Last week the subject was too much water, the kind that dilutes your blood until the brain swells. This week is the follow up question I get more than any other in clinic: how much should a person actually drink?

The famous answer is eight glasses a day. It is repeated everywhere, printed on water bottles, and built into apps that buzz at you. It also has no evidence behind it. What it has instead is a good origin story, two kidney doctors, and a qualifier that got lost along the way.

As the story is sometimes told, a nephrologist was once asked how much water a person needs and answered, offhand, "oh, about eight glasses a day." That version is almost certainly apocryphal. No one is on record saying it. But an offhand remark hardening into a figure the whole country recites, in an age before social media, is a remarkable thing, so the tale survives.

The documented trail is nearly as loose. In 2002 the kidney physiologist Heinz Valtin went looking for the science behind "8 by 8" and found none. The number traces to a 1945 Food and Nutrition Board note that suggested about one milliliter of water per calorie of food, roughly two and a half liters a day. The very next sentence, the one everyone forgot, said most of that water is already in the food you eat. Strip the qualifier away and a total that counted your meals becomes an instruction to drink eight more glasses on top of them. A Harvard nutritionist's personal habit of six plus glasses a day, solemnly recorded in his obituary, probably helped it spread.

If one kidney doctor is blamed for the myth, another deserves credit for the era's other great hydration story. In 1965, Dr. Robert Cade, a nephrologist in the University of Florida's renal division, set out to replace what the Gators' football players were losing in the Florida heat. He measured the salt and sugar in the sweat soaked into their jerseys, then mixed water, sodium, sugar, and a squeeze of lemon to put it back.

He first tested it on the sideline during a game against LSU. The drink took the mascot's name, and Gatorade went on to become the best selling sports drink on earth. Worth remembering what it was for: athletes losing liters an hour, not the rest of us sitting at a desk. That is a distinction the marketing has always been happy to blur.

Those figures are population averages, not a prescription. For a healthy person with working kidneys, thirst is a reliable guide. The same machinery that makes water intoxication hard to achieve also makes chronic under hydration hard to stumble into.