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Comment by wlesieutre | Hacker News Reader

"Don't swim for 30 minutes after eating, you'll get cramps and drown" is one I've heard in the US from multiple people

"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.

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.

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.

I don't recall the original interaction anymore but I remember confronting my mother in my early 30's about her telling me the one about gum staying in your stomach for 7 years. She had no recollection of having done that, and argued that being a nurse she would have never told us that.

But apparently younger me was convinced that story came from her and not from the schoolyard rumor mill.

Now that I have my own children, I'm convinced this myth is a delay tactic perpetuated by parents :)

Yep, white lies. The underlying rationale is that kids do throw up in pools and you don't want to get into a public argument with a kid about how they're not going to throw up, can they go in pretty please.

It's like the thing about getting arrested for turning on the lights inside a moving car. No, you won't get arrested, but the reflection on the windshield can be annoying, and getting distracted by kids going ham on flicking the lights on and off is obviously dangerous.

See also, school "permanent records".

I think the white lie is even simpler.

As a kid, I always wanted to go swimming (swimming pool or sea) as soon as I finished eating. My parents told me not to because of the cramps. "You must wait an hour before swimming".

It wasn't about throwing up and as an adult I finally understand it: they wanted to have a chat at the table after eating, take it slowly, maybe they were a bit drowsy. What they didn't want to do was to look after kids doing potentially dangerous activities such as swimming. So that hour of waiting? A period of grace for them to enjoy their meal and maybe rest for a short while.

Exactly! Parental relief.

Nah, it's definitely the hygiene thing. Pools often forbid food and drinks at the deck for the same reason: if someone dirties the pool water, they basically need to close off the whole pool and recycle the pool water for a few hours, which is super inconvenient for all parties involved (and obviously embarrassing for the parents of the kid who made the mess). You'd be surprised at how often these things happen.

Ask me how I know :)

In British Columbia, Canada, the BC Motor Vehicle Act mentions interior lamps in connection with larger passenger vehicles. In addition to requiring that they have them, it has this requirement that they be on while driving at night:

  Interior lamp
    10.20 (1) A commercial passenger vehicle shall be equipped with a
              lamp or lamps within the vehicle so arranged to illuminate
              the aisle of the vehicle to the rear of the driver.

           (2) A person driving a commercial pass enger vehicle having a
               seating capacity including the driver of more than 12 occupants
               shall insure that the light or lights referred to in subsection
               (1) are illuminated at all times that passengers are being
               carried between sunset and sunrise.

AFAIK, that's meant for buses, and it's the same rationale as airplane floor lights (i.e. illumination is necessary to avoid falls from tripping in the dark, e.g. if the passenger is going to the toilet, which some buses have).

I can't find any other mention of interior lamps in the same traffic code document.

We had a child swim in our pool immediately after eating and she threw up. Probably where that one came from. Kind of like how it is bad luck to open an umbrella inside. It's not bad luck, but somebody might get poked in the eye.

I'm from the Netherlands; I always heard that's because you might get a bit too excited and throw up your food. The 30 minutes are to allow your stomach to settle a bit.

An amusing thing about such myths is that instead of abandoning them outright, we wrap them in auxiliary myths such as not wanting kids to throw up.

On the other hand, how long does it take to know that your kids haven't gotten food poisoning from the food that would be served near a swimming pool?

It's presumably not just food poisoning--it'd be exercise-induced.

Even if wrong, it seems harmless enough.

Even if I believed that, I wouldn't, say, avoid jumping out of a burning ship into the water just because I just had a meal.

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