Dirty fasting: butter in your coffee isn't a fast. It's breakfast in a mug.
Dirty fasting lets in "a few" calories, usually fat, and still calls it a fast. Fat fasting goes further: days of almost nothing but fat. The marketing is loud. The evidence is quiet.
The short version
- What it is
- "Fasting" with some calories allowed: butter or MCT oil in coffee, a splash of cream, a snack. Fat fasting: several days of mostly fat.
- Good for
- Selling MCT oil and grass-fed butter.
- Bad at
- Being a fast. Also possibly bad for your LDL cholesterol.
- Evidence
- Very weak. A few small studies, a case report, a lot of blogs.
- Our take
- Excluded. Our fast is water, black coffee and green tea. Nothing else.
What dirty fasting actually is
"Clean" fasting means no calories during the fast: water, black coffee, plain tea. Dirty fasting means you allow some calories, often "under 50" or "under 100", and pretend that still counts. The most famous version is butter coffee (sold as "bulletproof coffee"): black coffee blended with butter and MCT oil, a refined fat made from coconut or palm oil.
Fat fasting is a cousin from the keto world: a few days of eating almost nothing but fat, at low total calories, to "kick-start" ketosis or break a weight-loss plateau. No proper trials have tested it.
The logic goes like this: fat barely raises insulin, so you "stay in fasting mode" while you eat it. That's half true and fully misleading.
How it works in your body
Pure fat does raise insulin less than sugar or protein. And yes, your liver turns MCT oil into ketones fast. But a fast isn't only about insulin. The whole point of a fast is that your body runs on its own fat. Give it fat from a mug and it burns that first.
Look at the numbers. One tablespoon of butter has about 100 calories.1 A typical butter coffee with a tablespoon of butter and a tablespoon of MCT oil comes to over 200 calories . That's not "a few calories". That's a small breakfast, minus the protein, minus the fibre, minus the vitamins.
What you eat, what you don't
Drink during the fast (the clean way)
- Water, still or sparkling
- Black coffee
- Green tea, plain black tea, herbal tea
- A pinch of salt in water if you get a headache
- Then break the fast with a real meal: eggs, fish, greens, olive oil
Forget it
- Butter coffee, "bulletproof" coffee, ghee coffee
- MCT oil, coconut oil or cream in your coffee
- "Fasting snacks", fat bombs, keto bars
- Bone broth sipped all morning "because it's only 40 calories"
- Sweeteners and "zero" drinks
- Days of eating only fat
A clean fast, start to finish
Here's what the alternative looks like on an ordinary 16-hour day, with nothing to blend:
- 8 pm: dinner ends. Kitchen closed.
- 7 am: a big glass of water. Black coffee or green tea.
- 10 am: hungry? More water, sparkling if it helps. Hunger comes in waves and passes.
- 12 noon: break the fast with a real meal: eggs, fish or tofu, greens, olive oil.
- Afternoon and evening: normal, sensible meals until 8 pm.
If noon feels impossible, make the fast shorter. Don't make it dirtier.
Dirty fasting in Singapore and in Austria, Germany, Switzerland
Singapore: a classic kopi comes with sugar and condensed or evaporated milk, which ends a fast on its own. Order kopi-o kosong (black, no sugar) or teh-o kosong. Nanyang-style beans are often roasted with sugar and margarine, but what ends up in the cup is as close to black coffee as a kopitiam gets. Skip the "keto" bulletproof drinks in cafés.
Austria, Germany, Switzerland: the café culture is built on milk: Melange, Cappuccino, Milchkaffee, Café crème. During the fast, order a Verlängerter or Kaffee schwarz, or an Americano. "Butterkaffee" and MCT oil sold in drugstores and health shops are the same idea with a German label. Our verdict doesn't change with the language.
Alcohol and dirty fasting
OK, now and then
- One glass of dry wine with a meal, inside your eating window
- Spirits with soda water and lime, with food
Out
- Any alcohol during the fast. "It's only one" is dirty fasting's favourite lie.
- Drinking on an empty stomach
- Cream liqueurs, cocktails, sweet wine
50 Club: one drink with dinner inside the eating window, never during the fast.
Alcohol puts fat burning on hold while your liver deals with it: two drinks cut it by about 73% in one small study.6 All the details are in our alcohol guide.
What dirty fasting is good for, with proof
Making a fast easier to start
To be fair: if a splash of milk is the difference between doing a 14-hour fast and doing none, the milk wins. That's the only honest case for it, and it's why our 50 Club allows a splash.
MCT oil and weight: a small effect
A meta-analysis of 13 trials found that replacing ordinary fats with MCT oil led to slightly more weight loss, about half a kilo.2 The authors called it modest and asked for independent confirmation. Note the word replacing: MCT instead of other fat, not on top of a fast.
The uncomfortable truth
"Butter coffee keeps you in fasting mode and sharpens your brain." A 2023 review went through the claims made for bulletproof coffee. It found no good evidence of better thinking or energy, mixed results on fullness, and case reports of raised cholesterol.3 In the one small trial we found, adding butter and MCT oil to coffee for 12 weeks nudged a cholesterol marker (ApoB) slightly up, while it went down in the plain coffee group. The authors said it "may be safe" but called for bigger studies.4
And butter itself? In a controlled trial, butter raised total and LDL ("bad") cholesterol compared with olive oil.5 The WHO recommends keeping saturated fat to 10% of your calories or less.7 A couple of tablespoons of butter in your morning coffee eats a big chunk of that before you've had any food.
The risks
- Cholesterol. Daily butter and coconut-based fats add a lot of saturated fat. Butter raises LDL compared with olive oil.5 If your LDL is already high, this is the wrong direction.
- Calories you don't notice. 200+ calories in a mug, every day, adds up. Many people wonder why they don't lose weight while "fasting".
- Crowding out real food. A fat-only breakfast has no protein, no fibre and few vitamins.
- Stomach trouble. Larger amounts of MCT oil can upset your stomach. Another reason not to drink it.
- Fat fasting on medication. Days of near-zero carbs and low calories can cause lows on diabetes drugs, and SGLT2 drugs plus very low carbs carry a risk of serious acid build-up. Not without your doctor.
Allergies: watch the staples
| Dirty-fasting staple | Allergen (EU list of 14)8 | Swap |
|---|---|---|
| Butter, ghee, cream | Milk | Black coffee. That's the point. |
| Collagen or protein "coffee creamers" | Often milk, sometimes fish | Black coffee, protein at your first meal |
| Fat bombs, keto bars | Often tree nuts, peanuts, milk, soy | A real meal inside your eating window |
| Nut-milk lattes | Tree nuts | Black coffee or plain tea |
Don't do dirty or fat fasting if you…
- have high LDL cholesterol or heart disease
- take insulin, sulfonylureas, SGLT2 or GLP-1 drugs
- have had your gallbladder removed or have pancreas problems
- are pregnant or breastfeeding
- are under 18 or have ever had an eating disorder
Healthen me asks about all of these and gives you a different plan if any apply.
How we use dirty fasting in your plan
We don't. It's excluded. Our fasting rule is short and clean: during the fast, water, black coffee and green tea. Nothing else. If that's too hard, the answer is a shorter fast, not butter.
When you break the fast, your first meal is real, low-carb food with protein: eggs, fish, greens, olive oil. That gives you the calm blood sugar butter coffee promises, plus everything it doesn't. More on how the window works in time-restricted eating.
This article is general information, not medical advice. If you have a medical condition or take medication, talk to your doctor before changing how you eat.
Sources
- US Department of Agriculture. FoodData Central: Butter, salted (SR Legacy, FDC ID 173410). fdc.nal.usda.gov, FDC ID 173410
- Mumme K, Stonehouse W. Effects of medium-chain triglycerides on weight loss and body composition: a meta-analysis of randomized controlled trials. Journal of the Academy of Nutrition and Dietetics 2015;115(2):249–263. doi.org/10.1016/j.jand.2014.10.022
- Goldman DM et al. Assessing the validity of Bulletproof Coffee's claims. Beverages 2023;9(4):101. doi.org/10.3390/beverages9040101
- Hoang T et al. The effects of medium-chain triglyceride oil and butter on lipid profiles. Cureus 2024;16(6):e62556. pmc.ncbi.nlm.nih.gov/articles/PMC11254513
- Engel S, Tholstrup T. Butter increased total and LDL cholesterol compared with olive oil but resulted in higher HDL cholesterol compared with a habitual diet. American Journal of Clinical Nutrition 2015;102(2):309–315. doi.org/10.3945/ajcn.115.112227
- Siler SQ, Neese RA, Hellerstein MK. De novo lipogenesis, lipid kinetics, and whole-body lipid balances in humans after acute alcohol consumption. American Journal of Clinical Nutrition 1999;70(5):928–936. pubmed.ncbi.nlm.nih.gov/10539756
- World Health Organization. Saturated fatty acid and trans-fatty acid intake for adults and children: WHO guideline. Geneva: WHO, 2023. who.int/publications/i/item/9789240073630
- Regulation (EU) No 1169/2011 on food information to consumers, Annex II (the 14 allergens). eur-lex.europa.eu/eli/reg/2011/1169/oj