Do you really need eight glasses of water a day?

The eight glasses rule has no clear scientific origin, and it is often traced to a 1945 recommendation that specified a fluid intake but also noted that much of it comes from food, a qualifier that dropped away as the number circulated. The National Academies do publish adequate intake figures, around 3.7 liters daily for men and 2.7 for women, but those are totals from all sources, and roughly 20 percent typically comes from food, with fruits, vegetables, soup, and even coffee counting. Coffee and tea are net hydrating despite their mild diuretic effect, which is another persistent myth worth discarding.

Actual needs vary enormously with body size, activity level, heat, altitude, illness, pregnancy, and diet, so a single number for everyone was never plausible. For healthy adults, thirst is a reasonably reliable signal, and pale yellow urine is a better practical check than counting glasses, though certain medications and supplements alter urine color. The groups where thirst is less reliable and intake deserves deliberate attention are older adults, whose thirst sensation blunts with age, young children, athletes, and people working in heat. Overdoing it is also possible: drinking far more water than the kidneys can excrete dilutes blood sodium and causes hyponatremia, which is rare but has killed endurance athletes and participants in water-drinking contests, so more is not automatically better.