeps you in ketosis and can raise ketones. | Keeps you in ketosis. |
| A strict fast (water only, autophagy) | Breaks it. | Breaks it. |
This matches the ratings in our what breaks a fast tool, where you can also check butter, coconut oil, collagen and other coffee add-ins.
What Is MCT Oil?
MCT stands for medium-chain triglycerides: fats whose fatty acids are 6 to 12 carbon atoms long. The main ones are:
- Caproic acid (C6)
- Caprylic acid (C8)
- Capric acid (C10)
- Lauric acid (C12)
Most MCT oil sold as a supplement is made from coconut or palm kernel oil and is mainly C8 and C10. Unlike most fats, MCTs go straight from the gut to the liver, where they are burned quickly or turned into ketones. They are still fat and still have calories: they are absorbed and used for energy, not “bypassed”.
What Is Bulletproof Coffee?
Bulletproof coffee is brewed coffee blended with MCT oil and unsalted butter (often grass-fed) until it froths like a latte. Entrepreneur Dave Asprey popularized it as part of his “Bulletproof Diet”, and it is common on keto and with one-meal-a-day eating.
To make one: blend a cup of hot coffee with 1 tablespoon of MCT oil and 1 tablespoon of butter for about 30 seconds. Start with a teaspoon of MCT oil if you’re new to it; more can upset your stomach.
Calories in Bulletproof Coffee
| Recipe | Calories | Saturated fat |
|---|---|---|
| 1 tbsp butter (USDA: 102 calories, 7.2 g saturated fat) | 102 | 7 g |
| 1 tbsp MCT oil (varies by brand) | About 100 to 130 | About 14 g |
| 1 tbsp of each, in black coffee | About 200 to 230 | About 21 g |
| 2 tbsp of each | About 400 to 460 | About 42 g |
MCTs are saturated fats too. For comparison, the US Dietary Guidelines advise keeping saturated fat under 10% of calories, about 22 grams a day on a 2,000-calorie diet. One standard bulletproof coffee uses up nearly all of that.
Benefits of MCT Oil and Bulletproof Coffee While Fasting
They Can Make a Fast Easier
MCTs can make you feel full sooner than other fats. (1) If a morning coffee with MCT oil helps you reach a late first meal without snacking, it can help with weight loss despite its calories. Some people call this a “fat fast”: the calories are small and all from fat, so insulin stays low.
Small Help With Weight Loss
In a review of 13 trials (749 people), replacing other fats with MCTs led to about 0.5 kg (1 lb) more weight loss over the study periods, with no change in blood lipids. (2) That is a small effect, and it comes from swapping fats, not adding them. Adding 200 to 450 calories of bulletproof coffee to your normal diet works against weight loss.
Ketones
MCTs are turned into ketones quickly, which is why they are popular on keto. Ketosis on its own doesn’t cause fat loss; your calorie balance still decides that.
Downsides
- Stomach upset. Cramps, nausea and diarrhea are common at higher doses, especially on an empty stomach. Start with a teaspoon and build up slowly. See diarrhea during fasting.
- Lots of saturated fat. Butter-based bulletproof coffee every day adds a lot of saturated fat, which can raise LDL cholesterol in some people. Get your cholesterol checked if you drink it daily.
- Not a meal. Bulletproof coffee has calories but no protein, fiber, vitamins or minerals to speak of. Replacing breakfast with it means missing those.
- It isn’t a true fast. If you fast for autophagy or follow a water-only fast, both end it.
How to Use MCT Oil While Intermittent Fasting
- Decide what your fast is for. For a strict fast, take MCT oil in your eating window instead. For weight loss or keto, a small amount in your morning coffee is a reasonable trade-off.
- Start with 1 teaspoon and increase slowly to see what your stomach tolerates.
- Count it. Include the calories in your daily total. Our calorie deficit calculator sets your target.
- Skip the butter if you want the MCT effect with less saturated fat.
Learn more about fasting
FAQs
Can I have bulletproof coffee while fasting?
Only if your fast allows calories. It breaks a strict fast but keeps insulin low and keeps you in ketosis, so many keto and intermittent fasters use it. Count its 200 to 450 calories.
Does a teaspoon of MCT oil break a fast?
A teaspoon has about 40 calories. It technically breaks a strict fast, but for weight loss or keto most people find it a small, acceptable cost.
Does MCT oil put you into ketosis?
It raises ketone levels, and it helps you stay in ketosis on a low-carb diet. On a normal diet with plenty of carbs, it won’t keep you in ketosis on its own.
Does MCT oil burn belly fat?
Not on its own. Swapping other fats for MCTs led to slightly more fat loss in trials, about 0.5 kg on average. (2) Your total calories matter far more.
What is the difference between bulletproof coffee and regular coffee?
Black coffee has about 2 calories and doesn’t break a fast. Bulletproof coffee adds MCT oil and butter, which brings it to 200 calories or more.
Should I cook with MCT oil?
It isn’t a good cooking oil: it has a low smoke point. Add it to coffee, smoothies or dressings instead.
The Bottom Line
MCT oil and bulletproof coffee both break a strict fast because they have calories. They barely raise insulin and they keep you in ketosis, so for weight loss or keto a small amount can make a fast easier to stick to. Count the calories, start with a teaspoon, and go easy on the butter.
References
Fitness Volt is committed to providing our readers with science-based information. We use only credible and peer-reviewed sources to support the information we share in our articles.
- St-Onge MP, Jones PJ. Physiological effects of medium-chain triglycerides: potential agents in the prevention of obesity. J Nutr. 2002;132(3):329-332.
- Mumme K, Stonehouse W. Effects of medium-chain triglycerides on weight loss and body composition: a meta-analysis of randomized controlled trials. J Acad Nutr Diet. 2015;115(2):249-263.
