The short answer: black coffee does not break a fast for most fasting goals.
The Science
An 8 oz cup of black coffee contains approximately 2 calories and zero protein, fat, or carbohydrates. This minimal caloric load does not meaningfully stimulate an insulin response, meaning it does not disrupt the metabolic state of fasting.
Autophagy and Coffee
Interestingly, a 2014 study found that caffeine may actually promote autophagy in mice (Pietrocola et al., Cell Cycle 2014). While human studies are limited, this suggests black coffee may not hinder — and could even support — cellular repair during a fast.
What Breaks Your Fast
- Milk or cream — adds calories + fat → may trigger insulin response
- Sugar or sweeteners — caloric sweeteners break a fast; some artificial sweeteners may raise insulin
- Bulletproof coffee — high-fat coffee technically breaks a caloric fast but is used in some fat-fasting protocols
- Flavoured syrups — always contain sugar or artificial sweeteners; avoid during fasting window
Types of Coffee — Quick Guide
| Coffee Type | Breaks Fast? | Why |
|---|---|---|
| Black coffee | ✅ No | Zero calories, no insulin spike |
| Plain espresso | ✅ No | Same as black coffee |
| Black decaf | ✅ No | Near-zero calories |
| Coffee + milk | ❌ Yes | Calories + protein → insulin response |
| Coffee + sugar | ❌ Yes | Calories break fast |
| Bulletproof coffee | ❌ Yes (calories) | High fat = caloric fast broken |
| Oat milk latte | ❌ Yes | Carbs + calories |
Bottom Line
Black coffee, plain espresso, and black tea are generally considered safe during intermittent fasting. Add nothing to your coffee during your fasting window — not even a splash of milk.