Water is the gold standard for fasting, but it’s not your only option. Black coffee, plain tea, and a few other zero-calorie drinks can keep you hydrated and even support some of the metabolic benefits of fasting, as long as you avoid anything that triggers a meaningful insulin response. The key is knowing which drinks are truly safe and which ones carry hidden trade-offs.
Water Comes First
Plain water should be your primary drink during any fast. When you stop eating solid food, your body loses a significant source of hydration: roughly 20 to 30 percent of daily water intake normally comes from food. Without that contribution, your body retains less water and passes more urine, which raises the risk of dehydration. Symptoms to watch for include dizziness, headaches, nausea, constipation, and drops in blood pressure.
There’s no single magic number for how much to drink, but aiming for at least the same amount you’d drink on a normal eating day is a reasonable baseline. If you’re fasting for 16 to 24 hours, keeping a water bottle nearby and sipping consistently throughout the day matters more than hitting a specific volume. Adding a pinch of salt to your water can help with electrolyte balance, especially during longer fasts or in warm weather.
Black Coffee and Plain Tea
Black coffee contains essentially zero calories and does not break a fast. Beyond being “allowed,” coffee may actually enhance one of fasting’s most sought-after benefits: autophagy, the cellular recycling process your body ramps up when it’s not processing food. Caffeine has been identified as a potent stimulant of autophagy, helping clear fat from the liver through a specific waste-processing pathway inside cells. Coffee also contains chlorogenic acid, a polyphenol that independently enhances autophagy in human cells.
Plain green, black, and white teas are equally safe during a fast. They contain minimal calories and offer their own polyphenols. One note on green tea: drinking brewed green tea is safe, but concentrated green tea extract supplements are a different story. Research at Rutgers University found that high-dose green tea extract supplements raised a liver stress enzyme by up to 80 percent in people with certain genetic variations. That risk doesn’t apply to simply drinking a cup of green tea.
The rule for both coffee and tea is simple: nothing added. Cream, milk, sugar, honey, and flavored syrups all contain enough calories and macronutrients to trigger an insulin response and interrupt the fasted state.
What About Artificial Sweeteners?
This is where things get nuanced. Not all zero-calorie sweeteners behave the same way in your body.
- Stevia: Does not raise insulin or blood glucose in meaningful amounts. In people with type 2 diabetes, stevia actually reduced post-meal blood sugar by 18 percent when consumed with food, without changing insulin levels.
- Erythritol: Even at doses up to 50 grams (far more than you’d use in a drink), erythritol did not alter plasma insulin, blood glucose, glucagon, or other metabolic hormones in healthy people.
- Monk fruit: Studies comparing monk fruit to sugar found no differences in glucose or insulin responses, making it a safe choice during fasting.
- Sucralose: The evidence here is mixed. One study in obese individuals found that sucralose increased insulin by 20 percent, while other studies in healthy people found no effect. If you’re fasting specifically for insulin-related goals, sucralose is the least predictable option.
There’s also the question of whether tasting something sweet on your tongue triggers a small insulin release before anything even hits your stomach. This “cephalic phase” response appears to be more relevant in people who are overweight or obese. Research shows that when sweeteners bypass the mouth entirely (delivered via capsule or catheter), this early insulin bump disappears, confirming that oral stimulation plays a role. For most people using a small amount of stevia or monk fruit in tea, this effect is likely minimal, but it’s worth knowing about if your fasting goals are tightly focused on insulin control.
Sparkling Water Has a Catch
Carbonated water contains zero calories and won’t trigger an insulin response, so it technically doesn’t break a fast. Many people reach for it because the fizz makes plain water feel more satisfying. But there’s an unexpected downside: carbonation appears to increase levels of ghrelin, the hormone that makes you feel hungry.
A study found that men who drank carbonated water, whether sweetened or unsweetened, had triple the blood levels of ghrelin compared to those who drank still water or degassed sparkling water. The carbon dioxide itself drove the effect. If you’re already struggling with hunger during your fasting window, sparkling water could make it worse. If hunger isn’t an issue for you, it’s a perfectly fine option.
Apple Cider Vinegar
A tablespoon of apple cider vinegar diluted in water contains about 3 calories, which is negligible from a fasting perspective. The more interesting question is whether it adds metabolic value. In a randomized controlled trial of people with diabetes, daily apple cider vinegar consumption lowered fasting blood glucose by an average of 23 points and reduced HbA1c (a marker of long-term blood sugar control) by 1.4 percentage points over the study period. Both changes were statistically significant.
These results came from people with diabetes who were also eating, so the effects during a true fasted state in healthy individuals are less clear. Still, nothing about apple cider vinegar disrupts the fasted state, and its acetic acid content may complement the blood sugar improvements that fasting itself provides. If you use it, dilute it well to protect your tooth enamel and stomach lining.
Lemon Water
Squeezing half a lemon into your water adds roughly 5 to 6 calories, almost entirely from citric acid and trace sugars. This amount is too small to produce a meaningful insulin response or interrupt fat burning. Some people believe lemon polyphenols improve insulin sensitivity, and there is early cell-culture research showing that bioactive compounds from lemon can counteract insulin resistance in fat cells. But there’s little evidence that drinking lemon juice replicates this effect in the human body.
Lemon water is fine during a fast for flavor and hydration. Just don’t expect it to add significant metabolic benefits beyond making your water more drinkable, which, if it helps you drink more water, is a benefit in itself.
Drinks That Will Break Your Fast
Anything with meaningful calories from protein, fat, or carbohydrates will trigger digestion and an insulin response. The obvious ones include juice, soda, milk, smoothies, protein shakes, and alcohol. Less obvious culprits include coconut water (which contains natural sugars), bone broth (which has protein and calories, typically 40 to 50 per cup), and coffee drinks with added cream, butter, or MCT oil. Bulletproof coffee, despite its popularity in fasting communities, delivers hundreds of calories from fat and shifts your body into a fed metabolic state.
Herbal teas are generally safe, but check the ingredients. Teas made from dried fruit, licorice root, or other naturally sweet ingredients can contain small amounts of sugar. Pure herbal teas like peppermint, chamomile, and rooibos are calorie-free and won’t interfere with your fast.