Lately it has been in vogue to dismiss the advice to drink eight glasses of water a day as a 'medical myth.'

Books and medical-journal articles have declared there's no scientific evidence for claims that '8 x 8' -- eight ounces, or 237 milliliters, of water eight times a day -- can bring a wide range of benefits, from speeding weight loss to ridding the body of toxins, fighting constipation, fatigue and dry skin and hastening recovery from colds and the flu. Headlines have jeered that 8 by 8 'doesn't hold water' and 'water advice doesn't wash.'

It's really more a dispute over whether the glass is half-empty or half-full.

Many studies have linked drinking extra water with health benefits, but critics generally dismiss them as statistically insignificant, inconclusive or not widely applicable.

For instance, a 10-year study of nearly 48,000 men published in the New England Journal of Medicine in 1999 found that the risk of bladder cancer fell 7% for every cup subjects drank per day. Other studies have found that the more water subjects drank, the fewer precancerous colon polyps they had. And a study of 20,000 Seventh-day Adventists in California in the American Journal of Epidemiology found that women who drank at least five glasses of water a day had a 41% lower risk of fatal heart disease, and men had a 54% lower risk, compared with those who drank just two glasses a day.

Physiologist Heniz Valtin of Dartmouth Medical School discussed these studies and more in a seminal 2002 article in the American Journal of Physiology -- and still concluded that they didn't support a universal recommendation that everyone drink 8 by 8, just people 'known to have a propensity for the disease(s) in question.' Six years later, Dr. Valtin says, 'I haven't seen a single scientific report that disagrees with my conclusion.'

Similarly, an editorial in the Journal of the American Society of Nephrology in April brushed off small studies suggesting that water increases thermogenesis (calorie burning), reduces migraines and increases blood flow to the skin. The piece repeated Dr. Valtin's conclusion: 'There's no clear evidence of benefit from drinking increased amounts of water.' But, it added, 'we concede there is also no clear evidence of lack of benefits.'

That frustrates researchers in the field. 'There's enough data to suggest that we should go get more data,' says Jodi Stookey, a scientist at Children's Hospital and Research Center in Oakland, Calif. Her studies have found that substituting water for sweetened beverages helped female dieters cut overall calories (whereas diet drinks seem to stimulate more eating) and that women on four popular diets who drank at least one liter of water a day lost more weight than those who didn't, regardless of their diet.

It's impossible to sort out cause and effect from casual associations in such studies, and participants aren't always accurate when asked to remember what they ate or drank. So what would satisfy skeptics as evidence? 'Only large and expensive randomized trials could settle these questions definitely,' the April editorial notes. 'Given that water cannot be patented, such trials seem unlikely.'

Urologists do agree that extra water can reduce the recurrence of kidney stones. But they don't know if water will prevent them in the first place. 'Kidney stones occur in 4% to 5% of the population, so it's not practical to tell everyone to drink that much,' says Stanley Goldfarb, a kidney specialist at the University of Pennsylvania Health System and co-author of the editorial.

He also disputes the notion that drinking extra water improves the function of various organs -- because it doesn't stick around. 'You don't accumulate those eight glasses of water. They're in the toilet,' he says, noting that the body has an exquisite system of self-regulation, excreting anything it doesn't need. And that doesn't mean the body is flushing out extra toxins, either, he says; the same toxins are just diluted in more water.

Experts also agree that people's water needs vary considerably: you'll need more if you're playing in the U.S. Open than if you're sitting at a computer in an air-conditioned office. But most nephrologists, and the National Academy of Sciences, say that thirst alone is a perfectly sufficient guide to how much you need. 'Thirst is one of the most powerful human motivators -- it's way above sex,' says Dr. Goldfarb.

Actually, that's a point of contention, too. Fereydoon Batmanghelidj,an Iranian-born doctor who extolled the benefits of water in two books, called relying on thirst to regulate hydration 'the greatest tragedy in medical history.' He and other water aficionados believe that by the time people feel thirsty, they are already dehydrated. This camp believes that the body needs roughly two liters of water a day to replace what it loses and that other beverages, particularly caffeinated ones, don't count.

A variety of official government recommendations concur with the two-liter total but say that includes the considerable amount of water that is naturally found in solid food. 'Even a slice of white bread is more than 30% water,' says Dr. Valtin.

Still, some diet experts firmly believe that drinking extra water helps people feel fuller and makes the body retain less fluid, even though some concede the benefit may be as much behavioral as metabolic.

'If your usual habit is to have a bowl of popcorn or peanuts on your desk, sipping on a glass of water instead is satisfying and keeps your hands and mouth busy,' says Karen Miller-Kovach, chief scientific officer at Weight Watchers, which recommends six glasses a day.

Some marathoners have died from downing large quantities of water very quickly. The human body can absorb only about one quart of water per hour, max, and after that, the brain can swell dangerously, says Dr. Goldfarb. Another danger is hyponatremia, in which sodium levels fall precipitously -- but that's also rare in healthy people. Drinking eight glasses a day 'is not likely to harm you, but it's very unlikely to help you in any way that the conventional wisdom has been claiming,' says Dr. Goldfarb.

Some boosters, however, say they have all the evidence they need. 'When I'm drinking a lot of water during the day, I feel better,' says Theo Robbins, a Manhattan attorney. 'I'm less inclined to get a headache. I have more energy. I'm able to focus a little better, and those are important things.'

最近，摒棄每天八杯水的所謂“醫學神話”已成為一種時尚。

有關書籍和醫學期刊文章已經宣稱，“8 x 8”飲水原則──即每天八杯水，每次八盎司（237毫升）──能夠為人體健康帶來諸多益處，從加速減肥，排除體內毒素，對抗便秘、疲勞和皮膚干燥，到加快感冒和流感的康復等等，此類說法缺乏科學根據。媒體更戲稱“8 x 8”飲水原則“并非滴水不漏”，而且也“無法除污”。

其實，這種爭論更多的是有關看問題的角度。

許多研究將多飲水與有益健康聯系在一起，但是批評人士通常認為此類研究數據不充分，結論不確定，或者并非廣泛適用。

比如說，1999年發表在《新英格蘭醫學期刊》(New England Journal of Medicine)上的一項針對近4.8萬名男性的10年期研究顯示，人們每天每飲一杯水，罹患膀胱癌的風險就會下降7%。其它研究則發現，飲水越多，出現具有癌癥前期特征的結腸息肉的機率就越小。發表在《美國流行病學期刊》(American Journal of Epidemiology) 上的一項針對加利福尼亞州兩萬名耶穌復臨論者的研究發現，與每天只飲兩杯水的人相比，每天至少飲用五杯水的女性患致命性心臟病的機率要低41%，男性要低54%。

達特茅斯醫學院(Dartmouth Medical School)的生理學家亨氏•瓦爾丁(Heniz Valtin)在2002年發表于《美國生理學期刊》(American Journal of Physiology)上的一篇開創性文章里就上述研究進行了探討，并且得出結論稱，這些研究并不能支持每人每天飲用八杯水的普遍性建議，只是適用于那些已知有可能患有某些疾病的人。六年后，瓦爾丁博士稱，我尚未見到任何科學報告與我得出的結論有悖。

相似地，《美國腎臟學會期刊》(Journal of the American Society of Nephrology)在4月份的一篇社論中駁斥了有關小規模研究所稱“飲水能夠增加生熱作用（燃燒卡路里），減少偏頭痛發作，并且加快皮膚血液循環”的結論。這篇社論重申了瓦爾丁博士的結論：目前沒有明確的證據顯示多飲水有益健康。但是，社論補充稱，我們也承認同樣沒有明確的證據顯示多飲水不會有益健康。

這讓該領域的研究人員感到頗為沮喪。加州奧克蘭兒童醫院和研究中心科學家裘迪•斯圖基(Jodi Stookey)表示，目前已有充分的資料顯示我們應當采集更多的數據。斯圖基的研究發現，以水代替加糖飲料有助于女性減肥者降低整體卡路里攝入（而低卡路里飲料似乎會刺激食欲）。她的研究還發現，采用四種流行減肥食譜、每天至少飲用一升水的女性，其體重下降的速度要快于飲水不足一升的女性，無論后者采用何種減肥食譜。

要想從此類研究得出的偶然相關性當中找出因果關系似乎不大可能，而且參與者在被問及他們的飲食或者飲水問題時，記憶并不總是非常準確。那么，什么樣的研究才能滿足懷疑人士對證據的要求呢？《美國腎臟學會期刊》4月份的社論指出，只有大型的、花費高昂的隨機試驗才能明確地回答這些疑問。鑒于水是無法申請專利的，此類試驗似乎是不可能的。

泌尿科醫師確實認同，多飲水能夠降低腎結石的復發機率。但是，他們并不清楚水是否可以預防腎結石。賓西法尼亞大學醫療體系(University of Pennsylvania Health System)的腎臟專家、同時也是上述社論的作者之一的斯坦利•戈德法布(Stanley Goldfarb)表示，目前人口中有4%至5%患有腎結石，因此建議每人每天都喝八杯水是不現實的。

戈德法布博士還駁斥了有關多飲水能夠改善各種器官功能的說法，原因是器官無法留住水份。他說，你無法將這八杯水留在體內。它們最后還是會被排出去的。戈德法布博士指出，人體有一套精密的自我調節系統，能夠自動排除它不需要的任何東西。他說，但這并不意味著人體能夠排出更多的毒素；同樣的毒素只是在更多的水里被稀釋了。

專家們也認為，人體對水的需求會變化很大：如果你在參加美國網球公開賽，而不是在一間有空調的辦公室里坐在電腦前，你肯定需要更多的水。但是，大多數腎臟學家以及美國國家科學院(National Academy of Sciences)均認為，口渴本身就完全足以指導人體的飲水需要。戈德法布博士稱，口渴是人體最有力的激發因素之一──遠遠超過性。

實際上，這也是一個爭論的焦點。出生于伊朗的Fereydoon Batmanghelidj博士曾經在兩本書里對飲水的益處大加推崇。他說，依賴口渴來調節飲水量是“醫學史上最大的悲劇”。他以及其他飲水愛好者認為，等到人們感到口渴時，他們已經處于脫水狀態了。這個陣營認為，人體每天需要大約兩升水來補充其流失的水份，其它飲料，尤其是咖啡因飲料，都不算在內。

有大量政府官方建議認同兩升水的飲水原則，但這其中包括固體食物本身就有的大量水份。瓦爾丁博士表示，即便是一片白面包，其含水量也超過了30%。

盡管如此，一些減肥專家堅定地認為，多飲水有助于人們產生飽腹感，使人體保留較少的液體，即便有些專家也承認，多飲水帶來的益處可能既有新陳代謝方面的，也有行為意義上的。

減肥專業公司Weight Watchers的首席科學家凱倫•米勒-科維奇(Karen Miller-Kovach)表示，如果你通常的習慣是在桌邊放上一碗爆米花或者花生的話，那么小啜一杯水也能令你感到滿足，而且能使你的手和嘴忙個不停。Weight Watchers推薦每天飲用六杯水。

一些馬拉松運動員曾經因為快速大量飲水而死去。戈德法布博士稱，人體每小時最多只能吸收大約一夸脫水，如果超過這個限度，大腦就會危險地膨脹。另一種危險則是低血鈉癥(hyponatremia)，在這種情況下鈉的水平會急劇下降──不過這在健康人群中也很罕見。戈德法布博士表示，每天飲用八杯水“不大可能會對人體構成傷害，但是它也極不可能給你帶來傳統理念所宣稱的種種好處。”

不過，一些支持者表示，他們擁有他們需要的所有證據。曼哈頓的律師希奧•羅賓斯表示，當我每天大量飲水的時候，我感覺更好。我不容易頭痛了，更有精神了，注意力也更集中了，這些都很重要。