What you can have during a fast depends entirely on why you’re fasting. A medical fast before bloodwork or surgery has strict rules (usually nothing but water). Intermittent fasting for weight loss or metabolic health is more flexible. And religious fasts follow their own guidelines. Most people searching this question are practicing some form of intermittent fasting, so that’s the focus here: what you can put in your body during your fasting window without undermining the metabolic benefits you’re after.
Water, Coffee, and Tea
Plain water is always fine during a fast. Still or sparkling, it has zero calories and no effect on insulin. Staying hydrated actually makes fasting easier, since mild dehydration can mimic hunger signals. Adding a pinch of salt or a squeeze of lemon won’t meaningfully affect your fasted state.
Black coffee and plain tea (green, black, herbal) are also widely considered safe during a fast. Both contain zero calories when you skip the cream and sugar. Coffee can even support fasting by temporarily blunting appetite. Just avoid flavored varieties that sneak in sweeteners or oils.
Why the Type of Fast Matters
The confusion around “what breaks a fast” exists because fasting produces several distinct benefits, and different substances interfere with different ones. The three main goals people fast for are:
- Fat burning and ketosis: Your body switches from burning glucose to burning stored fat. Anything that spikes blood sugar or insulin can interrupt this shift.
- Autophagy: The cellular cleanup process where your body recycles damaged components. This is generally believed to require very low nutrient intake, particularly low protein and carbohydrate levels.
- Calorie restriction: Simply consuming fewer total calories for weight loss. Small amounts of calories during the fasting window may not matter much here, as long as you stay in a significant deficit overall.
If your primary goal is weight loss through calorie restriction, a splash of cream in your coffee probably won’t derail your results. If you’re fasting specifically for autophagy, you’ll want to be stricter. There is no single universally agreed-upon calorie threshold that “breaks” a fast. The commonly cited “50 calorie rule” circulates online but lacks direct clinical support. A major trial published in the New England Journal of Medicine that studied time-restricted eating allowed only noncaloric beverages outside the eating window, suggesting researchers treat even small calorie amounts as potentially meaningful.
Artificial Sweeteners and Sugar Substitutes
This is where things get complicated. Not all zero-calorie sweeteners behave the same way in your body.
Erythritol, a sugar alcohol found in many “keto-friendly” products, has a glycemic index of 0 and an insulinemic index of 2 (compared to 100 for glucose on both scales). Studies in both lean and obese subjects, including those with diabetes, show that doses of erythritol up to 75 grams don’t affect blood glucose or insulin levels. If any sweetener is unlikely to disrupt a metabolic fast, erythritol is a strong candidate.
Other sweeteners are less clear-cut. Research on sugar-free energy drinks found that even calorie-free versions promoted insulin resistance, elevated blood sugar, and increased triglycerides at rates comparable to their sugary counterparts over time. The sweeteners and additives in these drinks appeared to cause metabolic disruption despite containing no sugar. This doesn’t mean a single packet of sweetener in your morning tea will wreck your fast, but regular consumption of artificially sweetened drinks during fasting windows may work against your goals.
Some medical fasting protocols, like those from Marshall Medical, go further and exclude all artificial sweeteners during fasting, including stevia, sucralose, aspartame, and saccharin. If you want to play it safe, stick with plain beverages.
Fats: Butter, MCT Oil, and Bone Broth
Adding butter or MCT oil to coffee (the “bulletproof coffee” approach) is popular among people combining intermittent fasting with a ketogenic diet. Technically, any calories break a strict fast. A single tablespoon of MCT oil contains roughly 121 calories.
However, pure fat has minimal impact on insulin. MCT oil in particular converts rapidly into ketones, which can actually help sustain ketosis. Some experts suggest that small amounts of MCT oil may not significantly disrupt autophagy either, since fat provides energy without raising insulin levels the way protein or carbohydrates do. The practical benefit is real: a small amount of fat can make a long fasting window much more tolerable by increasing satiety, which helps you fast longer overall.
The trade-off is straightforward. If you’re fasting strictly for autophagy or following a clinical protocol, skip the fat. If you’re fasting for weight management and ketosis, one to two teaspoons of MCT oil or a small pat of butter in your coffee is unlikely to undermine your results and may actually help you stick with the routine. Bone broth falls in a similar gray area: it contains calories and some protein, so it breaks a strict fast, but the amounts are small enough that many people use it as a tool during extended fasts.
Supplements, Vitamins, and Medications
Pure vitamins and minerals in capsule or tablet form are generally fine during a fast, as long as they don’t contain added sugars, starches, or fillers sweetened with stevia or monk fruit. Read the label. Gummy vitamins are out since they contain sugar and calories. Capsules with binding agents are usually negligible.
Fat-soluble vitamins (A, D, E, and K) are a different story. These need dietary fat to be absorbed properly. Vitamin D in particular is best taken with food. If you take these during your fasting window, your body won’t absorb them well. Save them for your eating window. The same applies to fish oil, CoQ10, and any supplement that contains fats or oils as part of its formula.
Supplements containing amino acids or protein (like collagen powder or BCAAs) will break a fast. Protein triggers an insulin response and activates pathways that counteract autophagy.
Electrolyte supplements (sodium, potassium, magnesium) without added sugar are fine and often helpful during longer fasts, when mineral depletion can cause headaches, dizziness, or muscle cramps.
Medications
Most prescription medications can be taken during a fast, but some need food to work properly or to protect your stomach. NSAIDs like ibuprofen are among the most common offenders. They irritate the stomach lining, and taking them on an empty stomach increases the risk of nausea, ulcers, and gastric bleeding. If you regularly take ibuprofen or similar pain relievers, take them during your eating window with food or a full glass of milk. The same goes for any medication your pharmacist has labeled “take with food.” If you’re unsure about a specific prescription, the timing of your eating window can usually be adjusted to accommodate it.
Sugar-Free Gum and Mints
A single piece of sugar-free gum contains roughly 5 to 10 calories, mostly from sugar alcohols like xylitol or sorbitol. One or two pieces won’t meaningfully affect your insulin levels or caloric intake. If chewing gum helps you get through your fasting window without snacking, the trade-off is worth it for most people. Just don’t go through half a pack, as the calories and sugar alcohols add up, and large quantities of sugar alcohols can cause digestive discomfort.
What Clearly Breaks a Fast
For the sake of clarity, here’s what will pull you out of a fasted state regardless of your goals:
- Any food with calories: Even “healthy” snacks like a handful of nuts or a piece of fruit fully break a fast.
- Milk, cream, or creamers: These contain protein, fat, and often sugar. Even a tablespoon of milk adds enough protein and carbohydrate to trigger a small insulin response.
- Sugary or diet sodas: Sugar-sweetened beverages obviously break a fast. Diet versions, while calorie-free, may promote insulin resistance with regular use.
- Juice, smoothies, or protein shakes: All contain significant calories, sugar, or protein.
- Alcohol: Contains calories, requires liver processing, and disrupts the metabolic state fasting creates.
A Practical Framework
Rather than obsessing over whether a specific item “breaks” your fast, match your strictness to your goal. If you’re fasting for weight loss, small amounts of zero-calorie sweeteners, a splash of cream, or a piece of gum are unlikely to matter. Your overall calorie deficit across the day is what drives results. If you’re fasting for metabolic benefits like improved insulin sensitivity and ketosis, stick with water, black coffee, plain tea, and possibly small amounts of pure fat. If autophagy is your specific aim, water and plain beverages only is the safest approach, since the science on what preserves or disrupts autophagy in humans is still limited.
Consistency matters more than perfection. A fasting routine you can sustain five days a week with minor “impurities” will deliver better long-term results than a strict protocol you abandon after two weeks.