During intermittent fasting, you can have water, black coffee, plain tea, and most zero-calorie beverages without breaking your fast. But beyond that short list, the answer depends on your fasting goal. Someone fasting for weight loss has more flexibility than someone fasting for cellular cleanup (autophagy), and that distinction changes which drinks, supplements, and small additions are on or off the table.
Water, Coffee, and Tea
Plain water is always fine, including sparkling or mineral water. Black coffee and unsweetened teas, whether black, green, or herbal varieties like peppermint, are also widely considered safe during a fasting window. They contain essentially zero calories and don’t trigger a meaningful insulin response. Coffee may even support fasting by mildly suppressing appetite and boosting fat burning.
The key word is “plain.” Adding sugar, milk, cream, or flavored syrups introduces calories, protein, and carbohydrates that will pull you out of a fasted state. A splash of milk in your coffee might seem negligible, but even small amounts of protein and lactose (milk sugar) can stimulate insulin release.
What About Cinnamon, Lemon, or Apple Cider Vinegar?
A pinch of cinnamon in your coffee or tea adds a trace amount of calories (roughly 2 per half teaspoon) and is generally considered fine during a fast. It won’t trigger an insulin response in that quantity.
Lemon water is a popular fasting companion. A squeeze of lemon in water adds about 1 to 3 calories and may actually help: lemon juice has been shown to reduce the glycemic response when consumed with meals, meaning it can support blood sugar stability. It won’t break your fast. Apple cider vinegar diluted in water is similarly low-calorie, and a meta-analysis of seven studies found that regular ACV consumption reduced fasting blood sugar and long-term blood sugar markers. A tablespoon diluted in water during your fast is a common practice. Just dilute it well to protect your tooth enamel.
Artificial and Natural Sweeteners
This is where things get more nuanced. Not all zero-calorie sweeteners behave the same way in your body.
Monk fruit extract appears to be one of the cleanest options. The body doesn’t recognize its sweet compounds (mogrosides) as carbohydrates, so they don’t trigger an insulin response. Animal research has shown monk fruit extract may actually suppress blood sugar spikes. Stevia is similarly low-risk, with most evidence suggesting it doesn’t raise insulin levels in the small amounts used to sweeten a drink.
Other sweeteners are less clear-cut. Some sugar alcohols like maltitol contain calories and can affect blood sugar. Diet sodas sweetened with aspartame or sucralose are technically calorie-free, but there’s ongoing debate about whether the sweet taste alone can trigger what’s called a cephalic phase insulin response, where your body releases a small amount of insulin just from tasting something sweet. If your goal is a strict metabolic fast, plain beverages are the safer bet.
Bulletproof Coffee and MCT Oil
Bulletproof coffee, made by blending butter or ghee and MCT oil into black coffee, is popular in fasting and keto circles. The logic is that pure fat doesn’t spike insulin the way protein or carbs do, so it keeps you in a “fat-burning” state even though it adds significant calories (often 200 or more per cup).
There’s some truth to this. MCT oil rapidly elevates ketone levels, which can support fat oxidation and may even promote cellular cleanup through effects on your cells’ energy-sensing pathways. For someone fasting primarily for weight loss or appetite control, bulletproof coffee can make longer fasts more sustainable without derailing fat burning.
But it does break a strict fast. Any caloric intake interrupts the complete nutrient deprivation that drives autophagy. If cellular repair is your goal, skip the butter and oil and stick with black coffee.
Bone Broth
Bone broth sits in a gray area. A cup contains roughly 40 to 50 calories along with protein and all nine essential amino acids, including glycine, which has been shown to support healthy insulin levels. That protein content means bone broth breaks a strict fast.
For fat loss, though, it can be a useful tool. If you’re struggling with hunger toward the end of a long fasting window, a mug of bone broth can improve satiety and prevent the kind of snack spiral that leads to overeating when you finally break your fast. Think of it as a strategic compromise: not a “clean” fast, but potentially better for long-term adherence than white-knuckling it through hunger and then binging.
Supplements and Vitamins
Most capsule-form vitamins and minerals are fine during a fast. They contain negligible calories and won’t affect your insulin levels. Electrolyte supplements (sodium, potassium, magnesium) without added sugar are also safe and can help with the headaches, dizziness, and fatigue some people experience during longer fasts.
Gummy vitamins are a different story. Most contain between 2 and 8 grams of sugar per serving. Even sugar-free versions typically use fruit juice concentrates or sugar alcohols as sweeteners. Save your gummy vitamins for your eating window.
Protein-based supplements like branched-chain amino acids (BCAAs) also break a fast. Research published in Nature found that BCAAs can acutely impair insulin sensitivity and raise blood glucose. In one study, the amino acid valine alone elevated blood glucose by nearly 100% within an hour. Collagen powders and protein shakes have the same issue: amino acids trigger an insulin response. Take these during your eating window.
The “50-Calorie Rule”
You may have seen the claim that anything under 50 calories won’t break a fast. This idea circulates widely in fasting communities, but it doesn’t come from any specific study. The scientific literature on intermittent fasting defines fasting periods as having “little to no caloric intake,” without naming a precise calorie threshold.
In practice, small amounts of calories from something like a squeeze of lemon or a pinch of cinnamon are unlikely to meaningfully affect your insulin levels or fat burning. But treating 50 calories as a hard cutoff is misleading, because the type of calorie matters as much as the amount. Ten calories from sugar will spike insulin more than 10 calories from fat. A few calories from coffee compounds behave differently in your body than a few calories from a cookie.
Rather than counting to 50, a better rule of thumb is to avoid anything with protein, carbohydrates, or sugar during your fast. Small amounts of fat are less disruptive, and trace calories from coffee, tea, or lemon are functionally irrelevant.
Quick Reference by Fasting Goal
- Weight loss: Water, black coffee, plain tea, lemon water, ACV, zero-calorie sweeteners like monk fruit or stevia, electrolytes, and even bone broth or a small amount of MCT oil if it helps you stick with the fast.
- Autophagy and cellular repair: Water, black coffee, plain tea, lemon water, ACV, and zero-calorie electrolytes only. Any calories from protein, carbs, or fat will reduce or interrupt the cellular cleanup process.
- Blood sugar management: Water, black coffee, plain tea, and ACV (which may actively help). Avoid all sweeteners, even calorie-free ones, until more is known about their insulin effects.
The eating window is where your nutrition happens. The fasting window is about giving your body a break from digestion and insulin signaling. Keep it simple: the fewer ingredients you add, the cleaner the fast.