Water, black coffee, and plain tea are the safest drinks during a fast, with zero calories and no meaningful effect on insulin. Beyond those three, your options expand or narrow depending on why you’re fasting and how strict you want to be. Most beverages that stay under roughly 1 to 5 calories per serving will keep you in a fasted state for fat-burning purposes, but even small amounts of protein or sugar can interrupt deeper cellular processes like autophagy.
Why What You Drink Matters
Fasting works by keeping insulin low. When insulin drops and stays low, your body shifts to burning stored fat for energy and eventually ramps up autophagy, a cellular cleanup process where damaged components get recycled. Anything that triggers an insulin response, even a modest one, can slow or stop these processes. Carbohydrates cause the sharpest insulin spikes, but protein also stimulates insulin to a lesser degree. Fat has the smallest effect of the three.
This means the main things to watch for in any drink are calories, sugar (including hidden sugars like maltodextrin and dextrose), and protein. Pure electrolytes, minerals, and most zero-calorie compounds pass through without disrupting your fast.
Water and Sparkling Water
Plain water is the gold standard. Still or sparkling, it has zero calories and no insulin effect. You can drink as much as you want. Sparkling water with “natural flavors” and no sweeteners (like plain LaCroix or Perrier) is also fine, as these typically contain no calories or sugar. Check the label: if the nutrition panel reads zero calories and zero sugar, you’re in the clear.
Watch out for flavored sparkling waters that add sweeteners, juice, or citric acid blends. Some brands sneak in small amounts of sugar or calories that aren’t obvious from the front label. If it lists anything beyond carbonated water and natural flavors, read the nutrition facts closely.
Black Coffee
Black coffee does not break a fast. A standard cup contains roughly 2 to 5 calories, almost entirely from trace amounts of protein and oils in the beans. In a clinical trial with healthy women, a single cup of black Arabica coffee actually lowered blood glucose and cortisol levels without increasing insulin. Coffee also appears to support autophagy rather than hinder it, making it one of the few drinks that may actively complement fasting.
The key word is “black.” Adding milk, cream, sugar, or flavored syrups introduces enough calories and macronutrients to trigger an insulin response. Even a tablespoon of cream adds about 50 calories and some protein. If you can’t stand black coffee, a tiny splash of heavy cream (under a teaspoon) is the least disruptive option, but purists avoid it entirely.
Tea
Green tea, black tea, herbal tea, and white tea are all safe during a fast when consumed plain. Like black coffee, unsweetened tea has negligible calories and no meaningful insulin impact. Green tea in particular contains compounds that may support fat oxidation during a fast.
Avoid tea lattes, chai with milk, and any bottled tea products. Bottled teas almost always contain added sugar, sometimes 20 to 30 grams per bottle. Brew your own and skip sweeteners.
Electrolyte Drinks
Electrolytes themselves (sodium, potassium, magnesium) contain zero calories and won’t break a fast. In fact, supplementing electrolytes is often a good idea during longer fasts, since your body excretes more sodium and potassium when insulin is low. This is why many people feel lightheaded, crampy, or fatigued after 16 to 24 hours of fasting.
The problem is what companies mix in with those electrolytes. Many popular electrolyte powders and sports drinks contain sugar, dextrose, maltodextrin, or other caloric fillers that will absolutely break your fast. Look for sugar-free electrolyte mixes with no calories on the label. You can also make your own by adding a pinch of salt and a squeeze of lemon to water.
Lemon Water and Apple Cider Vinegar
A squeeze of fresh lemon in your water adds roughly 1 to 3 calories, which is too little to affect insulin or disrupt a fast. It can make plain water more palatable during long fasting windows, and the small amount of citric acid won’t cause problems.
Apple cider vinegar diluted in water (1 to 2 tablespoons in a glass) is another popular fasting drink. It contains about 3 calories per tablespoon and essentially no sugar. The acetic acid in vinegar has actually been shown to lower blood glucose and insulin levels in small studies, which is the opposite of breaking a fast. Dilute it well to protect your tooth enamel and stomach lining, and don’t drink it straight.
What About Diet Soda and Zero-Calorie Sweeteners
This is where things get more nuanced. Monk fruit sweetener is not absorbed by the upper digestive tract and has no direct impact on blood glucose. It’s one of the cleanest options if you need sweetness. Stevia behaves similarly, with minimal to no insulin response in most research.
Artificial sweeteners like aspartame and sucralose (found in most diet sodas) are more debated. They contain zero calories and don’t raise blood glucose directly, so they technically won’t break a fast from a caloric standpoint. However, some evidence suggests they may trigger an insulin response through taste receptors or affect gut bacteria in ways that could undermine fasting benefits. If your goal is maximum autophagy or gut rest, skip them. If you’re fasting primarily for weight loss, a diet soda is unlikely to derail your results.
Bone Broth
Bone broth contains 8 to 12 grams of protein per serving, which is enough to stimulate some insulin release and interrupt autophagy. It does not spike blood sugar significantly, as it has negligible carbohydrates, and continuous glucose monitor data typically shows a flat response. But the amino acids in bone broth, particularly branched-chain amino acids like leucine, activate a pathway called mTOR that directly suppresses autophagy.
For strict fasting, bone broth breaks the fast. For more flexible approaches, like a modified fast or a fasting-mimicking protocol, many people use bone broth to ease hunger during longer fasts (24 hours or more) while accepting the tradeoff. It won’t knock you out of fat-burning mode the way a meal would, but it does end the deeper cellular cleanup phase.
Quick Reference by Fasting Goal
- Fat loss and insulin management: Water, black coffee, plain tea, zero-calorie electrolytes, lemon water, apple cider vinegar, monk fruit or stevia sweetened drinks, and diet soda are all generally fine.
- Maximum autophagy: Stick to water, black coffee, plain tea, and zero-calorie electrolytes. Avoid all sweeteners (even zero-calorie ones), bone broth, and anything with protein.
- Gut rest: Water and plain electrolytes only. Coffee and tea stimulate digestive secretions, so skip them if gut rest is the specific goal.
The simplest rule: if a drink has calories from sugar or protein, it will break your fast to some degree. The closer you stay to zero calories with no sweetness, the cleaner your fast will be.