What Breaks a Fast? The Definitive Answer

Quick answer: Calories break a fast, and protein breaks it hardest — amino acids raise insulin and shut down the cellular cleanup that fasting is meant to start. Water, black coffee, plain tea, salt and plain creatine are safe. Sugar, cream, protein powder, collagen and BCAAs are not. Sweeteners, gum, and vitamins sit in a gray zone whose answer depends on why you are fasting.

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. If you have one specific item in mind, you can look it up by name and see a separate verdict for strict fasting, fat loss, and autophagy.

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. Salt is sodium chloride: no calories, no protein, no carbohydrate, nothing for insulin to respond to.

Salt matters more while you fast, not less. As you burn through glycogen your body releases the water stored with it, and that water carries sodium out with it. Low insulin adds to the effect, because insulin is part of what makes the kidneys hold sodium back. Lose enough and you get the headache, the lightheadedness, the cramps and the brain fog that people mistake for fasting simply being hard. A pinch of salt in water fixes more fasting misery than any other single change.

Read the label on flavored electrolyte products, though. Pure sodium, potassium and magnesium powders are fine; plenty of commercial mixes carry sugar or maltodextrin and those break the fast. How much you need depends on your body weight and how long you fast, which the electrolyte target calculator works out for you. 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.

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.

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, without argument. Diet soda is a sweetener question, covered below.
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.

Protein and Amino Acids: The Clearest Fast-Breaker

Protein is the one category with no honest gray zone, and understanding why explains the ranking of everything else on this page. Fat is calorie-dense but metabolically quiet. Carbohydrate raises blood glucose and insulin the way everyone expects. Protein does something less intuitive: it raises insulin sharply while lowering blood glucose, so it can end a fast thoroughly while a glucose meter shows nothing alarming.

The clearest demonstration is a 2004 study in the American Journal of Clinical Nutrition. Nilsson and colleagues fed twelve volunteers meals of milk, cheese, whey, cod and wheat gluten against a white-bread reference. The whey meal produced a 90% higher insulin area under the curve than white bread, while its glucose response was 57% lower. The insulin response tracked the rise in plasma amino acids, leucine most strongly. The same group repeated the test in 2007 with glucose-equivalent drinks, removing food as an explanation: whey raised insulin 60% above the glucose reference, and a drink of branched-chain amino acids alone also raised insulin significantly. Worth reporting from that same paper: a five-amino-acid mixture raised insulin 31%, and that result was not statistically significant. Whey and BCAAs at real doses do this. A trace of amino acid does not.

Protein powder

USDA lists generic whey-based powder at 352 calories per 100 g with 78 g of protein — about 113 calories and 25 g of protein in a 32 g scoop. Two caveats: that entry sits in USDA's SR Legacy tier, frozen in 2018, and it is a composite rather than the tub in your cupboard. Read your own label; scoops run anywhere from 25 g to 45 g.

It breaks a fast under every definition, and there is no fasting-friendly version. If a powder is worth taking for muscle, it signals protein; if it does not signal protein, it is not doing much for muscle.

Collagen

Collagen gets an exemption in fasting circles it has not earned. A scoop of collagen peptides is close to pure protein — ten grams of powder is roughly ten grams of protein, and protein carries about four calories a gram. Collagen is not the leucine-rich protein whey is, so the signal is weaker. Weaker is not absent. Collagen in your morning coffee ends the fast; black coffee does not.

BCAAs and EAAs

The "zero calorie" label on a BCAA tub argues the wrong point. Calories were never the interesting part of the protein question — the amino acid signal is, and BCAAs are that signal sold on its own. Leucine triggers insulin release directly, and it is the nutrient sensor that tells the cell food has arrived. EAAs get the same answer.

Be straight about the limits here. The chain from leucine to mTOR to suppressed autophagy is well described in cells and in animals. No human trial has isolated protein as the variable during a fast, and no routine test can measure autophagy in a living person. That is strong mechanistic ground, not proof, and it is more than enough reason to keep protein out of an autophagy fast.

The fasted training problem

Most people asking about protein mid-fast train fasted in the morning and worry about muscle. The tradeoff is real, and the answer is scheduling rather than chemistry. Train near the end of your fast and eat straight after: you get the fasted session and the protein without pretending the shake was free. If you do shake at 7am, your eating window opened at 7am — log it that way and the fasting timer keeps the boundary honest. Total daily protein matters far more for holding muscle than whether it landed within an hour of lifting.

Sweeteners: What the Human Studies Found

The calorie question is settled and boring. High-intensity sweeteners are hundreds of times sweeter than sugar, so they are used in milligram amounts. What fills out a packet is bulking agent — dextrose, maltodextrin, or erythritol — put there so the packet pours like sugar. Under 21 CFR 101.9, anything under 5 calories per serving may be labeled zero. "Zero calories" is a rounding rule, not a measurement, and four packets across a morning is four servings. Liquid drops carry the least hidden material.

The real argument is whether sweet taste alone releases insulin. It is a weaker argument than you will read elsewhere.

The study everyone cites is about sucralose. In Diabetes Care in 2013, Pepino and colleagues gave 17 adults with obesity 48 mg of sucralose or water ten minutes before a 75 g glucose load. Against water, sucralose produced a 20% greater insulin area under the curve. Two things get misreported constantly: that 20% is the area under the curve, not a "20% spike", and the design is sucralose followed by sugar — not sucralose in black coffee at hour 14 of a fast.

The same lab then tested our actual question. In 2019, Nichol and colleagues had 21 adults swallow sucralose, drink water, or taste sucralose and spit it out. Swallowed sucralose raised the glucose response. Tasted and spat, it did not raise insulin. The sweet-taste hypothesis, tested head-on, failed. The broadest acute evidence agrees: a 2023 network meta-analysis of 36 trials in 472 participants — aspartame, sucralose, acesulfame K, saccharin, cyclamate, stevia and blends — found these drinks have no acute metabolic or endocrine effects, behaving like water.

Stevia

Stevia has the honest counterweight, so here it is. A 2022 crossover in Clinical Nutrition ESPEN gave 33 women with insulin resistance 60 mL of water carrying just 15.3 mg of stevia. The stevia condition produced a higher C-peptide response than control, and C-peptide is released alongside insulin. That is the closest published test to "stevia in water while fasting", and it did not find nothing. Set against pooled data from dozens of trials, the fair reading is that any insulin effect of stevia is small, inconsistent, and probably depends on who you are. Not zero. Not a spike either.

Erythritol and monk fruit

Erythritol is a sugar alcohol rather than a high-intensity sweetener, so it goes in by the spoonful — and US labeling law assigns it zero calories per gram. The practical catch is that retail "monk fruit sweetener" is overwhelmingly erythritol with a trace of monk fruit extract. An honest answer about monk fruit is usually an answer about erythritol.

Which fast are you running?

On a strict fast, sweeteners are out by definition. A strict fast is a rule about inputs, not calories, and its whole value is that you never have to litigate it. On a fat-loss fast, they do not break it: no meaningful calories, no reliable insulin signal, and a sweetened black coffee that carries you to noon is doing more good than harm. On an autophagy fast, nobody knows — there is no human evidence in either direction and no test that would tell you your status. Unknown is not the same as safe.

One thing outranks the whole insulin argument: what sweet taste does to your appetite. If it makes the last hours of your fast harder and your first meal bigger, the sweetener is costing you more than its calories ever could.

Supplements: The Label Decides

A supplement breaks a fast if it carries meaningful calories, protein or amino acids, or sugar. By that test most plain vitamins and minerals pass easily. It is the format and the fillers that catch people out.

Fine during a fast: vitamin D3, magnesium, zinc, B vitamins and vitamin C in capsule or tablet form all carry no calories. Fat-soluble vitamins (A, D, E, K) absorb far better with dietary fat, so your first meal is the better time anyway — an absorption argument, not a fasting one.

Breaks a fast: gummy vitamins, almost without exception, because the gummy is made of sugar or sugar alcohol. Liquid vitamins in a juice or syrup base. Greens powders and meal replacements that carry calories. Anything protein-based. Fish oil sits in between: the fat is real calories, so take it with food if you are strict about it.

Nobody needs to worry about pill fillers. Magnesium stearate, cellulose and silicon dioxide arrive in milligram quantities, carry no usable energy, and do nothing to insulin.

Creatine

Plain creatine monohydrate is inert here: no calories, no protein, no carbohydrate, no insulin response, and no evidence it touches autophagy. It is a molecule, not a macronutrient, and it works in the muscle cell regardless of your metabolic state.

The problem is never the creatine, it is the tub. Pre-workouts and flavored blends routinely add dextrose or maltodextrin to push absorption, and those break a fast outright. Creatine gummies usually carry sugar. Unflavored monohydrate or plain capsules are the clean options. On timing: insulin does improve creatine uptake into muscle, which is why some loading protocols pair it with carbohydrate — so taking it with your first meal may saturate you faster. Fasted creatine still works. Neither choice is wrong.

Gum, Toothpaste, and Mouthwash

Brush your teeth. Toothpaste used normally and spat out does not break a fast by any reasonable measure. A pea-sized amount is a fraction of a calorie, you swallow almost none of it, and fluoride has no metabolic effect at all. Mouthwash follows the same rule: swish and spit. Whitening strips carry no calories and you are not eating them. Skipping oral hygiene over a fasting worry would cost you far more than the residue ever could.

Religious fasting runs a different rulebook. Some protocols prohibit swallowing anything during the window, saliva included, and brushing before or after the window is the usual answer. That is a spiritual protocol, not a metabolic one, and this page cannot settle it.

Gum

Gum is the genuinely gray one. Regular gum has sugar in it and will break your fast. Sugar-free gum is a couple of calories a piece, sweetened with sorbitol, xylitol or aspartame — all low-glycemic, none of them a meaningful insulin load at that dose.

The argument against sugar-free gum is the chewing itself: the anticipation of food. That is the same sweet-taste argument the sucralose experiment above tested directly and failed to support. One or two pieces will not undo a fat-loss fast, and if gum is what carries you past hour 15 without eating, chew it. On a strict water fast, skip it — the point of that protocol is having no variables to argue about. Nicotine gum, for what it is worth, has no sugar and no meaningful calories; it does not break a fast either.

Mints are the closer call, because a mint is a small sweet you swallow rather than something you spit out, and we settle them on their own page.

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.

This page is about the hours while the fast is running. The moment it ends is a different question with a different answer, and the answer depends almost entirely on how long you went: how to break a fast builds the sequence for eight lengths from twelve hours to five days, and says plainly which steps have a study behind them and which are only what people do.

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." What it will not do is check the coffee for you — the app logs no food or drink, only a daily protein and fibre entry — so keeping the fasting hours clean is still your job. The visual countdown and streak tracking keep you honest and motivated.

Frequently Asked Questions

Does toothpaste break a fast?

No. You spit it out, so it never really enters your digestive system. A pea-sized amount is a fraction of a calorie even if you swallow some, and fluoride has no metabolic effect. Brush your teeth normally.

Do BCAAs break a fast if the label says zero calories?

Yes. Calories are the least important part of the protein question. BCAAs deliver the amino acid signal that raises insulin and switches off the cleanup a fast is meant to start, and in a 2007 trial using glucose-equivalent drinks a branched-chain amino acid drink raised insulin significantly above the glucose reference on its own.

Do vitamins break a fast?

Most do not. Plain capsules and tablets — D3, magnesium, zinc, B vitamins, vitamin C — carry no calories and no insulin response. Gummy vitamins do break a fast, because the gummy is made of sugar. Anything in a juice or syrup base does too. If the label lists calories, treat it as food.

Does salt break a fast?

No. Salt is a mineral with no calories, no protein and no carbohydrate, so there is nothing for insulin to respond to. It becomes more useful the longer you fast, because low insulin and glycogen loss both drive sodium out.

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?

Not on calories, and the insulin case against it is weaker than commonly claimed — the direct test of sweet taste without swallowing found no insulin rise. Out for a strict fast on principle. The better reason to skip it on any fast is appetite: if the sweet taste sharpens cravings, it costs you more than the arithmetic says.

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.