Dirty Fasting vs Clean Fasting: The 50-Calorie Rule
Dirty fasting means allowing a small number of calories during your fasting window, usually under 50 or under 100. Clean fasting means water, black coffee, and plain tea only. Neither cutoff comes from a clinical trial. The studies that tested fasting either allowed zero calories or roughly 500 to 650. The space between is internet consensus.
Key Takeaways
- The "50-calorie rule" and "100-calorie rule" trace to no study. Real fasting trials used 0 kcal or ~500 to 650.
- "Clean fasting" has two definitions: zero calories (Johns Hopkins) or zero calories plus zero sweeteners and flavors (Gin Stephens).
- Whether dirty fasting "works" depends on the goal. For calorie reduction, a splash of cream is trivial. For insulin or autophagy, the evidence is thin.
- FDA labeling lets anything under 5 kcal per serving be listed as zero (21 CFR 101.9).
What Is Dirty Fasting?
Dirty fasting allows a small caloric intake during the fasting window. Healthline (2021) says people "typically consume up to 100 calories" and notes there is no research on this practice. Fitnessvolt sets the line at under 50, citing no source. Dr. Berg says 50 to 100 calories, preferably from fats.
What Is a Clean Fast?
Clean fasting has two versions that disagree on sweeteners.
Version 1: Zero calories. Johns Hopkins Medicine describes the fasting window as water and zero-calorie beverages such as black coffee and tea. Diet soda or a zero-calorie sweetener would pass.
Version 2: Zero calories, zero sweeteners, zero flavors. Gin Stephens' clean fast from Fast. Feast. Repeat. (St. Martin's Press, 2020) goes further. As summarized by Shortform, the allowed list is unflavored water, black coffee, plain tea, mineral water, salt, electrolytes, and doctor-approved medication. Not allowed: flavored waters, diet sodas, creamers, sweeteners "even if zero calorie," gum, mints. Stephens' rationale is that sweet taste may trigger insulin. That theory is not proven, but many fasting communities enforce it. See also: which teas keep a fast clean.
Where the 50-Calorie Rule Came From
No identifiable study. The thresholds circulate in IF blogs, but none trace back to a trial. Here is what clinical trials actually used:
| Trial | Design | Fast-day calories | What was excluded |
|---|---|---|---|
| Stekovic et al., 2019 (Cell Metabolism) | 36 h fast / 12 h eat, n=60, 4 wks | 0 kcal | Diet sodas, "calorie-free" beverages |
| Trepanowski et al., 2017 (JAMA Intern Med) | Alternate-day fasting, n=100, 1 yr | ~25% of energy needs | N/A (structured meals) |
| Harvie et al., 2011 (Int J Obes) | 2 restricted days/wk, n=107, 6 mo | ~2,710 kJ (~650 kcal) | N/A (structured meals) |
Research brackets the question at 0 and at hundreds of calories. Nothing sits at 50. The strictest protocol (Stekovic) banned even zero-calorie diet drinks. For more, see what breaks a fast, goal by goal.
One wrinkle: FDA labeling allows anything under 5 kcal per serving to print as zero. A Splenda packet lists 0 calories but contains dextrose and maltodextrin, so the true figure is somewhere under 5 kcal.
Dirty Fasting vs Clean Fasting, Item by Item
Data from USDA FoodData Central and brand labels, October 2026.
| Item | Serving | kcal | Carbs (g) | Protein (g) | Zero-cal clean fast | Stephens clean fast | Under 50 kcal | Under 100 kcal |
|---|---|---|---|---|---|---|---|---|
| Black coffee | 8 fl oz | 2 | 0 | 0.3 | Yes | Yes | Yes | Yes |
| Splenda packet | 1 packet | 0* | 0* | 0 | Yes | No (sweetener) | Yes | Yes |
| Whole milk | 1 tbsp | 9 | 0.7 | 0.5 | No | No | Yes | Yes |
| Half-and-half | 1 tbsp | 20 | 0.6 | 0.5 | No | No | Yes | Yes |
| Heavy cream | 1 tbsp | 51 | 0.4 | 0.4 | No | No | No | Yes |
| Lemon juice | 1 fl oz | 7 | 2.1 | 0 | No | No | Yes | Yes |
| Sugar | 1 tsp | 16 | 4.2 | 0 | No | No | Yes | Yes |
| Honey | 1 tbsp | 64 | 17.3 | 0 | No | No | No | Yes |
| Butter | 1 tbsp | 102 | 0 | 0.1 | No | No | No | No |
| Coconut oil | 1 tbsp | 121 | 0 | 0 | No | No | No | No |
| Brain Octane MCT oil | 1 tbsp | 130 | 0 | 0 | No | No | No | No |
| Kettle & Fire chicken bone broth | 16.9 oz | 80 | 1 | 19 | No | No | No | Yes |
| Roon pouch | 1 pouch | 0 | 0 | 0 | Yes | No (sweetener, flavor) | Yes | Yes |
* Label zero per FDA rounding; actual is under 5 kcal.
Butter coffee math: 1 tbsp butter (102 kcal) + 1 tbsp Brain Octane MCT oil (130 kcal) = ~232 kcal, past both dirty-fasting cutoffs. See MCT oil, butter, and cream by the tablespoon.
Does Dirty Fasting Still Work?
That depends on the goal.
For calorie reduction, a tablespoon of half-and-half is 20 kcal, which is noise. Trepanowski and Harvie both allowed hundreds of fast-day calories and still showed weight changes similar to daily restriction.
For "keeping insulin low," the sweetener question has no clean answer. Sucralose alone did not raise insulin in a 2011 crossover (Ford et al., n=8). But sucralose before a glucose drink raised peak insulin secretion rate about 22% in a 2013 crossover in obese subjects (Pepino et al., n=17), which tested sucralose plus glucose, not sucralose alone. Different questions. See the sucralose and insulin studies.
For autophagy, be skeptical. A 2025 exploratory analysis (Bensalem et al., n=121) found that time-restricted eating may increase autophagic flux in humans, but the only signal was a post hoc difference against standard care at 6 months, with no significant rise within the fasting group itself. The authors say further studies are required.
For protein, a full carton of bone broth delivers 19 grams, a meal-sized dose for a fasting window.
Which Should You Choose?
If you are following a study protocol or a coach's plan, use its exact definition. Stekovic means zero calories, no diet drinks. Harvie-based 5:2 means roughly 650 calories on restricted days. If a splash of milk is the only way you get through the morning, one tablespoon is about 9 calories. If you follow Stephens' clean fast, the line is taste, not calories.
Johns Hopkins says people under 18, pregnant or breastfeeding, with type 1 diabetes on insulin, or with a history of eating disorders should not try intermittent fasting. Anyone on other glucose-lowering medication should talk to a clinician first. For extended protocols, see a 36-hour fast, hour by hour. For a broader item list, see what you can have while fasting.
The Number Matters Less Than the Rule You Actually Keep
The 50-calorie rule and the 100-calorie rule both emerged from internet consensus, not clinical evidence. Real trials either banned all calories or allowed hundreds. Nothing tested the magic of 50. That does not make dirty fasting useless. It means you should pick a rule, apply it consistently, and stop treating the number as science.
Frequently Asked Questions
Does cream break a fast?
Depends on the definition. Half-and-half (1 tbsp, ~20 kcal) passes both dirty-fasting cutoffs. Heavy cream (1 tbsp, 51 kcal) passes the 100-calorie rule but fails 50. Under Stephens' clean fast, any creamer breaks the fast.
How many calories break a fast?
No clinically validated number. The 50- and 100-calorie thresholds are community conventions. The strictest trial (Stekovic 2019) allowed zero and banned diet drinks. Less strict trials allowed 500 to 650 kcal.
Do artificial sweeteners break a clean fast?
Under the zero-calorie definition, no. Under Stephens' clean fast, yes. Sucralose alone did not raise insulin in one study (Ford 2011, n=8), but sucralose before a glucose load raised peak insulin secretion rate by about 22% in obese subjects in another (Pepino 2013, n=17). Different conditions, not contradictory.
Is bulletproof coffee dirty fasting?
A standard recipe (1 tbsp butter + 1 tbsp MCT oil) totals ~232 kcal, exceeding both cutoffs. See MCT oil, butter, and cream.
Does bone broth break a fast?
A 16.9 oz carton of Kettle & Fire chicken bone broth has 80 kcal and 19 g protein, per the brand's label. Passes the 100-calorie rule, fails the 50-calorie rule and both clean-fast definitions. See bone broth and fasting.
Is dirty fasting still intermittent fasting?
Yes. Dirty fasting is a variation within intermittent fasting. The "dirty" part refers only to what you allow during the fasting window.
Zero Calories, With the Sweetener on the Label
Roon passes every calorie-based fasting definition in the table: zero calories, zero sugar. It does not pass Stephens' clean fast. Roon contains sucralose and natural-and-artificial flavoring, and there is no point hiding that.
If your rule is calories, the question is what you are replacing. Heavy cream adds 51 kcal. Butter coffee adds 232. Roon adds zero and delivers 80 mg caffeine, 60 mg L-theanine, 25 mg methylliberine (Dynamine), and 5 mg theacrine (TeaCrine) through a sublingual pouch.
The duration comes from methylliberine slowing caffeine clearance. A randomized, double-blind crossover PK study (n=12) found 100 mg methylliberine with 150 mg caffeine cut caffeine clearance from 41.9 to 17.1 L/hr and extended half-life from 7.2 to 15 hours, no higher peak. That study used 4x the methylliberine and ~1.9x the caffeine in a Roon pouch, so the numbers are directional. Felt window: 6 to 8 hours. Trade-off: stop at least 8 hours before bed.
From $9.16 per tin (12-tin subscription, $0.61/pouch). If you want caffeine that lasts through a fasting morning without adding calories, try Roon.
Related from Roon
- What Breaks a Fast? A Goal-by-Goal Guide
- Does Sucralose Break a Fast?
- What Can I Have While Fasting?
- Does MCT Oil Break a Fast?
Written by Roon Team
