Yes, you can drink during fasting, and staying hydrated is actually essential. The key is choosing drinks that don’t trigger a significant insulin response or add meaningful calories. Water, black coffee, and plain tea are all safe choices that keep your fast intact. Beyond those basics, the details matter: what you add to your drinks, which sweeteners you choose, and how strictly you define “fasting” all determine where the line falls.
Water Is Your Foundation
Plain water, still or sparkling, is the simplest choice during a fast. It has zero calories, no effect on insulin, and keeps you hydrated through what can be a physically demanding stretch. Sparkling water works well as a mid-morning craving disruptor, since the carbonation creates a sensation of fullness that can take the edge off hunger.
If you’re fasting for more than 16 hours, exercising, or in a hot climate, consider zero-calorie electrolyte water. Sodium, potassium, and magnesium don’t break a fast, and they help prevent the headaches, muscle cramps, and foggy thinking that sometimes show up in longer fasting windows. These symptoms are often mistaken for hunger when they’re really just dehydration and mineral depletion.
Black Coffee and Plain Tea
Black coffee is one of the most popular fasting drinks for good reason. It suppresses appetite, provides a mild energy boost, and may actually enhance some of the benefits people fast for in the first place. Research published in the journal Cell Cycle found that both caffeinated and decaffeinated coffee rapidly triggered autophagy (your body’s cellular cleanup process) in mice. The effect came not from caffeine but from polyphenols, the plant compounds naturally present in coffee beans. These polyphenols appear to mimic some of the metabolic signals of caloric restriction, activating the same energy-sensing pathways that fasting itself turns on.
Unsweetened tea, whether black, green, or herbal, is equally safe during a fast. Green tea contains its own set of polyphenols that may complement fasting benefits. Herbal options like chamomile or rooibos work well in the evening as a zero-calorie stand-in for a late-night snack. The one rule: nothing added. No honey, no sugar, no flavored syrups.
What About Milk, Cream, or Sweeteners?
This is where most people trip up. A latte or cappuccino contains enough milk to deliver a real dose of calories, protein, and sugar, all of which trigger insulin release and effectively end your fast. These are off-limits during a fasting window.
If you genuinely can’t tolerate black coffee, a small amount of fat is the least disruptive option. About 1 teaspoon (5 ml) of heavy cream or coconut oil is unlikely to significantly alter your blood sugar or total calorie intake. It’s a compromise, not a freebie, but it’s far less impactful than milk or a flavored creamer. Keep in mind that protein and carbohydrates are the strongest triggers of insulin, while small amounts of pure fat have a much weaker effect.
Artificial sweeteners are more nuanced. A study in the journal Appetite found that both stevia and aspartame produced significantly lower insulin responses compared to sugar. Stevia performed especially well, lowering post-meal insulin levels compared to both aspartame and sugar, and also reducing blood glucose relative to sugar. That said, “lower than sugar” isn’t the same as “no effect at all.” Zero-calorie sweetened drinks won’t add calories, but their intense sweetness can stimulate cravings in some people, making the fast harder to sustain psychologically. If your goal is strict autophagy, plain drinks are the safest bet. If you’re fasting primarily for weight loss and a diet soda keeps you on track, the tradeoff is reasonable.
Lemon Water and Apple Cider Vinegar
A squeeze of lemon in your water adds a negligible number of calories and won’t meaningfully affect your fast. It makes water more palatable, which helps if you’re struggling to drink enough.
Apple cider vinegar is a slightly more interesting case. It contains only trace amounts of carbohydrates and is unlikely to break your fast. There’s also evidence that vinegar helps stabilize blood sugar levels and increases feelings of fullness, both of which work in your favor during a fasting window. The standard approach is 1 to 2 teaspoons diluted in a large glass of water. Some people use it 10 to 20 minutes before their first meal to slow the blood sugar spike that comes with breaking a fast. Always dilute it well; undiluted vinegar can damage tooth enamel and irritate your throat.
Bone Broth: A Gray Area
Bone broth is the most debated fasting drink because it occupies a middle ground. A typical cup contains roughly 30 to 50 calories, along with small amounts of protein and amino acids. Protein is a direct activator of the nutrient-sensing pathways that fasting is designed to suppress. In practical terms, bone broth will likely blunt autophagy to some degree.
That said, many people use bone broth strategically during extended fasts (24 hours or longer) to maintain electrolytes and make the fast sustainable. If you’re doing a shorter daily fast of 16 to 18 hours, bone broth probably isn’t necessary. If you’re attempting a multi-day fast and bone broth is the difference between continuing and quitting, the small caloric cost may be worth it. Context matters more than purity here.
How Much Does It Take to Break a Fast?
There’s no single calorie number that flips a switch from “fasted” to “not fasted.” Fasting exists on a spectrum. Research on fasting-mimicking diets shows that protocols providing 20% to 25% of normal daily calories still produce some fasting benefits, including measurable autophagy. That doesn’t mean you should eat 500 calories and call it fasting, but it does mean that a teaspoon of cream in your coffee isn’t erasing the previous 14 hours.
What matters most is what you consume, not just how much. Insulin is the primary signal that shuts down autophagy, and amino acids (from protein) along with glucose (from carbohydrates) are the strongest insulin triggers. Pure fat has the weakest effect. This is why a splash of heavy cream is considered more acceptable than a splash of skim milk, even at the same calorie count: skim milk delivers more protein and lactose sugar relative to its volume.
For a practical rule of thumb, keeping your fasting window under roughly 10 calories from any single drink, and prioritizing fat over protein or carbs if you must add anything, will preserve most of the metabolic benefits people fast for.
Quick Reference by Drink
- Water (still or sparkling): Always fine. Zero calories, no insulin effect.
- Black coffee: Fine, and may enhance autophagy. Skip the sugar and milk.
- Plain tea: Fine. Black, green, white, and most herbal varieties all work.
- Zero-calorie electrolyte water: Fine, and helpful for longer fasts or active days.
- Lemon water: Fine. Negligible calories.
- Apple cider vinegar (diluted): Fine. Trace calories, potential blood sugar benefits.
- Diet soda or zero-calorie sweetened drinks: Technically zero calories, but may increase cravings. Use with awareness.
- Coffee with 1 tsp heavy cream: Minor compromise. Acceptable for most fasting goals.
- Coffee with milk, sugar, or flavored creamer: Breaks your fast.
- Bone broth: Contains calories and protein. Best reserved for extended fasts where you need the support.
- Juice, smoothies, sweetened drinks: Break your fast completely.