The Short Version
This claim is false. No scientific evidence supports "eight glasses of water per day" as the optimal intake for human health. The National Academies explicitly state there is no single daily water requirement, and a peer-reviewed review in the American Journal of Physiology found zero studies backing the "8×8" rule. Actual water needs vary significantly by sex, body size, activity level, climate, and diet, and roughly 20–30% of daily water intake comes from food. Every major health authority rejects this as a myth.