"I'm not thirsty"

If you're not thirsty, you can't be short of fluid. That's the belief, and it's a fair one, because thirst is what the body has for the job, and if the body wanted water it would say so. The belief has two problems, and one of them is measurable.

Fiona McKiernan, Jenny Houchins and Richard Mattes collected hourly ratings of hunger and thirst from 50 adults for seven days in a row, along with what they ate and drank. Hunger came in two clear waves a day. Thirst was stronger than hunger and roughly level across the day, and about three quarters of all fluid was drunk around meals. That's a description, not an instruction, but it tells you something about the quiet signal. It's not that thirst is absent between meals. It's there all day at a steady level, which is exactly the kind of signal you stop noticing, the way you stop noticing the hum of a fridge. Hunger interrupts. Thirst waits.

The second problem is that "not thirsty" doesn't mean "fine". In a study by Nathalie Pross and colleagues, 20 healthy women went without fluid for about 24 hours in a controlled setting. Increased thirst and darker pee were among the earliest signs, well before anything showed in the blood. That's the useful part. The quiet signal and the colour in the bowl arrive first, and if you've trained yourself to talk over the first, the second is still there, which is why chapter 18 uses it.

There's an honest objection here. Some people say they're never thirsty, and mean it, and a few of them are right that their signal is weak. Thirst does get less reliable with age, which chapter 20 comes back to. But for most adults under sixty, "I'm never thirsty" means "I've never listened", and the pale check will tell you which you are within a week.

The practical implication isn't to try harder to notice thirst. Noticing a steady quiet signal against a busy day is a skill most people won't learn and shouldn't have to. The implication is to stop relying on the signal at all, and drink on a schedule that's built into things that already happen, which is what the pair is for.