I have always been very doubtful about the claims that we are all dehydrated. I think it is a word misused in the same way that 'allergy' is. Real (clinical) dehydration is not the same thing as what quite often self-styled 'experts' describe as dehydration, usually without any tangible evidence or explanation as to exactly what they mean by it.
There are a couple of things that particularly irritate me. One is to make people think that a guideline about how much water to drink is an absolute instruction, so that they almost obsessively take their bottles of water about with them, sipping away however inappropriate the place, and preferring to slug out of the bottle even if there's a jug of water and glasses on the table, so that they can measure their intake.
Another is the ludicrous advice that your body does not know how much it needs. "If you become thirsty" they affirm, "this is a sign that your body is already dehydrated. You must drink before you are thirsty."
If you substituted 'hungry' for 'thirsty' and 'starved' for 'dehydrated' (oh go on, then, 'eat' for 'drink' too, if you are being pernickity), it would be apparent that this is complete rubbish. The reason that most of us are overweight (and awfully sweet of you not to gloat if you're reading this and you aren't) is that we eat when we're not really hungry and we don't stop when we're full. So why are we told to learn to 'read' our bodies regarding food, but to override the signals from our brain regarding water? If you offer a baby or a small child too much food to eat or more liquid to drink than it needs, it refuses the excess. An animal drinks when it's thirsty and not before. There's nothing more delicious or refreshing than a long draught of water when you are hot and thirsty.
There's a third annoyance, I realise, which leads to a fourth. When 2 litres a day was first given as a recommended daily amount of fluid, the scientist who said it said quite clearly that this was total fluid and included what liquid comes in food. This message didn't come across and people were told that it had to be water. Not soup, not juice, not tea, even a squeeze of lemon juice was suspect - unadulterated water was all that would do. Any other liquid was dehydrating and was a negative influence. This is obviously rubbish. The hydrating property of liquid is not eliminated by adding something else to it, even if that something does, like coffee or alcohol, have a dehydrating quality in itself. It may slightly reduce its hydratingness, but it does not dehydrate you. Some people drink no water at all - do they die of dehydration? Not if they drink several cups of tea, a few pints of beer, or whatever, a day they don't.
In addition, some smartypants soon thought that if 2 litres a day is good, then 4 litres must be better and started to recommend a glass of water every hour. There is such a thing as overhydration, and people have died from it - usually, it must be said, when their bodies have lost all sense of moderation because of the amount of Ecstasy they've taken. Too much of anything is not better than a healthy amount. I have read, too, that valuable trace elements can be flushed out of your body if you drink more than you need, which is the other side of the 'flushing out toxins' claim.
I do think that, if you work in a modern office which, as most of them are, is kept overheated in winter and air conditioned in summer, you need to keep up your fluid intake. I don't work in that environment, but when I'm in such a place I find I become thirsty, and headachy if I don't drink enough. I also think that too much coffee isn't a good thing, and I don't drink squashes and the like and don't think that too much fruit juice is good for you, so yes, water is the thing. But if you aren't in an office and you aren't out in the sun and you aren't exerting yourself much, you don't necessarily need as much to drink as if you are.
And it varies day to day. A few days ago, I found myself making mug after mug of herb tea all day and I drank a pint of water in the evening too. Yesterday, on the other hand, because it was just the way it went, I had two mugs of coffee and a mug of tea, and a quarter of a pint of water before I went to bed (because it was all that was in the glass and I wasn't going downstairs for more) and that was all. I pinched the back of my hand and the skin was elastic and my pee was normal, so I wasn't concerned. I may well want to drink more today to make up for it, however, and this will be fine. It'd be different if I had a kidney problem, for instance, and my body could not so easily regulate itself, but I don't.
Then there's the suggestion that we often eat when we're actually thirsty and that we can't tell the difference between hunger and thirst. No I don't. Yes I can. It may be the case with some people, and there are certainly some who drink water to quell hunger pangs - they are often the ones who obsessively diet to the American size zero. This is not healthy either.
This brief article was in The Times a couple of days ago - I've put a link but have also printed it in full.
There is no solid evidence that drinking eight 8oz glasses of water a day improves skin tone, aids dieting, prevents headaches, flushes out toxins or improves wellbeing. In research for the Journal of the American Society of Nephrology Dr Dan Negoianu and Dr Stanley Goldfarb found no study to support the claim.