The Definitive Guide to How Much Water During Intermittent Fasting

During a fast, your body undergoes a massive internal shift in how it handles fluids, meaning your standard eight glasses a day might not be enough. To maintain cellular function and energy levels, you generally need to increase your water intake by 20% to 30% compared to non-fasting days to compensate for the loss of food-based hydration.

The short answer

While there is no universal number, most people practicing intermittent fasting should aim for 3 to 4 liters (about 100 to 135 ounces) of water daily. This is because you are missing out on the 20% of daily water intake typically derived from solid foods like fruits and vegetables. During a fast, your insulin levels drop, causing your kidneys to flush out sodium and water at an accelerated rate. To stay balanced, you should drink roughly 500ml of water every two to three waking hours, adjusting upward if you are exercising or live in a hot climate.

Why this happens

The reason you feel thirstier or more easily dehydrated during a fast is rooted in glycogen storage. Your body stores carbohydrates in your muscles and liver as glycogen. For every gram of glycogen stored, your body holds onto about 3 to 4 grams of water. As you enter the fasting state and your body begins to burn through these glycogen stores for energy, that stored water is released and excreted through urine. This process is why people often see a rapid drop in 'water weight' during the first few days of a new fasting routine. However, this also means your internal reservoir of fluid is shrinking, leaving you more vulnerable to the physical symptoms of dehydration like headaches and fatigue.

What the research says

Biological mechanisms dictate how our bodies respond to the absence of food. When we look at the science of fluid dynamics during a caloric deficit or a total fast, several key shifts occur in the endocrine and renal systems:

• Evidence-based mechanisms of fasting hydration:

Myths people believe

The internet is full of conflicting advice regarding fasting. It is important to separate physiological facts from common misconceptions that could hinder your progress or health.

• Fasting hydration myths vs reality:

Hydrating during a fast is not just about the volume of water you swallow, but how well your body retains the minerals required to use it.

What to actually do

To optimize your hydration while fasting, you need a structured approach that accounts for the lack of meals. Follow these concrete steps to ensure your body stays balanced throughout your fasting window.

Staying consistent with these habits is the hardest part of any fasting protocol. Using a tool like GetHydrately can help you visualize your intake and ensure you are hitting your 3 to 4 liter goal without having to do the mental math. Remember that these guidelines are general; if you have underlying kidney issues or heart conditions, always talk to a clinician before significantly increasing your fluid or salt intake.