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Comment by jaggederest | Hacker News Reader

"Hydration", or "8 glasses of water a day" is another one.

Your kidneys work, if you aren't thirsty (barring certain medical conditions) you don't need to drink water.

There's no such thing as "dehydration without being thirsty" on a routine basis. You'll develop a pretty powerful thirst at 2% water depletion, and for most healthy adults you need to see 4% or more for negative effects.

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I’m not an expert, but 20 years ago I had a small part in DARPA research into monitoring soldiers’ hydration. The idea was that people in combat often don’t notice they are thirsty, or power through it, and as a result are operating at reduced cognitive and physical capacity. Of course, that’s combat. And for all I know the funding was based on the popular misunderstanding.

Yes, and marathon runners or other people in extreme situations need to drink before they're thirsty so that they keep their intake high enough. But those are really extreme situations, most people walking around with a water bottle aren't sweating out 2 liters per hour.

Nonsense. As we age, our sense of thirst diminishes. Senior citizens are at higher risk of dehydration.

https://health.clevelandclinic.org/drink-up-dehydration-is-a...

That's true, and that's why I said "healthy adults" - Senior citizens are in a special category where they may need to drink before they are feeling thirst, but most people are not senior citizens (yet?). The average person walking around with a water bottle doesn't need to drink before they're thirsty.

Is there such thing as a "blood hygrometer" (for lack of a better description) that could objectively tell you when you're dehydrated?

Dehydration skews a lot of blood values. Hematocrit is probably the most direct, since it's the percentage of red blood cells in a volume. Dehydration decreases the volume, and thus increases hematocrit. Electrolytes get out of whack too.

In practice, I don't think anyone really does this: thirst, urine/sweat, and weight are very easy to measure, and the obvious next step would be pulse or blood pressure.

I was thinking more along the lines of whether there is something like a continuous glucose monitor but for dehydration, which could beep or otherwise alert a semi-independent elderly person that they are becoming dehydrated so they could get some water before it started to stress their kidneys. Like those pill boxes with alarm clocks built in so they don't forget to take medication.

But I'm aware there are various issues with CGM's (a friend has one, which may act as an insulin pump as well, and complains that the needle needs to be moved around every so often as the attachment site stops giving accurate readings). The cost/benefit probably isn't there, if an elderly person cannot control their own hydration they probably also aren't able to adequately care for a CGM or similar monitor. If it was something you could implant into the body and poll wirelessly it might have some value.

That might be overkill.

I think you could get 99% of the way there with a reminder at 10am, 2pm, and 6pm to have a glass of water. With a few exceptions (e.g., heart failure), there's no real harm in moderately overdoing it--you'll just pee it out.

Drinking a lot of water can be helpful - if you expect to perspire heavily from temperature and/or physical exertion but that is a different kettle of fish.

It can be very difficult communicating people new to the Phoenix area just how much water they need to drink during the summer if they are going to exert themselves outdoors.

I am French and travelled menu times to Phoenix for business. I loved the place (Scottsdale) and was once invited to play golf there (my first golf experience).

I was slowly turning into lyophilized me, fortunately I could give up at the 9th hole and went to immediately drink 2 liters of water.

Drinking a lot of water prior to exertion isn't helpful. You'll just piss it out.

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