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The Institute of Medicine recommends 3.5L of beverages every day for adult males that are physically active (of course, there are a lot of factors that can change this recommendation somewhat). The recommended figures on this Stackexchange Personal Fitness site tend to be similar. Recently, however, news stories have been saying that you should only drink when you're thirsty, quoting an article that claims that the body shows signs of not wanting to drink after a certain amount.

For me personally, if I would drink as much as I want (i.e., only when I'm thirsty) without paying attention to the amount, it would be a 1-1.5L daily.

Is the human body's response really a reliable indicator for how much water you should drink?

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Personally I feel like the thirst response is the primary go-to for what hydration your body needs, but it's not exactly fool proof.

There was a 1991 study showing that, as men (and probably) women age, they have reduced thirst and are at increased risk of dehydration. The participants in the study you linked were young-ish:

Twenty healthy participants (11 female, 9 male; age range 19–54 y[ears of age].

Personally I look at it like this:

  • There is documented evidence that later in life your thirst response starts being unrepresentative of the amount of water you need.
  • I'm loading myself up with water retaining creatine on a daily basis.
  • When I drop into a ketogenic diet, the amount of uric acid goes up. The studies I've seen have related to high-protein/keto/low-carb diets says that your kidneys are fine, but I still like to know I've got more water moving through me.
  • Drink too much at night and you'll mess with your sleep waking up to pee.
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In practice, when you exercise a lot and lose water in the form of sweat, you will need to manage the amount of water you drink, you can't leave that to just feeling thirsty and drinking as a response to that. The rule of thumb is to drink 50% more than the lost water (which you can measure by weighing yourself before and after the exercise session). Also, you should not drink more than 0.8 liters per hour. So, if you have lost 1 liter, you need to drink about 1.5 liters and you should do that over the course of the next two hours.

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My wife drinks when she is thirsty as do I. My wife drinks a lot of coffee, however. A few times she has gone to the emergency room for unrelated issues (she has shunts) and she has been kept because she is dehydrated. I believe this is her normal state because coffee is a Diuretic (makes her urinate a lot) so she is losing water there and not taking enough in.

I drink when I am thirsty and have never had an issue with dehydration but I don't drink as much coffee.

So I think the answer is "it depends". If you are not normally thirsty for water for whatever reason (drinking the wrong things, not aware, etc...) then only drinking when you are thirsty can lead to dehydration. If you are aware of how much you drink and make sure you drink enough water throughout the day I believe we train our bodies to be thirsty when it needs to be.

Expanding a bit on why only drinking coffee made my wife dehydrated. It has been pointed out that - under normal circumstances - even though you urinate more when you drink coffee you are drinking enough water to not make it matter.

For someone who only drinks coffee, however, they will stop drinking when they have too much caffeine in their system because too much caffeine can cause unwanted side effects such as jitteriness, hypercactivity, headaches, etc... so if they find themselves thirsty they will realize they've had enough coffee to drink and not drink anything. In time, they have trained their thirst to be thirsty for caffeine, not the water the body needs. Such a person can not trust their thirst and so I got into the habit of drinking iced tea with lemon so my wife would start drinking it also getting the water intake she needs and the caffeine intake she desires. I'm happy drinking water when I get thirsty.

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  • Coffee being a diuretic is a bit of a myth. The mechanism by which it works increases frequency of urination, but does not affect volume. Excessive drinking of coffee can lead to dehydration only in the same way that drinking excessive amounts of water paradoxically can, by flushing your system of all of those useful salts used to retain the water you need.
    – Sean Duggan
    Nov 21, 2014 at 13:34
  • Umm.. diuretics increase the frequency of urination, that's what they do, that's the defintion. Diuretic (medicine) (chiefly of drugs) causing increased passing of urine. You confuse me when you say it's not a diuretic it only increases the frequency of urination O.o - If your are saying that coffee being a diuretic doesn't affect our hydration level, that's a totally different thing that I haven't researched so you may be right, if that is your point.
    – Serpardum
    Nov 23, 2014 at 15:47
  • A diuretic is any substance that promotes the production of urine. You can find similar descriptions in the medical dictionaries. Caffeine promotes the frequency of urination, but not increased production. It's a common misconception.
    – Sean Duggan
    Nov 24, 2014 at 0:17
  • I was thinking my comment to you over and I'm wrong. Coffee is a diuretic, same as water. The extra fluid lends to production of urine. It's caffeine that's not a diuretic. My apologies.
    – Sean Duggan
    Nov 25, 2014 at 15:37

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