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I am not sure if 'pre-hydrating' is a valid term to use for this, please correct me if you know the term.

While walking down a tiring route, I was so tired and low on water. All I could think of was, "ahh, I wish I had more water with me". I was carrying enough (for me), but a girl whom I met on the way wasn't, so I shared. When I reached a village after slogging for hours, sipping some after a typical interval and distance, I rested for a few minutes and drank a lot of water.

If I know that I am going to be trekking over the weekend, does it help if say from Thursday, I start drinking more water than I usually do?

This clicks my mind because I spend the whole day in air-conditioned office, so, naturally and unfortunately I drink less water than I am supposed to.

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Yes, but not days before, hours before. When I drink 2 whole glasses of water 1-2h before skating (13-20mi) I don't need to bring water with me. Even if I have to go to the bathroom before, afterwards, I no longer feel the need. It feels like my kidneys are working in reverse. I think they can do that, but I'm not certain. – Chloe Jan 14 at 19:41
    
The moment I read about the 'sharing of water with the girl' I knew the OP has to be an Indian. :) – Jay Jan 15 at 7:21
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@Jay: And, what made you think so? – WedaPashi Jan 15 at 8:12
    
I really don't know. Just a gut feeling. – Jay Jan 15 at 8:36
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I spend the whole day in air-conditioned office, so, naturally and unfortunately I drink less water than I am supposed to. This seems to be based on urban folklore that people need to drink more water than they would drink due to the natural sensation of thirst. Not true. ajpregu.physiology.org/content/283/5/R993.full – Ben Crowell Jan 17 at 19:38
up vote 45 down vote accepted

Your body does not store significant amounts of water (most of the, usable, water in our body is in blood plasma). Unlike energy which can be stored as fat or carbohydrates, water has no real storage mechanism. This is why you will die of thirst long before you die of hunger.

As such, once your body reaches its optimal hydration point it will expel excess water (you go to the loo). Your kidneys control this process. If you ingest large quantities of water your kidneys will react to expel the excess. No storage will happen.

This can also put extra strain on your kidneys and lower your blood salts level. So even if this was practical it'd be ill advised. Drinking far too much water in a short period of time (remember it's not stored, your kidneys will eject the extra water as fast as they can) can lead to hyponatremia or (in extreme cases!) water intoxication and even (eventually!) death! Though you would have to this to an incredibly extreme level to get anywhere near the Water Intoxication level. Hyponatremia can be nasty though and is reasonably easy to trigger:

13% of the athletes who finished the 2002 Boston Marathon were in a hyponatremic state ...

Signs and symptoms of hyponatremia include nausea and vomiting, headache, short-term memory loss, confusion, lethargy, fatigue, loss of appetite, irritability, muscle weakness, spasms or cramps, seizures, and decreased consciousness or coma

You lose water naturally though sweating, urine, moisture evaporated from your lungs, etc. (you also produce water when you convert glucose to energy but this will be outstripped by your loss of water, typically).

You need to replace lost water constantly. The key to optimal hydration is little and often, because too much and your body will expel it, but too little and you become dehydrated. There's no way to avoid this mechanism.

Related:

How can I tell if I am dehydrated or have low electrolytes (need salt, etc.)?

How do I achieve/maintain optimal hydration?

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you also produce water when you convert glucose to energy but this is a small amount in the scheme of things not so small. A quick calculation based on the glucose breaking equation gives ~375g per day – njzk2 Jan 14 at 17:13
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So that's about 0.375 Litres of water, your recommended intake is 2.7, so....... 13%?! Not that insignificant I suppose but still not a huge amount. Point taken though. – Liam Jan 14 at 17:21
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What about if you have some extra salt with that extra water: does the extra salt allow you retain the extra water? – ChrisW Jan 15 at 0:04
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Adding salt to water is a very good idea @ChrisW. this is basically what isotonic means (with the addition of some sugar also. The salt aids absorption of the water and replenishes salt lost though sweating, etc. If your in a hot climate you should always add salt (and sugar) to your water. You can by isotonic sachets to add. – Liam Jan 15 at 9:07
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That doesn't seem relevant in the context of the question? The OP wasn't asking about this. He just wanted to know if he could drink more water before a hike. This seems a different question/topic. – Liam Jan 15 at 10:55

Liam's right that you can't/shouldn't overhydrate, but based on my experience it's easily possible to start the day under-hydrated.

Maybe you're office-bound and don't drink enough water, or you've had a spot more alcohol the evening before than you should, or too much coffee... Either way, you start walking under-hydrated, and end up drinking more water than expected early in the day.

If I'm doing a day hike, I tend to take a 1 or 2 litre water bottle in the car (in addition to what I'm walking with...), and use this to improve hydration as I drive/before I start walking. Whatever's left in the bottle is great to rehydrate with when you return to the car.

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That makes sense. I'll upvote as soon as I can. (Daily voting limit reached) – WedaPashi Jan 14 at 12:49
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Just wanted to add you'd have to drink a lot of coffee to get dehydrated. Either way, +1. – Roflo Jan 14 at 15:49
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@Roflo - indeed. My problem is I'll drink a doppio instead of a half pint of water. So more coffee == much less water :( – Roddy Jan 14 at 17:12
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-1 because this propagates multiple myths about water. See lightandmatter.com/article/hiking_water.html , especially the section "Myth: caffeine causes dehydration." – Ben Crowell Jan 17 at 19:36
    
@BenCrowell. Alcohol isn't diuretic? I take it you've never had a hangover, then? – Roddy Jan 17 at 22:44

Not at all. It only takes a few hours for water to work its way from your gut into your system, after which any extra simply goes through your kidneys and out the other end.

However you can definitely drink extra water before you start and need less during the day. The water you lose in sweat will then be directly replaced by the water you've drunk in the morning. It doesn't completely remove your need for water during the day, but it certainly helps. I believe American trail hikers call this "camelling up".

Of course you do need to think about electrolytes. A reasonable breakfast will mostly take care of that though, as well as providing you with energy through the morning.

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If I know that I am going to be trekking over the weekend, does it help if say from Thursday, I start drinking more water than I usually do?

This clicks my mind because I spend the whole day in air-conditioned office, so, naturally and unfortunately I drink less water than I am supposed to.

Others have already (correctly) told you that we cannot store much water (for later use).

However, as you mention that you generally drink too few water, I think that making a point of drinking much water starting a day or two before the tour is actually a good idea. (Even better would be to drink enough water in general, though.)

While that does not build up a reserve of water for the tour, it allows you to start without a water deficit. Your kidneys do not just produce pee to keep the correct water level of the body, they also regulate pH (acid-base homeostasis) and electrolytes and expel different kinds of water-soluble waste (see e.g. https://en.wikipedia.org/wiki/Renal_physiology). To do that, first so-called primary urine is produced which contains lots of water (ca. 180 l/day is healthy) which is then concentrated about 100x to keep the water in the body. This concentration step is why drinking and peeing more water is easier for the kidneys. A rule of thumb is that your kidneys are happy if you pee ca. 2 l/day (this looks clear unless you ate something that has/creates colored urine and is really a lot).

The often-cited problems with hyponatremia are typically in situations where people sweat in the liter/hour range and replace only pats of the water, but no electrolytes and/or are in somehow restricted situations (like the cited marathon race or in the (in)famous water-drinking competitions) in which it seems good to ignore all kinds of signals of the body.
The thing is, these situations are quite opposite to drinking more water during a day or two in preparation of a tour: that gives you time enough to act on all kinds of signal of what the body needs: Thirst is our signal for the need of water. But need for Na+ can be perceived as well, it leads to an appetite for salty food (https://en.wikipedia.org/wiki/Specific_appetite). There's also a sensation that prevents overhydration, e.g. I then dislike the thought of drinking more water.

IMHO there is no need at all to run into low Na+ trouble on tours: Just take some salt with you, and act on whether feel an appetite for saltor not: if it feels like a good idea to have salt on your cucumber or egg, that's what you should do. If the idea of cucumber without salt seems better, you're in need of water, but not of Na+ .

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Water intake before the trek is just one of the factors that allow one drink a little bit less during the first day; more importantly, the same factors let you start the trek with much more energy and tolerance to discomfort. That will eventually help you combat thirst as well, provided that you still don't expect miracles from your preparation and still carry enough water with you - basically as usual.

Eat foods rich in polysaccharides (pasta, bread, vegetables,...) for the last 1-2 meals the day before the trek. Glucose is stored in your body in the form of glycogen, and glycogen binds a lot of water. One to four, or you could say that 80% of what's labelled glycogen in your muscles and in your liver is actually water, invisible to your hydration control mechanisms. However, it takes time to accummulate glycogen and throughout that time the supply of glucose needs to stay away from both extremes; polysaccharides make that easier because they are digested slowly (compared to sugar) and they release glucose into your bloodstream gradually.

While you are doing so, follow the usual hydration advice as well; without sufficient (slow, gradual) water intake you will just not be able to build your glycogen reserves either.

Get enough sleep. I'm getting boring, right?

Granted, we are not talking about huge quantities of glycogen and water accummulated. But any form of physical exercise is just plain more enjoyable and healthier if you start it hydrated and full of energy. And learning to eat and drink properly is a life skill that only gets more and more useful as you will age.

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