Does bulletproof coffee break a fast?

What's in a cup

Ingredient	Calories	Fat
Butter (14 g)	About 100	11.5 g
Coconut oil (14 g)	About 120	13.5 g
MCT oil	Around 100 or more: check your label	Almost all of it
Black coffee	Almost none	None

Why it doesn't count as fasting

Fat on its own barely moves insulin, which is why butter coffee became popular with fasters. But a fast is also about where your fuel comes from. While you fast, insulin falls and your fat tissue releases stored fat for your muscles and liver to burn ². Drink a few hundred calories of butter and oil, and your body burns those first. The fat you'd like to lose waits its turn.

MYTH

Butter coffee keeps you fasting because it has no carbs.

FACT

Carbohydrates aren't the only thing that ends a fast. Calories from fat replace the fat your body would have burned from its own stores, even though insulin barely moves.

What about ketones from MCT oil?

MCT oil does raise ketones, which is part of its appeal. In a Canadian study of 9 adults, MCT oils raised blood ketones within hours, the C8 type far more than others, and about twice as much when taken without a meal ³. But those ketones come from the oil you drank, not from your fat stores. Ketones on a fast can be a sign your body has switched to burning its own fat; ketones after MCT oil mostly tell you that you had MCT oil.

If you love it

• Make it your first meal, the drink that breaks your fast, not something you have during it.
• Start your next fast after it, just as you would after any meal.
• Go slowly with MCT oil if you're new to it: large amounts upset some people's stomachs.
• Have a protein-rich meal later in your window. Butter coffee has almost no protein, and protein is what helps you keep muscle while you lose fat ⁴.

The fasting coffee

For a coffee that keeps your fast, drink it black, or with a small splash of milk if that's what makes it possible (see milk in coffee). Black coffee even nudges your body the same way as the fast: in a laboratory study, a large dose of caffeine doubled the rate at which fat was released into the blood ⁵. Stay at or under about four 240 ml (8 oz) cups a day ⁶.

Sources

1. U.S. Department of Agriculture, Agricultural Research Service. FoodData Central: Butter, salted (FDC ID 173410); Oil, coconut (FDC ID 171412). SR Legacy. fdc.nal.usda.gov/
2. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi.org/10.1002/oby.22065
3. St-Pierre V, Vandenberghe C, Lowry CM, et al. Plasma Ketone and Medium Chain Fatty Acid Response in Humans Consuming Different Medium Chain Triglycerides During a Metabolic Study Day. Front Nutr. 2019;6:46. doi.org/10.3389/fnut.2019.00046
4. Leidy HJ, Clifton PM, Astrup A, et al. The role of protein in weight loss and maintenance. Am J Clin Nutr. 2015;101(6):1320S-1329S. doi.org/10.3945/ajcn.114.084038
5. Acheson KJ, Gremaud G, Meirim I, et al. Metabolic effects of caffeine in humans: lipid oxidation or futile cycling? Am J Clin Nutr. 2004;79(1):40-6. doi.org/10.1093/ajcn/79.1.40
6. Wikoff D, Welsh BT, Henderson R, et al. Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and children. Food Chem Toxicol. 2017;109(Pt 1):585-648. doi.org/10.1016/j.fct.2017.04.002