Hydration during fasting is mostly about electrolytes — the right mineral water prevents headaches, cramps and the dreaded "fasting flu."
When you stop eating, insulin drops. Lower insulin causes the kidneys to excrete more sodium and water, which is why most fasting-related headaches and dizziness in the first 24–48 hours are actually mild electrolyte depletion — not low blood sugar. Choosing a water with meaningful sodium and magnesium fixes the problem.
For fasts beyond 24 hours, prioritize sodium, magnesium and potassium. Most bottled water is low in potassium, so cover that with a pinch of salt and an electrolyte tab — but use mineral water as the sodium and magnesium base.
Low-mineral water (Evian, Volvic) is fine for short fasts and doesn't break a fast in any meaningful sense. High-mineral water (Gerolsteiner, Donat Mg) becomes essential beyond 36–48 hours when sodium and magnesium losses add up.
| Fast type | Best water | Why |
|---|---|---|
| Short fasts (16–24h) | Evian, Volvic, Mountain Valley | Low-mineral still water — won't break the fast and adds no calories. |
| Extended fasts (24–72h) | Gerolsteiner, San Pellegrino | Moderate mineral content replaces electrolytes without insulin response. |
| Multi-day fasts | Donat Mg (small doses), Vichy Catalan | Higher sodium and magnesium prevent fasting-related cramping and headaches. |
