Water, black coffee, and plain tea are the safest drinks during a fast. Beyond those three, what you can add to your cup depends on why you’re fasting. Someone fasting for fat loss has more flexibility than someone fasting for cellular cleanup (autophagy), and the distinction matters for nearly every drink on this list.
Water Is the Foundation
Plain water is the only drink that universally keeps every type of fast intact. It has zero calories, triggers no insulin response, and keeps you hydrated while your body shifts into fat-burning mode. Sparkling water and mineral water are equally fine, as long as they contain no added sugars or flavors with calories.
Where water gets more interesting during fasting is with electrolytes. When you stop eating, your kidneys excrete more sodium, which pulls potassium and magnesium along with it. During longer fasts (24 hours or more), adding electrolytes to your water helps prevent headaches, fatigue, muscle cramps, and that general “fasting fog.” Aim for roughly 1,500 to 2,300 mg of sodium, 1,000 to 2,000 mg of potassium, and 300 to 400 mg of magnesium per day. Look for electrolyte packets or drops that contain zero calories and no sugar. A pinch of salt in your water handles the sodium piece on its own.
Black Coffee and Plain Tea
Black coffee is one of the most popular fasting drinks for good reason. It contains essentially zero calories, suppresses appetite, and may actually enhance some benefits of fasting. Your body produces ketones from fat during a fast, and caffeine appears to boost that ketone production, which supports mental clarity and energy. Coffee’s polyphenols (plant compounds that act as antioxidants) add another layer of benefit without adding calories.
Plain tea, whether green, black, white, or herbal, follows the same rules. No calories, no insulin spike, no broken fast. Green tea in particular contains compounds that may support the cellular recycling process called autophagy, which is one of the key reasons people fast in the first place.
The catch with both drinks: nothing goes in them. No milk, no cream, no sugar, no honey. Even a splash of cream adds enough calories and protein to trigger a metabolic response. If you’re fasting strictly for autophagy, keep it black or plain. If you’re fasting for weight loss, a tiny amount of cream (under 10 calories) is unlikely to derail fat burning, but it does technically end a clean fast.
Apple Cider Vinegar
A tablespoon of apple cider vinegar diluted in water contains roughly 3 calories, which is negligible enough that most fasting protocols consider it acceptable. The more interesting question is whether it helps. Acetic acid, the active compound in vinegar, slows the rate at which your stomach empties, which can reduce blood sugar spikes when you eventually eat. Research in diabetic patients has shown that regular apple cider vinegar consumption can improve blood glucose and lipid levels over time.
If you’re using vinegar during a fast, dilute one to two tablespoons in a full glass of water. Drinking it straight can damage tooth enamel and irritate your esophagus. The taste is strong, but many people find it curbs hunger during the hardest hours of a fast.
Diet Sodas and Artificial Sweeteners
This is where things get genuinely murky. Diet sodas, zero-calorie flavored waters, and sugar-free drink mixes technically contain no calories, but the sweeteners in them may still affect your body in ways that matter during a fast.
The research on artificial sweeteners and insulin is contradictory. Some studies have found that sucralose can increase blood glucose and reduce insulin sensitivity, particularly in people with obesity. Other studies on saccharin found lower glucose and insulin levels compared to water. Aspartame studies have gone both ways, with one showing lower insulin than sugar and another showing higher insulin than plain water. A systematic review in PLOS ONE concluded that no solid conclusions about any specific sweetener’s effect on glucose metabolism can be drawn from the existing evidence.
What this means practically: if your goal is weight loss and diet soda helps you stick to your fasting window, it’s probably not going to erase your results. If your goal is a clean fast for autophagy or maximum metabolic benefit, artificial sweeteners introduce enough uncertainty that plain water, coffee, and tea are better choices.
Stevia and Monk Fruit
Stevia and monk fruit sweeteners sit in a slightly different category from artificial sweeteners. Both are plant-derived, contain zero calories, and early research indicates that stevia does not significantly raise insulin or blood sugar levels. Monk fruit similarly appears to have minimal metabolic impact.
Adding a few drops of liquid stevia to tea or sparkling water is one of the more conservative ways to make fasting beverages more palatable without introducing the uncertainties that come with sucralose or aspartame. That said, “does not significantly raise insulin” is not the same as “has zero effect.” For the strictest interpretation of fasting, plain is still best.
Bone Broth: A Gray Area
Bone broth comes up constantly in fasting conversations, especially for people doing longer fasts of 24 hours or more. A cup of bone broth contains 40 to 50 calories along with protein and amino acids, which means it breaks a fast in the strict sense. Protein triggers some insulin release and activates growth-signaling pathways that pause autophagy.
For fat loss purposes, though, a small cup of bone broth during an extended fast can be a useful tool. The calories are modest, the protein helps preserve muscle, and the sodium and minerals address electrolyte needs. If a cup of broth at hour 20 keeps you from abandoning a 36-hour fast entirely, the tradeoff works in your favor. Just know that it does pull you out of a fully fasted metabolic state, even if briefly.
What Definitely Breaks a Fast
Some drinks are clearly off-limits regardless of your fasting goals:
- Juice (fruit or vegetable) contains sugar and enough calories to fully break a fast, even in small amounts.
- Milk and plant milks contain protein, fat, and often sugar. Even unsweetened almond milk has enough calories to count.
- Smoothies and protein shakes are meals in liquid form.
- Alcohol contains 7 calories per gram and requires your liver to stop fat metabolism to process it.
- Coconut water is surprisingly caloric, with roughly 45 calories and 9 grams of sugar per cup.
- Coffee with cream, sugar, or flavored syrups can easily add 50 to 200 calories per cup.
Matching Your Drinks to Your Fasting Goal
The cleanest fast allows only water, black coffee, plain tea, and calorie-free electrolytes. This is the approach if you’re fasting specifically for autophagy or gut rest, where even small metabolic signals from sweeteners or protein can interfere with the process you’re trying to trigger.
For fat loss through intermittent fasting (16:8, 18:6, or similar patterns), you have more room. Water, coffee, tea, apple cider vinegar, and stevia-sweetened drinks all keep you well within the range where your body continues burning fat. The occasional diet soda is unlikely to matter in the bigger picture. Bone broth is a judgment call that depends on how long your fast is and whether you need the help to get through it.
The simplest rule: if a drink has calories, it breaks a strict fast. If it has zero calories but contains sweeteners, it preserves most fasting benefits but introduces some metabolic uncertainty. If it’s water, coffee, or tea with nothing added, you’re on completely solid ground.