Everyone learns it as a rule: drink eight 8-ounce glasses of water a day, about two liters, on top of whatever you eat.

Nobody can point to the study that set the number, because there isn't one.

The advice is usually traced to a 1945 U.S. National Research Council recommendation: adults need roughly 2.5 liters of water a day. Then it adds a sentence most retellings drop: most of that quantity is already contained in the food you eat. The eight cups survived. The “it's in your food” clause did not.

In 2002 Heinz Valtin, a kidney physiologist, went looking for the evidence behind “8 x 8” and found none. No controlled study shows a healthy adult needs eight glasses of plain water daily. The body holds water within a wide range, and thirst is a decent guide.

In 2008 Negoianu and Goldfarb reviewed the same ground in the Journal of the American Society of Nephrology and reached the same verdict.

Then in 2022 Yamada and colleagues tracked water turnover directly, using isotope-labeled water, in 5,604 people aged 8 days to 96 years across 23 countries (Science). Turnover moved with body size, physical activity, pregnancy, air temperature, humidity, and altitude. It did not sit at one universal number.

Drink when you're thirsty. Watch your urine color if you want a gauge. But “eight glasses a day” is advice with no evidence under it, carrying a dropped clause for eighty years.

#water #health #mythbusting #factcheck

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