Water, black coffee and calorie-free supplements are usually fine; protein shakes and amino acids count as food. What breaks a fast depends on your goal.
Key Takeaways
Whether something breaks a fast depends on your goal, from weight management to a gut rest to stricter aims such as autophagy.
Water, black coffee and plain tea are essentially calorie-free and fine; adding milk, sugar or syrup adds calories.
Calorie-free vitamin, mineral and electrolyte supplements usually do not break a fast, but check the label for calories, sugars and sweeteners.
Protein shakes and amino acids, including BCAAs, carry calories and count as food, so they belong in your eating window.
Fasting is an optional tool rather than a rule, it is not suitable for everyone, and anyone with a health condition or medication should check with a GP or dietitian first.
Evan Lynch is a CORU-registered dietitian with a BSc in Food Science and Health, a Postgraduate Diploma in Dietetics, and an MSc in Sports Nutrition. Evan is the Head of Nutrition for Europe at AG1, where he supports strategic nutrition direction, education, and evidence-based communication. Alongside this role, he runs a private practice supporting clients across sports nutrition, gastrointestinal disorders, geriatric nutrition, and weight management.
Take up skipping breakfast or eating inside a set window and one question starts trailing you around. Does this break my fast? It usually lands on the morning coffee, the daily supplements, or the "zero-calorie" can in the fridge door.
The short version is that it depends on your goal, and it is less complicated than the forums make out. What follows is a plain guide to what does and does not break a fast, treating fasting as one optional tool rather than a set of commandments. For the bigger picture of how it all works, our intermittent fasting guide covers that ground, so this piece stays on the "what breaks it" question.
To break a fast is to give your body enough of something that it clicks out of the fasted state and starts processing what came in. There is no single tidy cut-off, because the metabolic effects of fasting stack up over hours,⊕ so where the line falls depends on why you are fasting in the first place:
So when someone asks whether coffee breaks a fast, the fair answer is a question back: for which goal? Keep that in your pocket for everything below.
For most people fasting for weight management or a gut rest, no. Water, black coffee and plain tea carry next to no calories, so they will not shift you out of the fasted state in any way that matters. Water is entirely fine, and staying hydrated takes the edge off the hunger and headaches that tend to show up early in a window.
Coffee and tea are fine as long as they are black. Add milk, sugar or syrup and you are adding calories. A splash of milk in one coffee is trivial for a weight-management goal, though not quite "nothing" for a strict one. Quick word on caffeine: healthy adults can have up to about 400 mg a day without safety concerns,⊕ roughly four cups of coffee, dropping to around 200 mg in pregnancy. Fasting does not change those numbers, but an empty stomach can make caffeine hit harder, so ease off if it leaves you wired.
Mostly no, with a couple of things to check.
A lot of people take a daily blend of vitamins, minerals and other nutrients and wonder whether it fits inside a fasting window. As with any supplement, it comes down to what is in it.
Plain vitamin and mineral supplements with no calories will not break a fast. Electrolytes, meaning sodium, potassium and magnesium, are often useful across longer windows and are fine so long as the product is calorie-free. Give a daily foundational blend the same test: many carry a small number of calories from their ingredients, which is negligible for a weight-management or gut-rest goal but does register if you are strict about autophagy. The label settles it, so glance at the calories and any added sugars or sweeteners before you assume.
The clearer exception is amino acids, BCAA supplements included. However often they are sold as "zero sugar", amino acids carry calories and prompt an insulin response,⊕ so in any strict sense they do break a fast. If your fast is about giving your metabolism a proper rest, file BCAAs under food.
A protein shake breaks a fast. It has protein and therefore calories, and it sets digestion and a metabolic response going. A shake is a small meal, so it belongs in your eating window, not your fasting one.
Artificially sweetened "zero-calorie" drinks are the grey area. A drink with no calories at all will not touch your energy balance, but the effect of sweeteners on appetite and metabolism is still up for debate, and for some people they just stoke cravings. A reasonable rule of thumb: for weight management, a calorie-free drink is unlikely to matter much; if you are being strict, plain water, black coffee or tea are the safe bets.
Strictly speaking, any calories at all break a fast. In the practical sense most people actually care about, very small amounts, the milk in a coffee or a calorie-free drink, do little to a weight-management or gut-rest goal. The tighter your objective, the lower the bar drops.
Worth being a bit wary of turning this into precise calorie-counting, though, because for some people that slides into an unhelpfully obsessive relationship with food. The spirit of the answer beats the decimal point: keep the fasting window low in calories, and do not let the number become the thing you fixate on.
Fasting is one approach to eating rather than a rule,⊕ and it is entirely optional. If it makes eating simpler and you feel well on it, good. If it leaves you anxious, preoccupied or unwell, it is not the tool for you, and nobody hands out prizes for persisting.
It is also not for everyone. The NHS advises that some people should not fast,⊕ including anyone pregnant or breastfeeding and anyone with a history of, or a leaning towards, an eating disorder. People with diabetes are told never to delay or skip meals without medical guidance, because fasting can affect blood sugar. If you have any health condition or take regular medication, check with your GP or a registered dietitian before you start, and tell your healthcare team if you change how you eat so any medicines can be adjusted.
And if fasting is turning into a way to restrict, or there is a history of disordered eating behind you, please step back and get support. Your GP can help, and the charity offers support across the UK.
What breaks a fast depends on your goal, but the everyday version fits in your head easily enough: water, black coffee and plain tea are fine, calorie-free supplements and electrolytes are usually fine, and anything carrying proper calories, protein shakes, amino acids, milky or sugary drinks, counts as food. Keep the fasting window low in calories, treat fasting as an optional tool rather than a rule to obey, and skip it altogether if it is not right for you. The foundations of good eating matter far more than the exact edges of the window.
