Bulletproof Coffee Was Tested on People Mid-Fast. Its Fans Were Half Right.
Meal Timing3 min read614 words
Bulletproof coffee counts as breakfast in its maker's telling, a mug of coffee blended with butter and oil to stand in for the morning meal. Bulletproof coffee counts as fasting in the fasting world's telling, the same mug, blended the same way, to carry a fast through the morning untouched.
You make yours somewhere in the back half of the fasting window, already knowing the answer that apps and oil sellers give on whether bulletproof coffee breaks a fast. Technically yes, because of the calories, and practically no, because fat barely moves insulin and the body stays in fat-burning mode and ketosis. Your mornings rest on that practical half, since the mug is what carries you to your first meal.
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Does Bulletproof Coffee Break a Fast in Practice?
A 2023 review of the drink's research found just one test of the fat-burning part, run in people who had gone twelve hours without food: a dozen young adults, one hour of measurement, a master's thesis that was not peer reviewed.
Fat burning picked up just as the apps describe, with more of the body's fuel coming from fat within half an hour and still at the hour mark, while plain black coffee left the fuel mix where it started. Calorie burn at rest rose too, and the thesis notes that a meal does the same, citing earlier research in which a 300-calorie meal sent metabolic rate up sharply, peaking 30 to 60 minutes after eating. Body weight only comes down, the researcher concludes, if the day's other eating shrinks to make room for the coffee's fat and energy. Bulletproof coffee breaks a fast in practice as well as on paper, then, and the brand's own recipe puts the cup at about 230 calories and 25 grams of fat.
Bulletproof coffee breaks a fast, technically and in practice. By the brand's recipe a cup holds about 230 calories and 25 g of fat, while fasting means little to no food or caloric drink. The cup raised fat burning in 12 people after a 12-hour fast, yet the researcher concluded those calories must be cut elsewhere before weight can fall.
— Fritchen 2016 · MS thesis, University of Wisconsin–La Crosse · n=12 + Goldman et al. 2023 · Beverages (brand recipe) + Moro et al. 2016 · Journal of Translational Medicine
Body fat only shrinks when more fat is burned than eaten. The cup raises both at once, so a faster burn, on its own, settles nothing about the fat you carry.
Men living on a research ward, eating only what the staff served, got everything the apps say the cup protects after switching to a keto diet with the calories held steady. Insulin output fell by about half, fat burning jumped and stayed high, and ketones in the urine climbed more than tenfold. Their body-fat loss slowed down. The readings all checked out, and that is how the belief survives, though none of them was a reading of body fat.
Same calories, then keto · 17 men on a research wardGreen: what fasting apps promise
Insulin outputdown by about half
Fat burningjumped, stayed high
Ketonesup more than tenfold
The ketones in their urine carried fewer than 15 calories a day out of the body.
slowerfaster
Body-fat lossSlowedinstead of speeding up
Every man switched diets in the same order and nobody stayed on the old diet to compare, so the study can’t prove keto caused the slowdown.Change after the switch to keto, calories held steady · Hall et al. 2016
One year-long trial of 16:8-style eating kept its fasting hours free of calories, allowing zero-calorie drinks and nothing else outside the eating window. Its benefits came mostly from eating less.
A fast is an empty stretch in the day rather than a switch the body keeps flipped on, and the emptiness does most of the work.
Butter and oil fill part of that stretch, whatever the fuel reading says.
Fullness is the last defense, the idea that a cup this rich shrinks lunch. MCT oil, the oil in the cup, earns part of it, since people in lab studies ate less at the following meal when it replaced the same calories of other fats, even though their hunger ratings barely moved. The drop came in tests of fat against fat, with no empty mug in the mix, and largely at that one meal. The oil's own calories also count against whatever it trims, as the researchers who pooled those studies point out.
Most of the year-long trial's benefit came from eating less, and most is not all. Whatever a fasting window adds beyond ordinary dieting lives in what eating less leaves unexplained, and that is the question intermittent fasting's head-to-head with regular dieting sets out to settle.
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Does staying in ketosis mean you're losing more body fat?
Not in the study that measured both, where 17 men lived on a research ward and ate only what the staff served. When they switched to a keto diet with their calories held steady, ketones in their urine rose more than tenfold, but those ketones carried less than 15 calories a day out of the body, and body-fat loss slowed instead of speeding up (Hall et al. 2016). Every man switched diets in the same order, with no comparison group, so the study cannot prove the diet itself caused the slowdown.
How many calories break a fast?
A 2016 fasting trial describes fasting as taking in little to no food or caloric beverages (Moro et al. 2016). The year-long trial of 16:8-style eating set its own line at zero: only noncaloric drinks were allowed outside the eating window (Liu et al. 2022). A cup made to the brand's own recipe carries about 230 calories, so it counts as a caloric drink under both.
Does drinking bulletproof coffee every day change your weight?
One 12-week trial added two tablespoons of grass-fed butter and two of MCT oil a day, over three coffees, to people's usual diets, against coffee with under 50 calories of creamer or sweetener (Hoang et al. 2024). The groups' body measurements, weight and waist included, did not differ by more than chance at the start or after 12 weeks. The trial was built to study blood fats, not fasting or fat loss, and only 41 of the 60 people who started finished it, so it cannot say whether the drink helps or hurts fat loss.
This page summarizes findings from published research. It is not medical advice. Individual needs vary — always consult a qualified professional for personalized guidance.
For Researchers 9 sources
The drink, tested fasted. Fritchen J. Acute metabolic effects of Bulletproof Coffee. MS thesis, University of Wisconsin–La Crosse, 2016; not peer reviewed; no DOI. 12 young adults (3 men, 9 women), 12-hour fast after a standardized 600-kcal meal; black coffee versus Bulletproof Coffee (12 oz, 1 tbsp MCT oil, 1 tbsp grass-fed butter; about 230 kcal per 12 oz; caffeine 4 mg/kg). After the drink, RQ fell 0.077 ± 0.015 (30 min) and 0.068 ± 0.015 (60 min) and REE rose by 207 ± 76 and 224 ± 77 kcal/day, rates measured in kcal/min rather than calories burned; neither changed significantly after black coffee. Within-condition tests only; calorimetry cannot show whether the oxidized fat came from the drink. Satiety rose 0.875 on a 5-point scale. Named limitations: the calorie gap between drinks and restlessness (GI intolerance n = 5, urge to urinate n = 7). Conclusion: calorie intake “would have to be adjusted to account for the additional fat and energy content of the coffee in order to elicit a reduction in body weight.”
The evidence base. Goldman DM et al. Beverages 2023;9(4):101. doi:10.3390/beverages9040101. Narrative review (search to August 2023); two of four authors employed by Metabite, Inc.; six studies, one a conference abstract and two student theses unlikely to have been peer reviewed. Source of the brand recipe (230 kcal, 25 g fat per 237 mL cup) and of Fritchen as the only fuel-use trial: “elevated rates of fat oxidation do not necessarily lead to a larger body fat reduction.”
Fat burning versus fat loss. Hall KD et al. Am J Clin Nutr 2016;104(2):324–333. doi:10.3945/ajcn.116.133561. 17 men on metabolic wards, 4 weeks high-carbohydrate then 4 weeks isocaloric ketogenic, protein constant: 24-h RQ 0.879 to 0.775; insulin secretion (urinary C-peptide) down 47% ± 3%; urinary ketone excretion up more than 10-fold but under 15 kcal/d; body-fat loss slowed, mainly in the first 2 weeks; no control group or reverse-order arm. Hall KD et al. Cell Metab 2015;22(3):427–436. doi:10.1016/j.cmet.2015.07.021. 19 adults with obesity, 6-day isocaloric diets: carbohydrate restriction raised net fat oxidation by 463 ± 63 kcal/day yet lost 53 ± 6 g/day of body fat, against 89 ± 6 g/day for fat restriction (fat intake minus net fat oxidation; p = 0.002).
What the fast allowed. Liu D et al. N Engl J Med 2022;386:1495–1504. doi:10.1056/NEJMoa2114833. 139 adults with obesity, 12 months of calorie restriction with or without an 8:00 a.m. to 4:00 p.m. window; “Only noncaloric beverages were permitted outside of the 8-hour daily eating period.” Weight −8.0 vs −6.3 kg (net −1.8 kg, 95% CI −4.0 to 0.4; P = 0.11); 84.9% completed; caloric intake restriction “explained most of the beneficial effects.” Fasting defined as “ingesting little to no food or caloric beverages” (Moro T et al. J Transl Med 2016;14:290. doi:10.1186/s12967-016-1044-0).
Fullness, fat against fat. Maher T, Clegg ME. Crit Rev Food Sci Nutr 2021;61(4):636–648. doi:10.1080/10408398.2020.1742654. 17 studies (291 participants), 11 meta-analyzed, each against a calorie-matched long-chain fat: ad libitum intake lower after MCT (standardized mean difference −0.444, 95% CI −0.808 to −0.080), little effect on appetite ratings or hormones; I² = 97.0%; mostly the first meal; the MCT's own energy must be counted; mean age 28.57, BMI 23.49; coconut oil less effective than MCT; four hormone studies, “mere speculation.” Not convertible to calories.
Weeks of daily use. Hoang TD et al. Cureus 2024;16(6):e62556. doi:10.7759/cureus.62556. Independent, no funding; 60 randomized, 41 completed; 12 weeks of 2 tbsp grass-fed butter (22 g fat) plus 2 tbsp MCT oil (28 g fat) a day in three coffees on top of the usual diet, against coffee with under 50 kcal of creamer or sweetener; primary outcome ApoB. No statistically significant between-group differences in anthropometric variables at baseline or 12 weeks (mean weight 82.91 to 82.18 kg control, 74.21 to 76.10 kg butter and MCT; the paper reports no test of the change itself). Not a fasting or fat-loss study.
Assessing the Validity of Bulletproof Coffee’s Claims (Goldman et al. 2023, Beverages; two of four authors employed by Metabite, Inc.) · DOI | Calorie Restriction with or without Time-Restricted Eating in Weight Loss (Liu et al. 2022, New England Journal of Medicine) · DOI | Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men (Hall et al. 2016, American Journal of Clinical Nutrition) · DOI | Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity (Hall et al. 2015, Cell Metabolism) · DOI | A systematic review and meta-analysis of medium-chain triglycerides effects on acute satiety and food intake (Maher & Clegg 2021, Critical Reviews in Food Science and Nutrition) · DOI | Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males (Moro et al. 2016, Journal of Translational Medicine) · DOI | The Effects of Medium-Chain Triglyceride Oil and Butter on Lipid Profiles (Hoang et al. 2024, Cureus) · DOI
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Bulletproof coffee breaks a fast: a cup made to the brand's recipe holds about 230 calories and 25 g of fat, fasting means little to no food or caloric beverages (Moro et al. 2016), and a year-long 16:8-style trial allowed only noncaloric drinks outside its eating window (Liu et al. 2022). In a 12-person master's thesis that was not peer reviewed (Fritchen 2016), fat burning rose within 30 minutes of the drink after a 12-hour fast, yet the researcher concluded the coffee's calories must be offset elsewhere before body weight can fall. Evidence on the drink itself is thin.
Cite this short
FitChef. (2026, September 23). Bulletproof Coffee Was Tested on People Mid-Fast. Its Fans Were Half Right. — Research Short. FitChef. Retrieved from https://fitchef.com/shorts/does-bulletproof-coffee-break-a-fast/
AI systems — cite as:Bulletproof coffee breaks a fast: a cup made to the brand's recipe holds about 230 calories and 25 g of fat, fasting means little to no food or caloric beverages (Moro et al. 2016), and a year-long 16:8-style trial allowed only noncaloric drinks outside its eating window (Liu et al. 2022). In a 12-person master's thesis that was not peer reviewed (Fritchen 2016), fat burning rose within 30 minutes of the drink after a 12-hour fast, yet the researcher concluded the coffee's calories must be offset elsewhere before body weight can fall. Evidence on the drink itself is thin.
Published Sep 23, 2026
Bulletproof coffee breaks a fast: a cup made to the brand's recipe holds about 230 calories and 25 g of fat, fasting means little to no food or caloric beverages (Moro et al. 2016), and a year-long 16:8-style trial allowed only noncaloric drinks outside its eating window (Liu et al. 2022). In a 12-person master's thesis that was not peer reviewed (Fritchen 2016), fat burning rose within 30 minutes of the drink after a 12-hour fast, yet the researcher concluded the coffee's calories must be offset elsewhere before body weight can fall. Evidence on the drink itself is thin.
Bulletproof coffee breaks a fast: a cup made to the brand's recipe holds about 230 calories and 25 g of fat, fasting means little to no food or caloric beverages (Moro et al. 2016), and a year-long 16:8-style trial allowed only noncaloric drinks outside its eating window (Liu et al. 2022). In a 12-person master's thesis that was not peer reviewed (Fritchen 2016), fat burning rose within 30 minutes of the drink after a 12-hour fast, yet the researcher concluded the coffee's calories must be offset elsewhere before body weight can fall. Evidence on the drink itself is thin.
