During a fast, you can have water, black coffee, plain tea, and most zero-calorie beverages without disrupting the metabolic benefits you’re fasting for. But the full answer depends on why you’re fasting, because different goals have different thresholds for what counts as “breaking” a fast.
Someone fasting for weight loss and fat burning has more flexibility than someone fasting strictly for cellular cleanup (autophagy). Here’s a practical breakdown of what’s safe, what’s borderline, and what will end your fast.
The Always-Safe List
Water is the obvious baseline. Still, sparkling, or mineral water are all fine. If you’re doing an extended fast (longer than 16 hours), adding electrolytes becomes important. Sodium, potassium, and magnesium are lost through normal body processes and through the increased water intake most fasters rely on. An unsweetened electrolyte powder or a pinch of salt in your water keeps you from feeling lightheaded or shaky without adding calories or triggering an insulin response.
Black coffee and plain tea (green, black, herbal) have essentially zero calories and don’t break a fast for most purposes. Coffee may actually support fasting by boosting fat oxidation and ketone production. One caveat: caffeine can affect how your body handles insulin in some individuals, with roughly 200 milligrams (about two cups of coffee) being enough to cause changes in certain people. If you notice jitteriness or energy crashes during fasted coffee drinking, you may want to switch to decaf or tea.
Sweeteners: Which Ones Are Safe
Artificial sweeteners like sucralose, aspartame, and acesulfame potassium don’t raise blood sugar on their own. That makes them technically acceptable during a fast if your goal is maintaining low insulin levels. However, the picture gets murkier with stricter fasting goals. Some evidence suggests that the sweet taste itself can trigger a small preparatory insulin release from your pancreas, even when no sugar actually arrives. This “anticipatory” response is small, but if you’re fasting for autophagy, it’s worth skipping sweeteners entirely.
Stevia and monk fruit extract fall into the same category. They won’t spike blood sugar, and for most intermittent fasters focused on weight management, they’re a reasonable way to make black coffee or tea more palatable. Just avoid sweetener packets that contain dextrose or maltodextrin as fillers, since those are actual sugars that will break your fast.
Apple Cider Vinegar and Lemon Water
Apple cider vinegar (a tablespoon diluted in water) contains roughly 3 calories and has been shown to lower both blood glucose and insulin levels. It’s one of the few things that may actively support your fasted state rather than threaten it. A small squeeze of lemon juice in water adds about 2 to 3 calories and a negligible amount of sugar. Neither of these will meaningfully interrupt a fast.
Fats, MCT Oil, and Butter Coffee
This is where fasting gets into a gray zone. Pure fats are the least likely macronutrient to spike insulin or activate the growth-signaling pathways (called mTOR) that fasting is designed to quiet. MCT oil in particular travels quickly to your liver and gets converted into ketones, which can actually deepen the fat-burning state your fast creates. A tablespoon or two of MCT oil can enhance ketosis without derailing a fast.
Butter coffee (coffee blended with butter or ghee and MCT oil) follows the same logic. You’re adding calories, so in the strictest sense you’re no longer fasting. But because those calories come entirely from fat, your body stays in a fat-burning, low-insulin state. If your goal is weight loss or sustained energy, butter coffee is a reasonable compromise. If your goal is autophagy, the cellular recycling process that peaks during zero-calorie fasts, any calories will blunt it to some degree.
Bone Broth
Bone broth contains protein, and protein triggers insulin and mTOR signaling more than fat does. A cup of bone broth typically has 30 to 50 calories and several grams of protein, which means it does technically break a fast. That said, glycine, the amino acid most abundant in bone broth, has shown promise for supporting insulin sensitivity and glucose tolerance. Some fasters, especially those doing longer 24- to 72-hour fasts, use bone broth strategically to maintain electrolytes and prevent muscle breakdown while accepting a partial compromise on the purity of their fast.
If you’re doing a standard 16:8 intermittent fast, bone broth is probably not worth the trade-off. Save it for your eating window. For extended multi-day fasts, a cup or two can make the experience far more sustainable.
Sugar-Free Gum
Sugar-free gum is safe for most fasting purposes. A 2015 study of 12 fasting participants found that chewing sugar-free gum for 30 minutes did not affect insulin levels. It may even help: that same study showed gum increased satiety and stabilized an appetite-suppressing hormone called GLP-1. A separate 2017 study of 33 people found that chewing sugar-free gum for an hour while fasting reduced hunger and led to eating less at the next meal. Each piece contains 2 to 5 calories, which is metabolically insignificant.
Vitamins and Supplements
Most supplements won’t break your fast, but timing matters for absorption. Fat-soluble vitamins (A, D, E, and K) need dietary fat to be absorbed properly, so taking them during a fast means you’re largely wasting them. Save those for your eating window when you’ll have a meal with some fat.
Water-soluble vitamins like B vitamins and vitamin C can be taken fasted, though some people experience nausea taking them on an empty stomach. Electrolyte supplements (sodium, potassium, magnesium, calcium) are not only safe during a fast but actively recommended, especially if you’re fasting longer than 16 hours or exercising in a fasted state. Plain creatine and basic mineral supplements are also fine.
Gummy vitamins, on the other hand, contain sugar and calories. They’ll break your fast. Same goes for any supplement in a soft gel with added oils, though the calorie count is usually small enough (under 10 calories) that it won’t meaningfully disrupt fat burning.
A Quick Reference by Fasting Goal
- Weight loss and fat burning: Water, coffee, tea, zero-calorie sweeteners, electrolytes, ACV, lemon water, MCT oil, butter coffee, and sugar-free gum are all reasonable. Bone broth is borderline.
- Insulin reduction and blood sugar control: Stick to water, coffee, tea, electrolytes, ACV, and lemon water. Avoid sweeteners if you want to be cautious, and skip anything with protein or significant calories.
- Autophagy and cellular repair: Water, plain coffee, plain tea, electrolytes, and ACV only. Any calories, including from fat, will reduce autophagy signaling to some extent. This is the strictest approach.
The practical takeaway: if you’re doing standard intermittent fasting for weight management, you have quite a bit of flexibility. Black coffee with a splash of MCT oil and an electrolyte drink will keep you comfortable and energized without undermining your results. The more specific or therapeutic your fasting goal, the more you should stick to plain, zero-calorie options.