Should we drink water before we feel thirsty?

Question

The claim is that we should be drinking water routinely to the point that we never feel thirsty, and that to not be doing so is to our detriment, at least enough for the notion to be popular amongst hikers, gym teachers, athletes and the like.

While I understand that from a medical point of view at the onset of thirst we are "mildly dehydrated" (thirst wouldn't make much sense if nothing was going on), the claim is that the sensation is indicative of an already-made mistake in behavior, and that drinking water now will be of significantly less benefit than if we had beforehand, preempting thirst.

Here is an example of one side of the claim:

If you wait until you are thirsty, your body is already dehydrated. It takes about TWO HOURS for anything you drink to have an effect on your body’s hydration level. By the time you feel thirsty, you are two hours behind! Drinking something even the second you realize you are thirsty means that you will still be dehydrated for at least two more hours. You should be drinking enough liquids throughout the day so that you rarely (or never) feel thirsty.

Here is an example of the other side.

Medically, dehydration is defined as a 5% increase in the concentration of solutes in your blood. (Often this can be more conveniently detected based on short-term weight loss.) Thirst sets in at about 2%, so you'll always feel strong thirst setting in long before you're dehydrated.

Answer 1

No, you do not drink water before you feel thirsty.

Source - Mythbusting sports and exercise products, British Medical Journal

Bottom line: drinking ahead of thirst

General public—Drinking ahead of thirst may worsen performance in endurance exercise and carries a rare but serious risk of hyponatraemia. The body’s internal mechanism for staying hydrated is cheaper, easier, and seems to be the best way to optimise performance

Professional athletes—Elite endurance athletes perform best when they drink to thirst; some studies suggest exercise induced dehydration can improve performance

The review did not find sufficient evidence to be able to say this finding is generalizable beyond endurance athletes:

A high quality randomised trial measuring the performance effects of different hydration regimes during shorter exercise (sprint-type) would determine whether the results of systematic reviews are generalisable beyond endurance athletes.

Answer 2

It depends.

While Tom77 has focussed on the subject of physical performance, there are other factors to be considered.

People with hyperuricemia, for example, are urged to ensure they drink at least two liters per day, preferably unsweetened, non-alcoholic beverages.

For some, this is significantly more than they would drink on their own, requiring them to consciously drink more than their thirst tells them to.

On the other end of the spectrum, patients undergoing dialysis treatment have to monitor their drinking habits to ensure they do not drink more than their kidneys can handle.

So, in the general case -- if you are healthy, and your thirst is "in sync" with your body's needs, you can probably trust it. But you should be aware that there are situations where your body will not tell you all you need to know.

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Sorry for the two German language sources linked; I had a somewhat hard time finding English equivalents. Will keep looking for them, though.