During the fasting window, stick to drinks with essentially no calories: plain water, sparkling water, black coffee, and plain unsweetened tea. Anything with meaningful calories — food, juice, smoothies, milk, or sweetened and creamy drinks — belongs in your eating window, because calories are what end a fast. A few things sit in a grey area (a splash of milk, BCAAs, diet soda, electrolytes), and whether they matter depends on how strict you want to be.
Key takeaways
- The fasting window is different from the eating window. This guide is about the fasting window, when the goal is to take in almost no calories. In your eating window you eat normally. [1]
- Safe, clean choices: plain water, sparkling or carbonated water (unsweetened), black coffee, and plain unsweetened tea. All are essentially calorie-free. [1]
- What breaks a fast: anything with real calories — food, fruit juice, smoothies, milk or cream, sweetened coffee or tea, and "bulletproof" style fatty coffee. [1]
- Grey areas: a tiny splash of milk, BCAA powders, diet soda, and sugar-free electrolytes. They may be fine for a relaxed fast but can affect appetite or the "clean fast" idea for some people.
- "Clean fast" means water and zero-calorie drinks only. A more flexible fast allows small extras. Neither is wrong — just be honest about which one you are doing.
- Keep caffeine moderate. Up to about 400 mg a day is not generally associated with problems for most healthy adults, but sensitivity varies. [2]
- If you are pregnant or breastfeeding, take medication, or coffee on an empty stomach bothers you, be more cautious and check with a clinician. [3]
- GoFasting can help you log your fasting window, water intake, calorie intake, weight, and steps, so a stray calorie-containing drink shows up instead of slipping by. Separately, notice how you feel.
What can you have during the fasting window?
The short version: water and other essentially calorie-free drinks. Intermittent fasting works by separating an eating window from a fasting window, and during the fasting window the aim is to take in almost no calories. [1] So the drinks that keep a fast intact are the ones your body barely registers as food.
The reliable options are:
- Plain water — still or sparkling. Drink it whenever you like; staying hydrated also helps with hunger between meals.
- Black coffee — nothing added. It is essentially calorie-free and can help some people feel more alert. [1]
- Plain unsweetened tea — green, black, herbal, hot or iced, with no milk or sugar. Also essentially calorie-free. [1]
That is the core list. It is short on purpose, because "essentially no calories" is a narrow bar to clear.
What actually breaks a fast?
A fast is defined by the absence of calories, not by any single ingredient. [1] Once a drink or snack delivers a meaningful amount of energy, your body starts processing it, and the fast is over. That makes the rule simple: if it has real calories, save it for your eating window.
Things that break a fast include:
- Food of any kind, including "just a small snack."
- Fruit, fruit juice, and smoothies — healthy, but full of natural sugars and calories.
- Milk, cream, or creamer added to coffee or tea, even in small amounts.
- Sweetened coffee and tea, sodas with sugar, and sports drinks.
- Milkshakes, protein shakes, and sweetened plant-based milks.
- "Bulletproof" or butter-and-oil coffee, which is essentially a small fatty meal.
None of these are bad choices in general. They simply belong on the eating side of your day, not inside the fast.
A quick guide to what you can have
| Drink or item | During the fasting window? | Why |
|---|---|---|
| Plain water, sparkling water | Yes | No calories; hydration helps with hunger. [1] |
| Black coffee (nothing added) | Yes | Essentially calorie-free; may boost alertness. [1] |
| Plain unsweetened tea | Yes | Essentially calorie-free. [1] |
| A tiny splash of milk in coffee | Grey area | Very few calories, but not a "clean" fast; can affect strict goals. |
| Diet soda / sugar-free drinks | Grey area | Zero calories, but sweeteners bother appetite or the stomach for some people. |
| BCAAs / amino acid powders | Grey area | Low calorie but not zero; a nudge toward "eating" for a strict fast. |
| Sugar-free electrolytes | Grey area | Useful on longer fasts; choose ones without added sugar. |
| Bone broth | Grey area | Has real calories, so it breaks a clean fast, but is sometimes used on longer fasts for salts. |
| Juice, smoothies, sweet drinks | No | Meaningful sugar and calories end the fast. [1] |
| Milk, cream, milkshakes, fatty coffee | No | Enough calories to count as food. [1] |
The grey areas, and how to decide
A few things sit between "clean" and "breaks it," and honest guidance says so rather than pretending the line is sharp for everyone.
- A tiny splash of milk. A dash in coffee adds only a few calories. On a relaxed fast that is usually a non-issue; on a strict, calorie-free fast, it is not "clean." If a little milk is the difference between sticking with fasting and quitting, most people would rather keep the splash.
- Diet soda and sugar-free drinks. They have no calories, so by the calorie rule they do not break a fast. But some people find sweet tastes increase cravings or bother the stomach, so notice your own response.
- BCAAs and amino acid powders. These carry a small number of calories and can register as a light "meal" for a strict fast. For most casual fasters they are not the thing to worry about.
- Electrolytes and bone broth. For short daily fasts you usually do not need either. On longer fasts, sugar-free electrolytes can help you feel steadier, and a small amount of bone broth is sometimes used for the salts it provides — but broth has calories, so it does break a truly clean fast. Keep any of this light, and see the safety note below.
The honest takeaway: apart from water, coffee, and plain tea, most extras are a judgment call about how strict you want to be — not a clear yes.
Clean fast vs a more flexible fast
Much of the confusion online comes from people using the same word for two different things.
- A clean fast means only water and truly calorie-free drinks — no milk, no sweeteners, no supplements. It is the strictest reading and the one that keeps the fasting window as "pure" as possible.
- A more flexible fast allows small extras like a splash of milk or a diet drink, accepting that the fast is not perfectly clean in exchange for being easier to stick with.
Neither is the "correct" one — they suit different goals. The useful move is to pick which one you are doing and be consistent, rather than drifting between them and wondering why results feel inconsistent. And it is fair to keep it simple: for most people, water, black coffee, and plain tea cover the fasting window with no grey areas at all.
How GoFasting can help you keep the line clear
Deciding what breaks a fast gets easier when your window and your intake are both visible. GoFasting can help you log your fasting window, water intake, calorie intake, weight, and steps, so a drink that quietly carries calories shows up as intake instead of slipping past. If you have something outside plain water, coffee, or tea, log it and let it count. Separately, pay attention to your own energy, hunger, and whether the routine still feels sustainable — those are your personal signals, not app metrics.
Track the basics, then judge what fits
- Fasting window — See exactly when the fast starts and ends.
- Water — Track hydration through the fasting window.
- Calories — Log anything beyond the clean list as intake, not "free."
- Weight and steps — Review trends over time, not a single day.
When to keep it moderate, or check with a clinician
⚠️ Coffee and tea are fine for most people in the fasting window, but caffeine still counts. For most healthy adults, up to about 400 mg of caffeine a day — roughly four to five cups of coffee — is not generally associated with problems, though sensitivity varies from person to person. [2] If you are pregnant or breastfeeding, take medication, have a heart or anxiety condition, or coffee on an empty stomach upsets your stomach, be more cautious with caffeine and talk with a clinician about what is right for you. [3]
Two practical points. First, "essentially calorie-free" is about calories, not caffeine — you can stay in a clean fast and still overdo coffee, so keep an eye on the total across the day. [2] Second, if you fast for longer stretches, hunger is expected, but feeling faint, dizzy, or unwell is a signal to stop and, if it keeps happening, to get advice; sugar-free electrolytes can help on longer fasts, but you should not push through symptoms just to keep the window "clean." Fasting is not recommended for everyone, so if you are unsure whether it suits you, check first. [1]
FAQ
Does black coffee break a fast?
No. Plain black coffee with nothing added is essentially calorie-free, so it is one of the standard fasting-window drinks. Adding milk, cream, sugar, or oil is what breaks it. [1]
Can I put a little milk in my coffee while fasting?
It is a grey area. A splash adds only a few calories, which is fine on a relaxed fast but not a clean, calorie-free one. If a small amount of milk keeps you fasting instead of quitting, that trade-off is reasonable. [1]
Does diet soda break a fast?
By the calorie rule, no — it has no calories. But some people find sweeteners increase cravings or bother the stomach, so notice your own response and treat it as a grey area rather than a clear yes.
Do BCAAs or supplements break a fast?
BCAA and amino acid powders carry a small number of calories, so for a strict, clean fast they count as a light break. For a casual fast they are usually not the main concern. Check the label for any added sugar or calories.
What can I drink on a longer fast?
Water is the main one. Sugar-free electrolytes can help you feel steadier, and some people use a little bone broth for the salts, though broth has calories and breaks a clean fast. Stop and seek advice if you feel faint or unwell. [3]
How much coffee is too much during a fast?
The calories are not the issue, but the caffeine is. Up to about 400 mg a day is not generally associated with problems for most healthy adults; sensitivity varies, and pregnancy, medication, or certain conditions call for less. [2][3]
Medical disclaimer: This article is for educational purposes only and is not medical advice. Speak with a qualified healthcare professional before fasting if you are pregnant or breastfeeding, take medication, have a medical condition, have a history of disordered eating, or are unsure whether intermittent fasting is appropriate for you.
References
- Johns Hopkins Medicine. Intermittent Fasting: What Is It, And How Does It Work? Accessed July 8, 2026 https://www.hopkinsmedicine.org/health/expert-qa/intermittent-fasting-what-is-it-and-how-does-it-work
- U.S. Food and Drug Administration. Spilling the Beans: How Much Caffeine is Too Much? Accessed July 8, 2026 https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much
- Mayo Clinic. Caffeine: How much is too much? Accessed July 8, 2026 https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/caffeine/art-20045678