During a water fast, you can drink plain water, black coffee, unsweetened tea, sparkling mineral water, and plain electrolyte drinks with no sweeteners. These are the core options that keep your fast intact by providing zero (or near-zero) calories. The details get more nuanced once you start asking about lemon juice, apple cider vinegar, bone broth, and sweeteners, so let’s break it all down.
The Safe List: What Won’t Break Your Fast
Plain water is the obvious foundation. Tap, filtered, spring, or mineral water all work. Beyond that, most fasting protocols also permit:
- Black coffee (no sugar, cream, or sweeteners)
- Plain tea (green, black, or herbal, with nothing added)
- Sparkling mineral water
- Plain electrolytes free of any sweeteners, including stevia, monk fruit, and sugar alcohols
The key rule is simple: if it has calories, sugar, or sweeteners of any kind, it’s off the table. That means no milk, no honey, no flavored water with added sugar, and no protein shakes.
Coffee and Tea: What You Can and Can’t Add
Black coffee and plain tea are widely accepted during fasting because they contain essentially no calories. The catch is that “black” really means black. A splash of cream, a spoonful of honey, or a pump of flavored syrup all introduce calories and can trigger a metabolic response that disrupts the fasted state.
A sprinkle of plain cinnamon is one of the few gray-area additions. It contains negligible calories and no sugar, so most people consider it acceptable. But flavored creamers, bulletproof coffee (coffee blended with butter or oil), and sweetened lattes are firmly in the “breaks your fast” category.
If you’re sensitive to caffeine, keep in mind that coffee on an empty stomach can cause acid reflux, nausea, or jitteriness. Herbal teas like peppermint or chamomile are gentler alternatives that won’t affect your fast.
Sparkling Water: A Caveat Worth Knowing
Sparkling water and seltzer are calorie-free, so they technically keep your fast intact. However, there’s a wrinkle. A study highlighted by UCLA Health found that carbonated water, whether sweetened or unsweetened, caused ghrelin levels to triple compared to flat water. Ghrelin is the hormone that tells your brain you’re hungry.
That doesn’t mean sparkling water “breaks” your fast in a metabolic sense. But if you’re already struggling with hunger, carbonation could make the experience significantly harder. Flat water or still mineral water won’t have this effect.
Sweeteners: Why Most Are Off Limits
This is where people get tripped up. You might assume that zero-calorie sweeteners are fine since they don’t contain sugar, but many fasting protocols exclude all of them. Marshall Medical’s fasting guidelines specifically list sucralose (Splenda), aspartame (Equal), saccharin (Sweet’N Low), stevia (Truvia, PureVia), and acesulfame potassium (Sunnett) as not allowed.
The reasoning varies. While the Mayo Clinic notes that artificial sweeteners don’t directly raise blood sugar, the concern during fasting goes beyond glucose. Some researchers believe sweeteners may trigger cephalic-phase insulin responses, where your body releases a small amount of insulin just from tasting something sweet. Others argue they stimulate appetite, making fasting harder. If your goal is maximizing the cellular cleanup process known as autophagy, the safest approach is to avoid all sweeteners entirely.
Sugar alcohols (like xylitol, sorbitol, and erythritol) are a separate category, and they can raise blood sugar. These are found in many “sugar-free” products and should be avoided during a fast.
Electrolytes: Essential for Longer Fasts
If you’re fasting for more than 24 hours, electrolyte management becomes critical. Without food, your body loses sodium, potassium, and magnesium faster than usual, and depletion can cause headaches, muscle cramps, dizziness, and fatigue.
General targets during an extended fast are roughly 4,000 to 7,000 mg of sodium per day, 1,000 to 4,700 mg of potassium, and 400 to 600 mg of magnesium. You can meet these by adding a pinch of sea salt or pink Himalayan salt to your water and using unflavored, unsweetened electrolyte supplements.
One important safety note: potassium salt substitutes (like Nu Salt) should not exceed half a teaspoon per day. And if you take blood pressure medications, particularly ACE inhibitors, ARBs, or diuretics, supplementing potassium without medical guidance can be dangerous.
Lemon Juice and Apple Cider Vinegar
A squeeze of lemon in your water is one of the most common fasting questions. Lemons have a low glycemic index (around 20 out of 100), and the small amount of juice from a wedge or half a lemon contains only a couple of calories. Most people consider this acceptable during a fast, especially since it can make plain water more tolerable over multiple days.
Apple cider vinegar is another popular addition. Diluting one to two tablespoons in water adds roughly 3 calories, which is negligible. Some evidence suggests it can help with blood sugar regulation by slowing gastric emptying and glucose absorption, though this effect is most relevant when taken with meals rather than during a fast. It won’t break your fast in any meaningful way, but drink it diluted to protect your tooth enamel and stomach lining.
Bone Broth: A Conditional Exception
Bone broth occupies a unique space. It contains calories and protein, so it technically breaks a strict water fast. However, some extended fasting protocols allow it after the 24-hour mark as a way to maintain electrolyte balance and make multi-day fasts more sustainable.
The trade-off is real, though. One cup of bone broth contains roughly 10 grams of protein. If you’re fasting specifically for autophagy benefits (the cellular recycling process that ramps up during prolonged fasts), keeping protein intake under 10 to 15 grams per day is important. That means one cup of bone broth per day at most, and even that may blunt some of the deeper fasting benefits.
What to Avoid Entirely
Some drinks seem like they’d be fine but aren’t. Diet sodas contain artificial sweeteners and often other additives. Coconut water has natural sugars and calories. Fruit juice, even freshly squeezed, is high in sugar. Milk and plant-based milks contain protein, fat, or both. Kombucha has sugar and calories from fermentation. Sports drinks like Gatorade are loaded with sugar or sweeteners. Alcohol is caloric and dehydrating.
The simplest test: if the label shows calories, sugar, protein, or fat, it breaks your fast. If it contains artificial sweeteners, it may not break your fast metabolically, but it goes against most strict fasting protocols and could make hunger worse.