It鈥檚 not called Adam鈥檚 ale for nothing. Water was presumably what our early ancestors drank, to the exclusion of everything else. If you stopped drinking it now you would be dead within a week. It is the only nutrient whose absence is lethal in so short a time.
But how much you should drink is surprisingly contentious. It is common to hear eight glasses a day 鈥� about 2 litres 鈥� even if you don鈥檛 feel thirsty. In 2002, physiologist Heinz Valtin of Dartmouth Medical School in New Hampshire . The closest he came was a 1974 book that casually advised six to eight drinks a day 鈥� not just water but also soft drinks, coffee, tea, milk and even beer.
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Read more: Good hydrations: From water to wine, how drinks affect health
We swallow 1.7 litres of fluids on average a day 鈥� and with them a lot of myths about what is, and isn鈥檛 good for us
As for its scientific validity, Valtin found none. As the Food and Nutrition Board of the US National Academies of Science, Engineering and Medicine advises: 鈥溾��. The only exception is some elderly people whose feedback mechanisms go awry, meaning they can become dehydrated without thirst.
Generally, there is little to gain by doing more than just quenching your thirst. Water doesn鈥檛 remove toxins from the skin, visibly improve your complexion or cure constipation. There is some support for the idea that drinking cold water makes you burn calories, and water with a meal does reduce overall calorie intake, perhaps because it helps fill you up or displaces calories from sugary drinks. But the overall influence of water on weight is far from clear.
There is a sliver of evidence that being well hydrated can protect against health problems including colorectal and bladder cancer, heart disease, hypertension, urinary tract infections and kidney stones. Good hydration makes it easier for the kidneys to extract waste, reducing wear and tear on them. Dehydration headaches do exist and water can cure them (although there are hundreds of other reasons why your head might ache), and drinking lots when you have a cold may loosen mucus, easing the symptoms.
Water may not be a cure-all, but the downsides of overdoing it are mild. Besides rare deaths through over-hydration among marathon runners and ecstasy users, the worst of it is that 鈥� they have too little sodium in their blood. This is not a major problem, but has been associated with mild cognitive impairment and an increased risk of falling in older people.
Overall, though, 鈥� intake and the evidence of positive effects is quite clear鈥�, according to a recent review.
Perhaps the most implausible claim of all has the strongest support: water can improve focus, at least among children. Several studies have found that having children aged 7 to 9 drink water improves their attention and, in some cases, recall. Perhaps children of this age are more prone to dehydration, which can cause a decline in alertness, concentration and working memory.
Tap or bottled?
For some, tap water is too clean, laced with chlorine-containing compounds used to sterilise it. For others it鈥檚 not clean enough, teeming with nasty pathogens and traces of chemicals. Then there鈥檚 the fluoride often added to it for dental health: , it remains controversial in some quarters today.
Whether for those reasons or simple taste, many people prefer to buy bottled water. Either way, that could be a waste of money, in most parts of the West at least. Around 25 per cent of bottled water sold in the US is simply tap water from municipal sources. A large proportion of bottled water is chlorinated just like tap water 鈥� for good reason. Water chlorination is impressively effective at preventing serious diseases such as dysentery, cholera and typhoid. Evidence that chlorination or compounds that reduce male fertility is 鈥渋nadequate鈥�, according to the World 91色情片 Organization, and the risk is extremely small compared with that from poorly sterilised water.
Tap water does contain traces of pharmaceuticals, toiletries and cosmetics, but the US Environmental Protection Agency says 鈥溾��. As for fluoridation, there is no evidence that this causes any health problems except where accidents lead to over-fluoridation, which can cause vomiting and diarrhoea.
There is stronger evidence that the minerals in some bottled waters, especially sodium, can be harmful. And while the health benefits of mineral-rich waters have long been touted, the enormous variation between brands makes this impossible to test.
As for taste, that is also impossible to test objectively. But if it is an issue, just chill your tap water: it makes bad flavours much less noticeable.
Water, water everywhere
Some 20 to 30 per cent of an average person鈥檚 fluid intake comes from food. But how much do individual foods contain?
100% Water
90-99 Fat-free milk, cantaloupe, strawberries, watermelon, lettuce, cabbage, celery, spinach, pickles, squashes
80-89 Fruit juice, yogurt, apples, grapes, oranges, carrots, pears, pineapple, broccoli
70-79 Bananas, avocados, cottage cheese, ricotta cheese, potato (baked), corn (cooked), shrimp
60-69 Pasta, legumes, salmon, ice cream, chicken breast
50-59 Ground beef, hot dogs, feta cheese, tenderloin steak
40-49 Pizza
30-39 Cheddar cheese, bagels, bread
20-29 Pepperoni sausage, cake, biscuits
10-19 Butter, margarine, raisins
1-9 Walnuts, peanuts (dry roasted), crackers, cereals, pretzels, peanut butter
0% Oils, sugar
This article appeared in print under the headline 鈥淲ater鈥�
