最近，摒弃每天八杯水的所谓“医学神话”已成为一种时尚。
有关书籍和医学期刊文章已经宣称，“8 x 8”饮水原则──即每天八杯水，每次八盎司（237毫升）──能够为人体健康带来诸多益处，从加速减肥，排除体内毒素，对抗便秘、疲劳和皮肤干燥，到加快感冒和流感的康复等等，此类说法缺乏科学根据。媒体更戏称“8 x 8”饮水原则“并非滴水不漏”，而且也“无法除污”。
其实，这种争论更多的是有关看问题的角度。
许多研究将多饮水与有益健康联系在一起，但是批评人士通常认为此类研究数据不充分，结论不确定，或者并非广泛适用。
比如说，1999年发表在《新英格兰医学期刊》(New England Journal of Medicine)上的一项针对近4.8万名男性的10年期研究显示，人们每天每饮一杯水，罹患膀胱癌的风险就会下降7%。其它研究则发现，饮水越多，出现具有癌症前期特征的结肠息肉的机率就越小。发表在《美国流行病学期刊》(American Journal of Epidemiology) 上的一项针对加利福尼亚州两万名耶稣复临论者的研究发现，与每天只饮两杯水的人相比，每天至少饮用五杯水的女性患致命性心脏病的机率要低41%，男性要低54%。
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达特茅斯医学院(Dartmouth Medical School)的生理学家亨氏•瓦尔丁(Heniz Valtin)在2002年发表于《美国生理学期刊》(American Journal of Physiology)上的一篇开创性文章里就上述研究进行了探讨，并且得出结论称，这些研究并不能支持每人每天饮用八杯水的普遍性建议，只是适用于那些已知有可能患有某些疾病的人。六年后，瓦尔丁博士称，我尚未见到任何科学报告与我得出的结论有悖。
相似地，《美国肾脏学会期刊》(Journal of the American Society of Nephrology)在4月份的一篇社论中驳斥了有关小规模研究所称“饮水能够增加生热作用（燃烧卡路里），减少偏头痛发作，并且加快皮肤血液循环”的结论。这篇社论重申了瓦尔丁博士的结论：目前没有明确的证据显示多饮水有益健康。但是，社论补充称，我们也承认同样没有明确的证据显示多饮水不会有益健康。
这让该领域的研究人员感到颇为沮丧。加州奥克兰儿童医院和研究中心科学家裘迪•斯图基(Jodi Stookey)表示，目前已有充分的资料显示我们应当采集更多的数据。斯图基的研究发现，以水代替加糖饮料有助于女性减肥者降低整体卡路里摄入（而低卡路里饮料似乎会刺激食欲）。她的研究还发现，采用四种流行减肥食谱、每天至少饮用一升水的女性，其体重下降的速度要快于饮水不足一升的女性，无论后者采用何种减肥食谱。
要想从此类研究得出的偶然相关性当中找出因果关系似乎不大可能，而且参与者在被问及他们的饮食或者饮水问题时，记忆并不总是非常准确。那么，什么样的研究才能满足怀疑人士对证据的要求呢？《美国肾脏学会期刊》4月份的社论指出，只有大型的、花费高昂的随机试验才能明确地回答这些疑问。鉴于水是无法申请专利的，此类试验似乎是不可能的。
泌尿科医师确实认同，多饮水能够降低肾结石的复发机率。但是，他们并不清楚水是否可以预防肾结石。宾西法尼亚大学医疗体系(University of Pennsylvania Health System)的肾脏专家、同时也是上述社论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%患有肾结石，因此建议每人每天都喝八杯水是不现实的。
戈德法布博士还驳斥了有关多饮水能够改善各种器官功能的说法，原因是器官无法留住水份。他说，你无法将这八杯水留在体内。它们最后还是会被排出去的。戈德法布博士指出，人体有一套精密的自我调节系统，能够自动排除恍枰娜魏味鳌Ｋ担獠⒉灰馕蹲湃颂迥芄慌懦龈嗟亩舅兀煌亩舅刂皇窃诟嗟乃锉幌∈土恕?br>
专家们也认为，人体对水的需求会变化很大：如果你在参加美国网球公开赛，而不是在一间有空调的办公室里坐在电脑前，你肯定需要更多的水。但是，大多数肾脏学家以及美国国家科学院(National Academy of Sciences)均认为，口渴本身就完全足以指导人体的饮水需要。戈德法布博士称，口渴是人体最有力的激发因素之一──远远超过性。
实际上，这也是一个争论的焦点。出生于伊朗的Fereydoon Batmanghelidj博士曾经在两本书里对饮水的益处大加推崇。他说，依赖口渴来调节饮水量是“医学史上最大的悲剧”。他以及其他饮水爱好者认为，等到人们感到口渴时，他们已经处于脱水状态了。这个阵营认为，人体每天需要大约两升水来补充其流失的水份，其它饮料，尤其是咖啡因饮料，都不算在内。
有大量政府官方建议认同两升水的饮水原则，但这其中包括固体食物本身就有的大量水份。瓦尔丁博士表示，即便是一片白面包，其含水量也超过了30%。
尽管如此，一些减肥专家坚定地认为，多饮水有助于人们产生饱腹感，使人体保留较少的液体，即便有些专家也承认，多饮水带来的益处可能既有新陈代谢方面的，也有行为意义上的。
减肥专业公司Weight Watchers的首席科学家凯伦•米勒-科维奇(Karen Miller-Kovach)表示，如果你通常的习惯是在桌边放上一碗爆米花或者花生的话，那么小啜一杯水也能令你感到满足，而且能使你的手和嘴忙个不停。Weight Watchers推荐每天饮用六杯水。
一些马拉松运动员曾经因为快速大量饮水而死去。戈德法布博士称，人体每小时最多只能吸收大约一夸脱水，如果超过这个限度，大脑就会危险地膨胀。另一种危险则是低血钠症(hyponatremia)，在这种情况下钠的水平会急剧下降──不过这在健康人群中也很罕见。戈德法布博士表示，每天饮用八杯水“不大可能会对人体构成伤害，但是它也极不可能给你带来传统理念所宣称的种种好处。”
不过，一些支持者表示，他们拥有他们需要的所有证据。曼哈顿的律师希奥•罗宾斯表示，当我每天大量饮水的时候，我感觉更好。我不容易头痛了，更有精神了，注意力也更集中了，这些都很重要。
Melinda Beck
（编者按：本文作者Melinda Beck是《华尔街日报》健康养生专栏“Health Journal”的专栏作家）
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.'