Water. I have nothing against it. There are seas and lakes of it. It flows in rivers, brooks and streams. Sometimes it falls from the sky – great news for plants but not if you've just put the washing out.

Without it, we wouldn't have lovely cups of tea, or chicken soup, showers, baths or – if you are Japanese – a shower and then a bath. And when you're thirsty, there is nothing more refreshing than a glass of it.

But drinking a glass every now and then is rather different from having to drink two litres a day, which is what we should all be drinking, if you believe them: the Water Fascists.

Water Fascists. I have everything against them.

They are everywhere, preaching about the need for two litres a day and its umpteen health benefits. It's the best thing for your skin. It's the bestthing for your liver. It's the best thing for your kidneys. Water is "the forgotten nutrient". Without it, your legs will drop off and your pelvis will explode at a time that might not necessarily be the most convenient for you (while you're in Sainsbury's, for example).

I approached a department store's cosmetics counter the other day to buy an exorbitantly priced skin cream. It probably wouldn't live up to a single one of its promises but, what the heck, I was going to buy it anyway.

But what happened next made my jaw drop. The girl behind the counter (the sort who looks as though she has applied her foundation with a JCB) interrogated me not only on my fluid intake, but also on my output! "What colour is your wee?" she asked. Listen, love, some things are private, you know. "Is it dark?" she persisted.

Not telling. Go away. Just rip me off quickly, cleanly and efficiently. "It should be a pale, straw colour," she concluded, decisively, probably because she was wearing a white coat. On the counter was a little sign that read: "Dehydration is every woman's number one enemy". Mine's actually the VAT man, but there you go.

Now, as I've said, I've nothing against water; it's just the Water Fascists. And what about my parents' generation, who as far as I can see drink little or no water? Indeed, I can't recall my mother ever drinking a glass of water.

I call her up. "Do you ever drink water, Mum?" "No," she replies. I tell her that, by rights, she should be dead. "Well, I'm not, but I'll just check with your father" she says. "Denis! Am I dead?" she bellows. Mum returns to the phone with the verdict: "He confirms that I am not."

See? Not only not dead, but also, at 82, still in full possession of her marbles. Furthermore, she doesn't suffer from fatigue, headaches or muscle pains, and neither does she look like a shrivelled raisin. She also plays tennis every day (badly, and with no backhand, but that's not the point).

Right, let's take the Water Fascists' main claims and see if they stand up. First, two litres a day. Essential? Yes, in fact. Can't pick a fight here. Your body does use two litres of fluids a day. You're actually losing moisture all the time. Breathe on a mirror, if you don't believe me. But does it have to be two titres of water? Well, according to them, yes. Tea and coffee don't count because they are diuretics, they say – they actually make you lose more water than you retain. True? No, not really. Okay, coffee is quite diuretic – although you will still always retain some fluid – while tea, according to a recent review of the literature in the European Journal of Clinical Nutrition, actually hydrates the body as effectively as water.

Indeed, as Tom Sanders, Professor of Nutrition and Dietetics at King's College London, says, "Tea is the most widely consumed beverage throughout the world, and there must be a reason for that. It's actually a functional food." Hydrating, you mean? "Yes."

He also adds that you get a heck of a lot of water from food. Fruit and vegetables can contain up to 95 per cent water. Cucumber is water, more or less, but in the shape of a stick. A jacket potato contains 70 per cent water. An egg is 70 per cent. Chicken is 65 per cent. On average, we all consume a litre of water every day through food, plus our bodies produce water metabolically.

So, is there any scientific research showing that above and beyond what we might eat and drink we also need to drink two litres of water a day? The answer, in short, is no.

According to Claire Williamson, nutrition scientist with the British Nutrition Foundation: "If you take into account what you eat, you only need1.2 litres of fluids a day, and that fluid does not have to be water. Tea, coffee, fruit juice – they all count, as does alcohol, so long as it's not too strong." But if I don't drink a ton of water my skin will shrivel off, won't it? Again, no. Or, as a recent study by the foundation puts it: "There currently appears to be little scientific evidence relating to the effects of water consumption on skin hydration, and whether drinking more or less water actually has any impact on skin appearance."

The next claim is the one that perplexes me most: if you are thirsty, you have left it too late; you are dehydrated already. Hang on. Isn't this like saying that if I feel hungry I've left it too late to eat? Surely the safest bet is just to follow your body's signals in these matters? I'm thirsty. I'll have a drink. Now I'm not thirsty. Problem solved. Aren't we in danger of putting thirst out of business otherwise?

Back to Professor Sanders, who says, "There is evidence to show that cognitive function improves on drinking water when you are thirsty. But there is no benefit from drinking lots of water when you are not thirsty."

I wonder, if you are going to drink water, is it better to drink bottled water? "We would never recommend bottled water over tap water," says Claire Williamson. "Although some people do prefer the taste." As for themineral content, according to the World Health Organisation, it is of no proven benefit whatsoever.

Lastly, have you actually tried drinking two litres of water a day? I did for nearly a week, most of which was spent seeking out public toilets and getting up 12 times a night. To make it more scientific, I got my friend Alison to try as well, which was rather cruel, I admit, as she cycles from north London into central London every day. She says that, on her first morning, if it hadn't have been for the nice man opening the Argos on the Holloway Road (who allowed her to use the facilities), there would have been a nasty and shaming accident by Highbury Corner.

One final thought: writer HG Wells once said that the prime aim of any industry is to sell you air and water. Now you can buy bottled water with added oxygen. Not even the great science fiction visionary could have seen that particular scam coming.

Liquid Lunch

The facts reveal that there is plenty of water in the food we eat every day

Egg: 70%

Cucumber: 95%

Fruit and vegetables: 95%

Chicken: 65%

Jacket Potato: 70%