„You must drink 8 glasses (2 litres) of water a day; thirst is already too late.”

The “8×8” rule has no scientific source. The 2022 Science study (n=5,604, 23 countries) found daily water turnover ranges from 1 to 6 litres; food supplies 20–30% of intake, and coffee and tea count. With healthy kidneys thirst is an accurate signal — exceptions: the elderly (blunted thirst), heat, exercise, kidney-stone tendency. Over-drinking (hyponatraemia) has killed marathoners.

Last reviewed: 6 September 2026

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