You're 14 hours into your fast and your stomach growls. You reach for a coffee, then pause. Will this ruin everything?
This is the most common question people have when they start intermittent fasting. The answer depends on what you're consuming and why you're fasting in the first place. Some things clearly break a fast. Others fall into a gray area that depends on your goals.
Here's a straightforward breakdown of what breaks a fast and what doesn't, backed by how your body actually processes different substances during a fasted state.
How Fasting Works in Your Body
When you stop eating, your body goes through a series of metabolic shifts. Blood sugar drops, insulin levels fall, and after roughly 12 hours, your body starts burning stored fat for energy. This state is called ketosis.
Around the 16 to 18 hour mark, a process called autophagy kicks in. Your cells start cleaning up damaged proteins and recycling them. This is one of the main reasons people fast beyond basic weight management.
Whether something "breaks" your fast depends on which of these processes you care about most:
- Fat burning: Anything that spikes insulin pulls you out of fat-burning mode
- Autophagy: Even small amounts of protein or certain amino acids can pause cellular cleanup
- Gut rest: Any food that requires digestion activates your gut, even in small amounts
Most people fast for fat loss and metabolic health. If that's you, the practical threshold is roughly 50 calories. Stay under that during your fasting window and you'll keep the core benefits intact.
Drinks That Won't Break Your Fast
These are safe to consume during your fasting window without disrupting fat burning or insulin levels.
Water
Plain water is always fine. Sparkling water and mineral water work too. Add a squeeze of lemon or lime if you want — the trace calories won't matter.
Stay hydrated. Dehydration is the top reason people feel terrible during a fast. Aim for at least 8 glasses spread throughout your fasting window.
Black Coffee
Plain black coffee has about 3 calories per cup and does not break a fast. Studies show caffeine may actually support fasting by boosting fat oxidation and suppressing appetite.
The catch: it must be black. No sugar, milk, cream, or flavored syrups. Even a splash of oat milk adds enough calories and protein to trigger an insulin response.
If you can't drink coffee black, try cold brew. It's naturally smoother and less bitter than hot-brewed coffee.
Plain Tea
Green tea, black tea, herbal tea, and white tea are all fine during a fast. Like coffee, they contain minimal calories and may help with appetite control.
Green tea in particular contains catechins that some research suggests support fat burning during fasted states.
Don't add honey, sugar, or milk. Plain only.
Apple Cider Vinegar
A tablespoon of apple cider vinegar diluted in water contains about 3 calories. It won't break your fast. Some people find it helps with hunger and blood sugar stability, though evidence is mixed.
The Gray Area: Things That Might Break Your Fast
These depend on your fasting goals and how strict you want to be.
Artificial Sweeteners
This one is debated. Diet soda and zero-calorie sweeteners technically have no calories. But some research suggests certain sweeteners (sucralose, aspartame) can trigger an insulin response in some people, even without calories.
If fat loss is your primary goal, most people are fine with occasional use. If you're fasting for autophagy or gut health, skip them.
Bone Broth
A cup of bone broth has around 40 to 50 calories with some protein. It will likely pause autophagy and gut rest. But for fat-burning purposes, the impact is small.
Many people doing longer fasts (24+ hours) use bone broth to ease hunger and maintain electrolytes. If you're doing a standard 16/8 fast, you probably don't need it.
Supplements and Medications
Fat-soluble vitamins (A, D, E, K) work better with food. Take them during your eating window.
Water-soluble vitamins, electrolytes, and most medications won't break a fast. If your doctor prescribed medication with food, follow their guidance — your health comes before your fasting schedule.
Branched-chain amino acids (BCAAs) contain calories and will break a fast.
Cream in Coffee
Even a small splash of cream (15-20 calories) technically breaks a fast. The fat and protein trigger a minor insulin response. If you're strict about fasting, skip it. If you're doing intermittent fasting mainly for weight management and that splash of cream keeps you consistent, the trade-off may be worth it.
Consistency matters more than perfection.
Things That Definitely Break Your Fast
Anything with significant calories, sugar, or protein will pull your body out of a fasted state.
- Any solid food — even a handful of nuts or a piece of fruit
- Sugary drinks — juice, soda, energy drinks, sweetened coffee
- Milk or cream — in amounts beyond a tiny splash
- Protein shakes and smoothies — these are meals, not drinks
- Alcohol — contains calories and your liver prioritizes processing it over fat burning
- Coconut oil or butter in coffee — "bulletproof coffee" is roughly 200-400 calories and breaks a fast, despite what some influencers claim
- Gummy vitamins — they contain sugar and calories
Common Mistakes That Accidentally Break a Fast
People often break their fast without realizing it. Watch out for these:
Flavored water. Many "zero calorie" flavored waters contain sweeteners that can spike insulin. Check the label.
Cough drops and mints. A single cough drop can have 10-15 calories from sugar. Sugar-free mints are generally fine.
Pre-workout supplements. Most contain calories, sugar, or BCAAs. Check the nutrition label before taking them during a fasting window.
"Just a bite." Tasting food while cooking counts. Even a small bite triggers digestion and insulin release.
Chewing gum. Regular gum has sugar. Sugar-free gum is technically fine, but the chewing motion can stimulate hunger in some people.
Does It Actually Matter If You Briefly Break Your Fast?
Here's some perspective. If you're fasting for general health and weight management, a small slip (like a splash of cream in your coffee) won't undo your progress. The bulk of fasting benefits come from the extended period without significant caloric intake, not from being 100% zero-calorie for every minute.
What matters more:
- Consistency over weeks and months beats perfection on any single day
- Staying in your eating window for the majority of your calories
- Not using "gray area" items as a loophole to snack throughout the day
If you're fasting for specific therapeutic reasons (autophagy, gut healing, or managing a medical condition), stricter adherence matters more. Talk to your doctor about your specific situation.
How to Track Your Fasting Window Accurately
Knowing exactly when your fast starts and ends removes the guesswork. A fasting timer shows you how far along you are and helps you decide whether that coffee or supplement fits within your plan.
FastFocus makes this simple. Set your fasting protocol (16:8, 18:6, 20:4, or others), start the timer with one tap, and see exactly where you are in your fasting window. The app tracks your streaks and history so you can spot patterns in what's working.
When you're not sure if something will break your fast, having a clear view of how far into your fast you are helps you make better decisions. Four hours in? Maybe hold off on the bone broth. Fourteen hours in with two to go? Water and black coffee will get you through.
Frequently Asked Questions
Does lemon water break a fast?
No. A squeeze of lemon in water adds about 1-2 calories. It won't affect insulin or fat burning. Lemon water is a popular way to make water more palatable during a fast.
Will black coffee with stevia break my fast?
Stevia is calorie-free and most studies show it doesn't trigger an insulin response. For most people, stevia in black coffee is fine during a fast. If you're fasting for autophagy, pure water and plain black coffee are safer choices.
Can I take my vitamins while fasting?
Water-soluble vitamins (B vitamins, vitamin C) are fine on an empty stomach. Fat-soluble vitamins (A, D, E, K) absorb better with food. Save those for your first meal after fasting.
Does toothpaste break a fast?
No. You don't swallow toothpaste (hopefully), so it has zero caloric impact. Brush your teeth normally.
How many calories will break a fast?
The commonly cited threshold is about 50 calories. Below that, your body stays in a fasted metabolic state for practical purposes. Above 50 calories, you're likely triggering enough insulin to shift out of fat-burning mode.
The Bottom Line
Stick to water, black coffee, and plain tea during your fasting window and you'll be fine. Most other things either break your fast outright or fall into a gray area that depends on your specific goals.
Don't overthink it. The biggest benefit of intermittent fasting comes from consistently maintaining your eating window over time, not from obsessing over whether 3 calories of lemon juice ruined your fast. If you're new to fasting, start with our beginner's guide and build from there.
If you want a simple way to stay on track, FastFocus gives you a visual timer, protocol options, and streak tracking to keep your fasting consistent — available free on iOS and Android.
