What Breaks a Fast? The Definitive Answer

Quick answer: Anything that triggers a significant insulin response or provides meaningful calories breaks a fast. Water, black coffee, and plain tea are safe. Anything with sugar, cream, calories, or protein above trace amounts will end your fasted state.

This is the question that generates more debate in fasting communities than any other. People want clear rules, but the answer depends on why you are fasting in the first place. Let us settle this with science rather than opinion.

The Short Answer

If it has calories, it breaks your fast. But the real picture has more nuance.

A strict fast means zero calories. Water only. But most people practicing intermittent fasting for health and weight loss are not monks. They want to know what they can get away with during fasting hours without undermining their results. Fair question.

Understanding Why It Matters

To determine what breaks a fast, you need to know what fasting is supposed to do:

Keep insulin low. Insulin is the primary storage hormone. When it is elevated, your body stores energy. When it is low, your body accesses stored fat. Any food that spikes insulin disrupts this process.
Maintain autophagy. This cellular cleanup process activates during extended fasting. Amino acids (protein) and significant caloric intake suppress the mTOR pathway and shut down autophagy (Alirezaei et al., Autophagy, 2010).
Sustain fat oxidation. During a fast, your body progressively increases its rate of fat burning. Consuming calories, particularly carbohydrates, halts this shift.

Different substances affect these mechanisms differently. That is why the answer is not always a simple yes or no.

What Does NOT Break a Fast

Water (Still and Sparkling)

Water has zero calories, zero insulin impact, and zero effect on autophagy. Drink as much as you want. Sparkling water is equally fine, and the carbonation can help suppress hunger.

Black Coffee

This is the one most people care about. Good news: black coffee does not break a fast. It contains virtually zero calories (about 2-5 per cup) and does not trigger a meaningful insulin response.

Better news: coffee actually enhances fasting benefits. Caffeine increases lipolysis (fat breakdown) and thermogenesis. A 2014 study in Food Science and Biotechnology found that coffee consumption during fasting increased fat oxidation by 10-29% (Acheson et al., 2014). Coffee also appears to stimulate autophagy, according to a 2014 study in Cell Cycle (Pietrocola et al., 2014).

For a full breakdown, read our guide on coffee and intermittent fasting.

The catch: Black means black. No sugar. No cream. No milk. No flavored syrups. Even a splash of milk (15-20 calories) adds enough protein and lactose to trigger a small insulin response.

Plain Tea

Green tea, black tea, herbal tea, and white tea are all fine during a fast. Like coffee, tea has negligible calories and may enhance fasting benefits. Green tea catechins, particularly EGCG, have been shown to increase fat oxidation independently (Hursel et al., American Journal of Clinical Nutrition, 2009).

No honey. No sugar. No milk.

Apple Cider Vinegar

One tablespoon of ACV contains about 3 calories and no sugar. It does not break a fast and may actually support fasting goals. A study in European Journal of Clinical Nutrition (2005) found that vinegar consumed with a meal reduced the glycemic response by 20-30% (Ostman et al., 2005). Dilute it in water if you use it.

Salt and Electrolytes

Plain salt, potassium, and magnesium supplements without added sugars or calories do not break a fast. In fact, maintaining electrolyte balance during fasting prevents common side effects like headaches, dizziness, and muscle cramps. Read more in our guide on electrolytes during fasting.

The Gray Zone

These items spark the most debate. Here is where the science currently stands:

Lemon Water

A squeeze of lemon in water adds 1-3 calories. This is negligible and will not meaningfully affect insulin, ketosis, or autophagy. Most fasting researchers consider lemon water acceptable. We cover this in detail in our lemon water and fasting guide.

Artificial Sweeteners

This is complicated. Artificial sweeteners contain zero calories, but some may still trigger an insulin response through cephalic phase insulin release, where your brain tells your pancreas to release insulin based on sweet taste alone.

A 2020 systematic review in European Journal of Clinical Nutrition found mixed results: sucralose appeared to trigger insulin release in some individuals while stevia did not (Nichol et al., 2020). The safest approach is to avoid sweeteners during your fast. If you must, stevia appears to be the least disruptive option.

Bone Broth

A cup of bone broth contains roughly 40-50 calories and small amounts of protein. This breaks a strict fast. However, some extended fasting protocols use bone broth specifically for electrolyte and amino acid support. For standard 16:8 intermittent fasting, save it for your eating window.

MCT Oil and Butter (Bulletproof Coffee)

Adding MCT oil or butter to your coffee provides 100-200+ calories. This absolutely breaks your fast from a caloric and insulin perspective. While it may keep you in ketosis (fat does not spike insulin significantly), it halts autophagy and stops your body from burning its own stored fat because you are giving it dietary fat instead.

The "fat does not break a fast" claim popular in keto circles is misleading. Fat breaks a caloric fast. It just does not break ketosis. Those are different things.

Supplements

Branched-chain amino acids (BCAAs): Break your fast. They are protein, and protein suppresses autophagy and triggers insulin release.
Creatine: 0 calories, minimal insulin effect. Technically fine, but better absorbed with food.
Multivitamins: May contain small amounts of sugar or calories. Fat-soluble vitamins need dietary fat for absorption. Take with meals.
Fish oil: Contains calories (40-50 per dose). Take during your eating window.
Psyllium husk fiber: Minimal calories but may trigger mild digestive activity. Borderline; best saved for eating window.

Gum

Sugar-free gum contains 1-5 calories and artificial sweeteners. The caloric impact is negligible, but the sweeteners may trigger cephalic insulin release. If chewing gum helps you get through your fast without eating, the trade-off is probably worth it. But it is not technically "clean" fasting.

What DEFINITELY Breaks a Fast

No ambiguity here:

Any food. Even a small snack.
Milk or cream in coffee. Even a splash.
Juice. Any kind.
Soda. Regular or diet (diet is debatable but not worth the risk).
Smoothies. Even "healthy" ones.
Protein shakes. These are food.
Alcohol. Any amount.
Sugar in any form. Including honey, maple syrup, and agave.
Bulletproof or butter coffee. Calories are calories.
Meal replacement drinks. They replace meals because they are meals.

The Practical Framework

If you are fasting for weight loss, the strict rule is: consume only zero-calorie beverages. A few calories from lemon, black coffee, or ACV will not derail your results, but anything above 10-15 calories per serving is too much.

If you are fasting for autophagy, be stricter. Avoid anything with protein or amino acids, as these activate mTOR and suppress autophagy most directly.

If you are fasting for insulin sensitivity, avoid anything sweet, even zero-calorie sweeteners. The cephalic insulin response may be small, but why introduce it when water and black coffee work fine?

For a comprehensive guide to fasting-friendly beverages, see our list of what you can drink during a fast.

The 50-Calorie Rule: Is It Real?

You may have heard that anything under 50 calories will not break your fast. This "rule" has no scientific basis. It likely originated from the 5:2 diet, which allows 500-600 calories on fasting days, but that is a modified fast, not a true fast.

There is no magic caloric threshold below which insulin stays completely flat. Even small amounts of protein (5-10 grams) can trigger measurable insulin release. The 50-calorie rule is a convenient fiction. Treat your fast as a fast.

How Fasted Helps

Fasted makes this simple. When your timer is running, you know you are in your fasting window, and the app shows your progress through different fasting zones. No second-guessing about whether you are "still fasting." Log your meals during your eating window and keep your fasting hours clean. The visual countdown and streak tracking keep you honest and motivated.

Frequently Asked Questions

Does toothpaste break a fast?

No. You do not swallow toothpaste, so it does not enter your digestive system. Brush your teeth normally. The small amount of sweetener you might taste does not cause a meaningful metabolic response.

Will a few calories from black coffee ruin my fast?

No. The 2-5 calories in a cup of black coffee are metabolically insignificant. Coffee is not just acceptable during a fast, it is actively helpful for fat burning and appetite suppression.

Can I have a tiny snack and still call it a fast?

No. A snack, no matter how small, triggers digestion, insulin release, and suppresses autophagy. If you need to eat, your fast is over. Break it properly with a real meal and start your next fast after.

Does diet soda break a fast?

Technically, diet soda has zero calories. But artificial sweeteners may trigger insulin release in some individuals, and the sweet taste can increase cravings, making fasting harder psychologically. Stick to water, coffee, or tea for the cleanest fast.

Is there any difference between what breaks a fast for men versus women?

The basic physiology is the same. Insulin responds to the same stimuli regardless of sex. However, women may be more sensitive to prolonged caloric restriction, which is a consideration for extended fasts (beyond 24 hours), not for standard daily IF protocols.