You must drink eight glasses of water a day
The old claim turned out to be wrong and was rejected by evidence.
TaughtDrink eight 8-ounce glasses of water daily, and coffee and tea do not count because caffeine dehydrates you.
NowThere is no scientific basis for the 8x8 rule. Heinz Valtin searched for the evidence in 2002 and found none. Food contributes substantial water, and caffeinated drinks are net hydrating.
What actually happened
Valtin, a kidney physiologist, went looking for the origin of the rule and could not find a single study supporting it. The best candidate he identified was a 1945 US Food and Nutrition Board recommendation of about 2.5 liters of water a day, which included the sentence, usually dropped in the retelling, that most of this quantity is contained in prepared foods.
The body's actual mechanism is thirst, and it works well in healthy people. Roughly 20% of typical water intake comes from food. Coffee and tea have a mild diuretic effect that is more than offset by the water they contain, so they count. The National Academies do publish adequate intake figures, around 3.7 liters a day of total water for men and 2.7 for women from all sources combined, but they are population averages, not a target to hit with a bottle. The one group who should ignore all of this is people told otherwise by a doctor for a specific condition.
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Sources
- American Journal of Physiology: Drink at least eight glasses of water a day. Really? (Valtin, 2002)journals.physiology.org
- National Academies: Dietary reference intakes for waternationalacademies.org
Who was taught this
Still standard through 2012, so anyone who finished school between 1950 and 2002 learned the earlier version.