Part of our Mushroom Coffee guide library.
Short answer: For most intermittent fasting plans, yes: plain black coffee has only about 2 calories per 8-ounce cup, so many people drink it during their fasting window. A strict zero-calorie fast would technically exclude it. What breaks a fast is usually what you add: a tablespoon of half-and-half adds about 20 calories, heavy cream about 51 and a teaspoon of sugar about 16. Fasting for a blood test is different: unless your provider says otherwise, drink only plain water, because coffee can affect the results.
Coffee is one of the most common ways people get through the morning on an intermittent fast. But does it count? Here is how many calories black coffee and common add-ins have, what the research says about coffee and insulin, and why the answer changes when you are fasting for a blood test. For mushroom coffee blends specifically, see does mushroom coffee break a fast?
What you'll learn
- Whether black coffee breaks a fast
- How many calories milk, cream, sugar and butter add
- What research says about caffeine and insulin
- Why you should skip coffee before a fasting blood test
Does black coffee break a fast?
An 8-ounce cup of brewed black coffee has about 2 calories (USDA FoodData Central). Whether that counts depends on the rules you follow. In a well-known U.S. trial of 16:8 time-restricted eating, participants were told to eat only between noon and 8 p.m. and to completely abstain from caloric intake the rest of the day (Lowe et al., 2020). By a strict rule like that, even black coffee technically has calories. Many intermittent fasting plans allow black coffee, plain tea and water during the fasting window, so check the plan you follow and stay consistent.
What breaks a fast: common coffee add-ins
The add-ins usually matter much more than the coffee itself. Approximate calories from USDA FoodData Central (USDA FoodData Central):
| Coffee or add-in | Amount | Calories (approx.) |
|---|---|---|
| Black coffee, brewed | 8 fl oz | 2 |
| Whole milk | 1 tablespoon | 9 |
| Half-and-half | 1 tablespoon | 20 |
| Flavored liquid creamer | 1 tablespoon | 38 |
| Heavy whipping cream | 1 tablespoon | 51 |
| Granulated sugar | 1 teaspoon | 16 |
| Honey | 1 tablespoon | 64 |
| Butter (as in "butter coffee") | 1 tablespoon | 102 |
| Whole milk (as in a latte) | 1 cup | 149 |
Check the label of your own creamer or plant milk, since brands vary. A latte or a butter coffee is closer to a small meal than to a cup of black coffee, so it breaks any calorie-based fast.
What about zero-calorie sweeteners?
Sweeteners such as sucralose, stevia or monk fruit add little or no calories, but fasting plans differ on whether they are allowed. If you are not sure, follow the strictest version of your plan, or save sweetened coffee for your eating window.
Does coffee affect insulin while fasting?
Some people fast for metabolic reasons and worry that coffee spikes insulin. Black coffee has almost no calories, but caffeine itself does have short-term effects: a meta-analysis of 7 randomized trials found that a single dose of caffeine reduced insulin sensitivity in healthy people (Shi et al., 2016). The authors noted that long-term trials are needed, and the studies did not test fasting specifically, so it is not known whether this matters for the goals of an intermittent fast.
Fasting for a blood test: skip the coffee
This is the one case where the answer is a clear no. If your provider asks you to fast before a blood test, MedlinePlus advises having nothing except plain water, because coffee, juice and soda can get into your bloodstream and affect your results. Fasting usually lasts 8 to 12 hours, and you should also avoid chewing gum, smoking and exercise. Ask your provider whether to take your usual medicines (MedlinePlus).
Coffee on an empty stomach
Caffeine increases the release of stomach acid, which can lead to an upset stomach or heartburn (MedlinePlus), and some people notice it more when they have not eaten. If coffee bothers you during a fast, try a smaller cup, a lower-caffeine option, or move it to your first meal. See when to drink mushroom coffee for more on coffee before food.
How to drink coffee while fasting
- Keep it black, or have it with water or plain tea, during the fasting window.
- Save milk, cream, sugar and butter for your eating window.
- Watch the caffeine. Without food, you may feel it more, and late cups can cut into your sleep. See how long caffeine stays in your system.
- Know your numbers. See how much caffeine is in a cup of coffee.
- Talk to your provider first if you have diabetes, take medicines that lower blood sugar or other regular medication, are pregnant or breastfeeding, or have a history of an eating disorder.
Common myths (quick reality check)
Myth: Any coffee is fine while fasting. Reality: Black coffee has about 2 calories a cup, but a tablespoon of heavy cream adds about 51 and a latte made with a cup of whole milk about 149.
Myth: Butter coffee keeps you in a fasted state. Reality: A tablespoon of butter adds about 102 calories, so butter coffee breaks any calorie-based fast.
Myth: Black coffee is fine before a fasting blood test. Reality: MedlinePlus advises only plain water before a fasting blood test, because coffee can affect the results.
Frequently asked questions (FAQ)
Can you drink coffee while fasting?
For most intermittent fasting plans, yes, if it is black. An 8-ounce cup has about 2 calories. A strict zero-calorie fast would technically exclude it.
Does black coffee break a fast?
Not by most intermittent fasting rules, since it has about 2 calories per cup. By a strict no-calorie rule, it technically does.
Does coffee with milk break a fast?
Yes, by calorie-based rules. A tablespoon of whole milk adds about 9 calories and a tablespoon of half-and-half about 20.
Can I drink coffee before a fasting blood test?
No, unless your provider says it is fine. MedlinePlus advises only plain water while fasting for a blood test, usually for 8 to 12 hours.
Does coffee raise insulin?
Black coffee has almost no calories, but a meta-analysis found that caffeine briefly reduced insulin sensitivity in healthy people. Whether that matters for fasting is not known.
Does mushroom coffee break a fast?
Most mushroom coffee blends have a few more calories than black coffee. See our guide to mushroom coffee and fasting for details.
Related guides
- Is coffee acidic?
- Does mushroom coffee break a fast?
- How much caffeine is in a cup of coffee?
- How long does caffeine stay in your system?
- When to drink mushroom coffee
- Does caffeine raise blood pressure?
Disclaimer and when to seek help
This article is for general education and is not medical advice. These statements have not been evaluated by the Food and Drug Administration. Our products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your health care provider before fasting if you have diabetes or another medical condition, take medication, are pregnant or breastfeeding, or have a history of an eating disorder, and follow your provider's instructions before any lab test.
References
- U.S. Department of Agriculture. FoodData Central (SR Legacy: coffee, brewed; milk, whole; cream, half and half; cream, heavy whipping; cream substitute, flavored, liquid; sugars, granulated; honey; butter, salted).
- Lowe DA, et al. "Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial." JAMA Internal Medicine, 2020;180(11):1491-1499. doi:10.1001/jamainternmed.2020.4153
- Shi X, et al. "Acute caffeine ingestion reduces insulin sensitivity in healthy subjects: a systematic review and meta-analysis." Nutrition Journal, 2016;15(1):103. doi:10.1186/s12937-016-0220-7
- MedlinePlus (U.S. National Library of Medicine). "Fasting for a Blood Test".
- MedlinePlus (U.S. National Library of Medicine). "Caffeine".
