Most things you eat or drink with calories will break your fast, but the picture gets more nuanced with common gray-area items like black coffee, artificial sweeteners, and supplements. Whether something “breaks” your fast also depends on why you’re fasting. A splash of cream might not derail weight loss goals, but it will interrupt autophagy, your body’s cellular cleanup process. Here’s a practical breakdown of what’s safe and what isn’t during a fasting window.
Why Your Fasting Goal Matters
Fasting triggers several distinct biological processes, and different foods or drinks interrupt them at different thresholds. The three main reasons people fast are weight loss (through calorie restriction and fat burning), insulin management (keeping insulin low to improve metabolic health), and autophagy (cellular repair that ramps up when nutrients are absent). Something that keeps you in fat-burning mode might still raise insulin slightly, and something that doesn’t touch insulin might still deliver enough calories or protein to shut down autophagy.
There is no universally validated “50-calorie rule,” despite how often you’ll see it repeated online. Clinical fasting studies consistently define fasting days as zero-calorie intake, permitting only water, plain tea, and black coffee. The idea that anything under 50 calories is fine comes from the intermittent fasting community, not from controlled research. If your goal is autophagy or strict metabolic fasting, the safest approach is zero calories.
Black Coffee and Plain Tea
Black coffee is one of the safest things to drink while fasting, and it may actually enhance your fast rather than undermine it. Research published in Cell Cycle found that coffee rapidly triggers autophagy across multiple tissues in mice, mimicking the cellular effects of nutrient deprivation. This happened with both regular and decaffeinated coffee, meaning the benefit comes from compounds in coffee itself, not just caffeine. Caffeine independently inhibits a growth-signaling pathway called mTOR, which further supports cellular cleanup.
Plain black or green tea is similarly permitted in virtually every fasting protocol used in clinical research. Both contain negligible calories and no macronutrients that would trigger an insulin response. The key word is “plain.” The moment you add sugar, honey, milk, or cream, you’ve introduced calories and proteins that will raise insulin and halt autophagy.
Things That Clearly Break Your Fast
Any food with meaningful calories, protein, carbohydrates, or fat will end your fasted state. But some items catch people off guard because they seem small or health-related:
- Gummy vitamins and chewable supplements: These are loaded with sugar and contain enough calories to spike insulin. They will break your fast every time.
- Collagen powder: A typical 13-gram serving contains around 47 calories, all from protein. Amino acids raise insulin and directly inhibit autophagy. Save collagen for your eating window.
- Bone broth: Even a small cup contains protein and fat, usually 30 to 50 calories. It breaks a fast.
- Milk, cream, or creamers in coffee: Even a tablespoon of half-and-half adds about 20 calories from fat and a small amount of protein, enough to trigger an insulin response.
- Fruit juice, smoothies, and sodas: These are essentially liquid sugar and will immediately end any fasted state.
- Bulletproof coffee or MCT oil: Despite claims that pure fat “doesn’t count,” adding oil to coffee delivers significant calories (over 100 per tablespoon of MCT oil) and will stop autophagy. It may keep insulin relatively low compared to carbs, but it is not fasting.
Artificial Sweeteners: A Gray Area
This is where things get complicated, because different sweeteners behave differently in your body. A clinical study published in Appetite compared stevia, aspartame, and regular sugar. Stevia produced significantly lower insulin levels than both aspartame and sugar at 30 and 60 minutes after a meal. Aspartame also produced lower insulin than sugar, but the gap was smaller.
What this means in practice: stevia in your coffee or tea is the least likely artificial sweetener to disrupt your fast from an insulin perspective. Aspartame (found in diet sodas and many sugar-free drinks) has a slightly larger effect. Sucralose, found in products like Splenda, has shown mixed results in other research, with some studies suggesting it can raise insulin in certain people.
If you’re fasting strictly for autophagy, even the sweet taste on your tongue could theoretically trigger a small preparatory insulin release, sometimes called the cephalic phase insulin response. For weight loss fasting, a packet of stevia in black coffee is unlikely to make a meaningful difference. For maximum metabolic benefits, plain is always safest.
Apple Cider Vinegar and Lemon Water
Apple cider vinegar is not only safe during a fast, it may complement your fasting goals. A tablespoon contains roughly 3 calories, virtually no sugar, and no protein. Research in the Journal of Diabetes Research found that vinegar actually lowered blood glucose levels and reduced postmeal insulin spikes when consumed before eating. It improved how efficiently muscles absorbed glucose from the bloodstream. Diluting a tablespoon in water during your fast won’t break it and may help with blood sugar control once you do eat.
Lemon water is similarly safe in small amounts. A squeeze of lemon juice (about half a lemon) adds roughly 5 calories and a negligible amount of sugar. This is not enough to trigger a meaningful insulin response. A full glass of lemonade with added sugar, however, is a different story entirely.
Medications and Standard Supplements
Most pill-form vitamins and medications contain minimal or zero calories and won’t break your fast. Fat-soluble vitamins like A, D, E, and K are better absorbed with food, so you may want to take them during your eating window for effectiveness rather than fasting concerns. Water-soluble vitamins like B and C can be taken anytime without issue.
Electrolyte supplements without added sugar (sodium, potassium, magnesium) are fine and can actually help prevent the headaches and fatigue that sometimes accompany longer fasts. Watch out for electrolyte drinks or powders that contain dextrose or maltodextrin, which are sugars that will raise insulin.
Fish oil capsules typically contain 10 to 15 calories from fat. For weight loss fasting, this is negligible. For strict autophagy fasting, even this small caloric load is technically breaking your fast. Branched-chain amino acids (BCAAs), which some people take for workouts, are protein and will absolutely break a fast.
Quick Reference by Fasting Goal
If you’re fasting for weight loss, your main concern is keeping calories very low. Black coffee, plain tea, water, stevia, apple cider vinegar, lemon water, and calorie-free electrolytes are all fine. A small splash of cream is unlikely to derail your results, though it’s not technically fasting.
If you’re fasting for insulin control, avoid anything with sugar, protein, or significant carbohydrates. Black coffee, plain tea, water, apple cider vinegar, and stevia are your safest options. Skip diet sodas with aspartame or sucralose if you want to be cautious.
If you’re fasting for autophagy, stick to water, black coffee (which actively supports autophagy), and plain tea. Avoid all calories, all sweeteners, all supplements with protein or sugar. Even small amounts of amino acids from collagen or BCAAs will signal your cells to stop the cleanup process and shift back into growth mode.