"I'd never manage eight glasses"

Eight glasses of water a day, and coffee doesn't count. Everyone's heard it, most people have failed at it, and the failure is a reason to do nothing, because if eight is the standard and you're managing two, why bother with three.

Heinz Valtin, a kidney physiologist, went looking for where the eight-glasses advice came from and whether anything supported it. He searched the scientific literature, the older paper records, and asked specialists in thirst and drinking. He found no scientific studies in support of it. What he found instead were surveys of thousands of healthy adults' food and fluid intake that suggested such large amounts weren't needed, along with studies showing that caffeinated drinks could be counted towards the total. His review was published in 2002, and the rule has outlived it comfortably, the way rules do.

What the NHS says now is smaller and easier. Aim for six to eight cups or glasses of fluid a day. Water, lower-fat milk, sugar-free drinks, tea and coffee all count. It's a guide, not a target, and you may need more if you're pregnant, in a hot place, active for long periods, or ill. And most people should aim to drink enough that their pee is a clear pale yellow. That last line is the one to keep, because it replaces counting with looking. A number tells you what you should have done. The colour tells you where you are.

The honest objection is that "six to eight cups of anything" sounds like nothing, and if it's nothing, how can the dry day be a problem. The answer is in chapter 1's exercise. Count yesterday. If you're at six with tea and coffee included and your pee is pale, you're fine, and this book is short and you can give it to someone. If you're at three, mostly before eleven, and your pee is dark, then the gap between three and six isn't large, and it's made of ordinary drinks you can attach to things you already do.

I think the eight-glasses rule has done harm in a specific way. It set a bar high enough that most people gave up on the whole idea, and it excluded the drinks people actually drink. The result is a lot of people who believe hydration is a project for other people, and who go through the day at three cups because six sounds like eight and eight is impossible.