Yes, you can absolutely drink water during the fasting window of a 16/8 intermittent fast. Water has zero calories and does not trigger an insulin response, so it won’t break your fast. In fact, staying hydrated during those 16 hours is important for both comfort and safety. Dehydration during caloric restriction is a recognized risk factor that can push normal cellular repair processes into harmful territory.
Why Water Matters During Fasting
When you eat normally, a significant portion of your daily water intake comes from food. Fruits, vegetables, soups, and even grains contain water that your body absorbs throughout the day. During a 16-hour fast, you lose that entire contribution, which means you need to compensate by drinking more than you might on a non-fasting day.
Fasting also changes how your body handles minerals. Sodium excretion increases during the early phase of fasting and continues even through prolonged caloric deprivation, eventually settling at a lower but persistent rate. Potassium follows a similar pattern, with rapid urinary losses early on that taper to a steady 10 to 15 milliequivalents per day. These losses come from your intracellular stores and are tied to the rate at which your body breaks down protein for energy. Drinking water helps your kidneys manage this mineral turnover and keeps you from feeling sluggish, dizzy, or headachy during your fasting window.
What You Can Add to Your Water
Plain water is the safest choice, but it’s not the only option. A pinch of salt in your water can help offset the sodium your body naturally excretes while fasting. Sparkling water, mineral water, and plain herbal teas are all fine since they contain no calories.
If you prefer flavored water, check the label. Products sweetened with erythritol are unlikely to be a problem. Studies in both lean and obese participants, including people with diabetes, have clearly shown that even large doses of erythritol (20 to 75 grams) do not affect blood glucose or insulin levels. That’s a much higher amount than you’d find in any flavored water. Stevia is similarly considered safe for fasting purposes. What you want to avoid are flavored waters that contain sugar, honey, juice, or high-calorie sweeteners, as those will break your fast.
Black coffee and plain green tea are also popular choices during the fasting window. Both contain essentially zero calories and can make the hours pass more easily.
How Water Helps With Hunger
One of the biggest challenges during the 16-hour window is managing appetite, and water is your best tool. Drinking a full glass of water stretches the stomach slightly, which sends signals to the brain that mimic the early stages of fullness. This effect is temporary, lasting roughly 20 to 30 minutes, but it can be enough to get you past a craving.
The hormonal picture is also more encouraging than you might expect. Research on water-only fasting found that even after eight full days without food, the main hunger-stimulating hormones (ghrelin and orexins) did not increase significantly. Leptin, the hormone that suppresses appetite, did drop, but the hunger center in the brain showed no acute activation. For a 16-hour fast, this means your body is not spiraling into unbearable hunger. The discomfort you feel is largely habitual, and water can smooth over those habitual cravings while your body adjusts.
How Much Water to Drink
There’s no single number that works for everyone, but a reasonable target during a 16-hour fast is around 8 to 12 cups (roughly 2 to 3 liters) across the full day, including your eating window. Spread your intake out rather than chugging large amounts at once. Sipping steadily through the fasting period keeps you hydrated without overwhelming your stomach.
Signs you’re not drinking enough include dark yellow urine, dry mouth, headaches, and difficulty concentrating. These symptoms are common in the first week of intermittent fasting and are almost always a hydration issue rather than a sign that fasting isn’t working for you. If you exercise during your fasting window, increase your intake accordingly, since sweat losses add to the fluid and mineral deficit your body is already managing.
What Will Actually Break Your Fast
The concern behind this question is really about whether anything in your water could disrupt the metabolic state that makes fasting beneficial. The short answer: only calories matter. Your body stays in a fasted state as long as it isn’t processing incoming energy from food or caloric drinks. Here’s a quick breakdown:
- Won’t break your fast: plain water, sparkling water, mineral water, black coffee, plain tea, water with a pinch of salt, water with lemon (a small squeeze, not half a lemon), erythritol-sweetened drinks, stevia-sweetened drinks
- Will break your fast: water with honey, coconut water, fruit juice, smoothies, milk or cream in coffee, protein water, any drink with calories above roughly 5 to 10
If a drink has a nutrition label showing more than a few calories per serving, treat it as food and save it for your 8-hour eating window.
Fasting, Hydration, and Cellular Repair
One of the reasons people practice intermittent fasting is to promote autophagy, the process by which your cells clean up damaged components and recycle them. Water does not interfere with this process. In fact, dehydration during caloric restriction can cause the opposite of what you want. Research has shown that when energy intake is low and dehydration sets in, the autophagy response can become excessive and defective, pushing cells toward self-destruction rather than healthy renewal. Staying well hydrated keeps this repair process functioning normally.
Think of water as the one thing that makes every benefit of fasting work better. It supports kidney function while your body excretes extra minerals, keeps your energy levels stable, reduces the intensity of hunger, and protects the cellular cleanup process that fasting is designed to trigger. There’s no version of 16/8 fasting where skipping water is a good idea.