Most things you consume during a fast won’t break it, as long as they contain virtually zero calories, no protein, and no carbohydrates. The key principle is simple: anything that triggers a meaningful insulin response or kicks your body out of its fasted metabolic state counts as breaking the fast. Water, black coffee, plain tea, and a handful of other items are safe. Everything else requires a closer look.
What “Breaking a Fast” Actually Means
When you fast, your body shifts from burning recently eaten food to burning stored energy. Insulin levels drop, fat burning ramps up, and cellular cleanup processes kick in. Breaking a fast means consuming something that reverses this shift, primarily by triggering insulin release. Even small amounts of protein, carbohydrates, or certain additives can signal your body to stop its fasting metabolism and start digesting.
This means the question isn’t really about calories alone. A tablespoon of coconut oil has calories but barely moves insulin. A sugar-free gummy vitamin might have only 10 calories but contains enough sugar and protein to provoke a response. What matters is the hormonal signal, not just the energy content.
Drinks That Keep You Fasted
Water is the obvious baseline, and you can drink as much as you want. Sparkling water and mineral water are equally fine as long as they contain no added flavors with sweeteners.
Black coffee is safe. It contains essentially no calories and does not raise insulin in any meaningful way. Some evidence suggests coffee may even enhance certain benefits of fasting by supporting fat oxidation. The catch is that it must be truly black: no cream, no sugar, no flavored syrups. Even a splash of milk adds enough protein and lactose to trigger a small insulin response.
Plain tea, whether black, green, white, or herbal, behaves similarly. Research on black tea consumption found no effect on fasting glucose or insulin levels, even over a full month of daily drinking. Herbal teas are fine too, but check the ingredients. Some blends include dried fruit pieces, honey crystals, or natural flavors that add trace sugars.
Artificial Sweeteners and the Sweet Taste Question
This is one of the most debated areas of fasting, and the research is more reassuring than most people expect. A systematic review and meta-analysis of randomized controlled trials found that low-energy sweeteners had no significant effect on blood sugar or insulin responses compared to control groups. The results held regardless of the type of sweetener, the dose consumed, or the participants’ baseline glucose and insulin levels.
There is a theoretical concern about the “cephalic phase insulin response,” the idea that simply tasting something sweet could trigger your brain to tell your pancreas to release insulin before any calories arrive. This response is real: orosensory signals do trigger neurally mediated insulin release. However, the magnitude of this effect from non-caloric sweeteners appears to be too small to meaningfully disrupt a fast in most people, based on the pooled clinical data showing near-zero insulin changes.
If your fasting goal is weight loss or general metabolic health, a diet soda or stevia-sweetened water is unlikely to cause problems. If you’re fasting for maximum autophagy (cellular cleanup) or doing a medically supervised protocol, some practitioners recommend avoiding all sweeteners to be safe. The stricter your goals, the more conservative you should be.
Apple Cider Vinegar and Lemon Water
A tablespoon of apple cider vinegar diluted in water contains roughly 3 calories and essentially no sugar. It won’t break your fast. In fact, small studies have shown it may lower both blood glucose and insulin, which aligns with fasting goals rather than working against them. If you drink it, dilute it well to protect your tooth enamel and stomach lining, and avoid varieties with added honey or fruit juice.
A squeeze of fresh lemon juice in water adds about 1 to 3 calories. This amount is negligible and won’t trigger insulin. Bottled lemon juice is usually fine too, but check labels for added sugars or preservatives. What you want to avoid are lemon-flavored waters that contain sweeteners or juice blends.
Electrolytes: Read the Label Carefully
Sodium, potassium, and magnesium in their pure forms contain zero calories and won’t affect your fast. This makes plain electrolyte supplements useful during longer fasts, where mineral depletion can cause headaches, dizziness, and muscle cramps. A pinch of salt in water is the simplest option.
Commercial electrolyte powders are where problems hide. Many popular brands include maltodextrin, dextrose, cane sugar, or fruit juice concentrate to improve taste. These are carbohydrates that will spike your blood sugar and break your fast immediately. Even “sugar-free” versions sometimes contain ingredients like maltodextrin as a filler, which has a higher glycemic index than table sugar. Look for products that list only mineral salts and perhaps citric acid for flavor, with no sweeteners of any kind. Some strict fasting protocols recommend avoiding even stevia and monk fruit in electrolyte mixes.
Supplements and Medications
Most standard pills and capsules taken with water are fine during a fast. A plain multivitamin tablet, vitamin D capsule, or magnesium pill won’t contain enough calories to matter. Fat-soluble vitamins (A, D, E, K) technically absorb better with food, so you may want to take them during your eating window for effectiveness rather than fasting concerns.
The supplements that will break your fast are easy to identify:
- Gummy vitamins contain sugar, gelatin (protein), and sometimes fat. They are essentially candy with vitamins added.
- Protein powders and amino acid supplements (like BCAAs) trigger insulin directly, even without carbohydrates.
- Oil-based supplements like fish oil or flaxseed oil capsules contain fat calories. A standard fish oil capsule has about 10 to 15 calories, which is borderline. One capsule is unlikely to fully break a fast, but taking several might.
- Collagen powders are pure protein and will break your fast.
- Any supplement with maltodextrin, pectin, cane sugar, or fruit juice concentrate in the ingredients list contains hidden carbohydrates.
For prescription medications, continue taking them as directed regardless of your fasting schedule. If a medication requires food, that takes priority over maintaining a fast.
The Gray Zone: Fats, Bone Broth, and Bulletproof Coffee
Pure fats like MCT oil, coconut oil, and butter occupy a gray area. They contain calories but provoke very little insulin response compared to carbohydrates or protein. Your body can convert MCT oil directly into ketones, which means it may support the fat-burning state rather than disrupt it. Some people add MCT oil or butter to their morning coffee during a fast, sometimes called “bulletproof coffee,” specifically to maintain ketosis while getting energy.
However, consuming fat does break a strict fast. Your body stops relying purely on stored energy and begins processing the incoming fat. If your goal is autophagy or a clean fast for medical testing, any caloric intake counts. If your goal is staying in ketosis or managing hunger while maintaining most metabolic benefits, small amounts of pure fat are a reasonable compromise. A tablespoon of MCT oil (about 100 calories) will keep you in ketosis but will technically end your fasted state.
Bone broth falls clearly on the “breaks your fast” side. Even a cup contains protein and some calories, enough to stimulate insulin and shift your body out of fasting mode.
A Quick Reference for Common Items
- Won’t break your fast: water, black coffee, plain tea, salt, apple cider vinegar (plain), a squeeze of lemon, zero-calorie sparkling water, plain electrolyte salts
- Probably won’t break your fast: diet soda, stevia or monk fruit in small amounts, a single fish oil capsule, zero-calorie sweeteners
- Will break your fast: cream or milk in coffee, bone broth, gummy vitamins, protein powder, BCAAs, collagen, juice of any kind, electrolyte powders with maltodextrin or sugar, any food
Match Your Rules to Your Goals
The strictness that matters depends entirely on why you’re fasting. Someone fasting 16 hours a day for weight management can comfortably drink black coffee with a zero-calorie sweetener and take plain supplements without undermining their results. Someone doing a 48-hour fast for deep cellular cleanup benefits should stick to water, plain salt, and nothing else.
The most common mistake is overthinking the small stuff while missing the obvious. A splash of lemon in your water won’t matter. The flavored creamer you add to coffee without thinking about it absolutely will. Focus on avoiding anything with protein, carbohydrates, or meaningful calories, and you’ll maintain your fast through the entire window.