During fasting as insulin drops, your kidneys excrete more sodium and water, and low sodium, potassium, and magnesium are behind the headaches, fatigue, cramps, and lightheadedness people often get on extended fasts. A mineral water can help a little, most mineral waters are fairly low in sodium and potassium. Professional fasting uses a deliberate electrolyte approach: a pinch of salt in water or an off the shelf electrolyte mix added to water.

Drinking water can dilute the sodium in your blood and the more you drink, the more you urinate, and urine carries some sodium, potassium, and magnesium with it. When you’re fasting, nothing is coming in, and low insulin already makes your kidneys shed more sodium. Drinking large amounts of plain water on top of that can leave you feeling worse.