In most types of fasting, water is not only allowed but encouraged. The major exception is religious dry fasting, such as during Ramadan, where all food and drink are restricted from sunrise to sunset. For intermittent fasting, medical fasting before blood tests, and pre-surgery fasting, plain water is perfectly fine and often recommended.
Water and Intermittent Fasting
If you’re doing intermittent fasting for health or weight loss, water does not break your fast. It contains zero calories, zero sugar, and zero protein, so it won’t trigger an insulin response or interrupt the metabolic processes that make fasting beneficial. Staying hydrated actually supports the goals most people are after: improved metabolism, reduced insulin resistance, lower inflammation, and cellular repair.
Dehydration during a fast can cause fatigue, headaches, irritability, lack of focus, constipation, and decreased urination. These symptoms are entirely avoidable by drinking water freely throughout your fasting window. There’s no physiological advantage to skipping water. According to the Cleveland Clinic, any weight you lose from avoiding fluids is water weight, not actual body fat, and it comes right back once you rehydrate.
What About Lemon Water or Other Additions?
Plain water is the safest choice during any fast. Once you start adding ingredients, the answer gets more nuanced. Strictly speaking, consuming any calories at all breaks a physiological fast. A squeeze of lemon in your water adds a negligible number of calories and is unlikely to meaningfully affect your results, but adding sugar, honey, or cream will kick you out of a fasted state.
As a general guideline, any beverage with more than a handful of calories per cup will break your fast. Black coffee and plain tea fall into a gray area (they contain trace calories), but sugary drinks, juice, milk, and flavored waters with sweeteners do not qualify as fasting-safe. If your goal is autophagy or strict metabolic fasting, stick with plain water.
Fasting Before Blood Tests
When your doctor asks you to fast before a blood test, you should avoid all food and beverages except plain water. Coffee, juice, soda, and flavored sparkling water can enter your bloodstream and skew results on tests like glucose panels and lipid profiles. Even lemon-flavored or artificially sweetened carbonated water can interfere.
Not only is plain water allowed before blood work, it actually helps. Staying well hydrated keeps more fluid in your veins, which makes it easier for the technician to draw your sample. If you’ve ever had a difficult blood draw, drinking a few glasses of water beforehand can make a noticeable difference.
Fasting Before Surgery
Pre-surgical fasting guidelines are more restrictive than most people realize, but they still permit water up to a point. The American Society of Anesthesiologists recommends that healthy patients can drink clear liquids, including water, fruit juices without pulp, and carbohydrate-containing beverages, until two hours before receiving anesthesia. The two-hour cutoff exists to reduce the risk of liquid entering the lungs during the procedure, which could cause choking or pneumonia.
In practice, many patients stop drinking far earlier than necessary, sometimes fasting from all liquids the night before. The ASA specifically notes that efforts should be made to allow clear liquids as close to two hours before the procedure as possible. If your surgery is scheduled for the afternoon, you likely don’t need to stop drinking water at midnight. Confirm the timing with your surgical team, but know that the guidelines are more generous than many people assume.
Religious Fasting: When Water Is Off Limits
Some religious fasts do prohibit water entirely. Ramadan fasting is the most widely practiced example. Muslims observing Ramadan abstain from all food and beverages, including water, from sunrise to sunset. This is classified as a dry fast, and it typically lasts 12 to 16 hours depending on the season and location.
Dry fasting carries real health risks that water-permitted fasting does not. The Cleveland Clinic warns that it can lead to dehydration, kidney problems, urinary issues, and nutritional deficiencies. The benefits that some proponents claim for dry fasting are not clinically proven, and the same metabolic benefits associated with fasting (improved immune function, reduced oxidative stress, better heart and brain health) can be achieved through forms of fasting that allow water. If you observe a religious dry fast, hydrating thoroughly before and after the fasting window becomes especially important.
How Much Water to Drink While Fasting
There’s no special water requirement during a fast, but most people should aim for the same general target as any other day: roughly 8 to 12 cups, adjusted for your body size, activity level, and climate. Some people find they need more water during fasting periods because they’re missing the fluid they’d normally get from food. Fruits, vegetables, soups, and other meals contribute a surprising amount of daily hydration, so when those are removed, your water glass needs to pick up the slack.
Pay attention to the color of your urine. Pale yellow means you’re well hydrated. Dark yellow or amber suggests you need more. If you’re experiencing headaches or fatigue during a fast, try drinking more water before assuming the fast itself is the problem.