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我們每天該喝多少水才夠?

How much water should you drink a day?
我們每天該喝多少水才夠?

In the early 19th Century, people had to be close to death before deigning to drink water. Only those “reduced to the last stage of poverty satisfy their thirst with water”, according to Vincent Priessnitz, the founder of hydropathy, otherwise known as “the water cure”.

19世紀初期,人們只有在垂死的時候才會“屈尊”喝水。水療法,顧名思義就是“用水治病”,其創始人普雷斯尼茨(Vincent Priessnitz)表示:“當時,只有窮得叮當響的人才會喝白開水解渴。”

Many people, he added, had never drunk more than half a pint of plain water in one sitting.

他補充說,很少有人一次喝下超過半品脫的白開水。

How times have changed. Adults in the UK today are consuming more water now than in recent years, while in the US, sales of bottled water recently surpassed sales of soda. We’ve been bombarded with messages telling us that drinking litres of water every day is the secret to good health, more energy and great skin, and that it will make us lose weight and avoid cancer.

但是時光飛逝,今時已不同往日。如今,英國成年人的飲水量在近幾年達到頂峰。最近在美國,瓶裝水的銷量超過了碳酸飲料。生活中,我們被各種各樣的信息狂轟濫炸:每天喝幾升水是保持健康活力和擁有美麗肌膚的秘訣,能減肥,使我們遠離癌癥。

Commuters are encouraged to take bottles of water onto the London Underground, pupils are advised to bring water into their lessons and few office meetings can commence without a giant jug of water sitting in the middle of the desk.

英國政府鼓勵上班族攜帶水瓶乘坐倫敦地鐵,小學生們也被鼓勵帶水到課堂飲用,而且幾乎所有公司開會時都會在桌子中間放一大壺水。

Fuelling this appetite for water is the “8x8 rule”: the unofficial advice recommending we drink eight 240ml glasses of water per day, totalling just under two litres, on top of any other drinks.

有一條非官方建議叫“8乘8法則”,建議我們除了任何其他的飲料之外,每天還要喝8杯240毫升的水,總量接近兩升。

That “rule”, however, isn’t backed by scientific findings – nor do UK or EU official guidelines say we should be drinking this much.

然而,這一“法則”并沒有任何科學支持。英國或歐盟的官方指南并沒有建議我們喝這么多水。

Where did it come from? Most likely, it seems, from misinterpretations of two pieces of guidance – both from decades ago.

那么,這種說法到底源自哪里呢?最有可能的原因似乎是人們誤讀了兩項幾十年前出臺的指導方針。

In 1945 the US Food and Nutrition Board of the National Research Council advised adults to consume one millilitre of liquid for every recommended calorie of food, which equates to two litres for women on a 2,000-calorie diet and two-and-a-half for men eating 2,500 calories. Not just water, that included most types of drinks – as well as fruits and vegetables, which can contain up to 98% water.

1945年,美國國家研究委員會(National Research Council)的食品與營養委員會(Food and Nutrition Board)建議,成年人每食用一卡路里食物,就要攝入一毫升液體。女性如果攝入2000卡路里的食物就要喝兩升液體,而攝入2500卡路里的男性則需要飲用2.5升液體。

In 1974, meanwhile, the book Nutrition for Good Health, co-authored by nutritionists Margaret McWilliams and Frederick Stare, recommended that the average adult consumes between six to eight glasses of water a day. But, the authors wrote, this can include fruit and veg, caffeinated and soft drinks, even beer.

1974年, 營養學家麥克威廉姆斯(Margaret McWilliams)和斯塔勒(Frederick Stare)合著的《健康好營養》(Nutrition for Good Health)一書建議,成年人平均每天飲用6至8杯水,但作者認為這應包括水果、蔬菜、咖啡因、飲料甚至啤酒。

In thirst we trust

相信口渴的感覺

Water is, of course, important. Making up around two-thirds of our body weight, water carries nutrients and waste products around our bodies, regulates our temperature, acts as a lubricant and shock absorber in our joints and plays a role in most chemical reactions happening inside us.

水很重要,約占人體體重的三分之二,將營養物質和廢物運送到身體各處,能調節體溫,在關節中充當潤滑劑和減震器,并在人體內大多數化學反應中發揮重要作用。

We’re constantly losing water through sweat, urination and breathing. Ensuring we have enough water is a fine balance, and crucial to avoiding dehydration. The symptoms of dehydration can become detectable when we lose between 1-2% of our body’s water and we continue to deteriorate until we top our fluids back up. In rare cases, such dehydration can be fatal.

我們在流汗、排尿和呼吸時會流失水分。保證身體水分充足,能夠避免脫水。當身體流失了1-2%的水分時,就會出現脫水癥狀,如果不補充水,情況就會持續惡化。在少數情況下,脫水可能致命。

Years of unsubstantiated claims around the 8x8 rule have led us to believe that feeling thirsty means we’re already dangerously dehydrated. But experts largely agree that we don’t need any more fluid than the amount our bodies signal for, when it signals for it.

未經證實的“8乘8法則”讓我們相信,感到口渴意味著我們已經處于危險的脫水狀態。但專家們則認為,當身體發出信號時,我們不需要攝入過多的水,適當即可。

“The control of hydration is some of most sophisticated things we’ve developed in evolution, ever since ancestors crawled out of sea onto land. We have a huge number of sophisticated techniques we use to maintain adequate hydration,” says Irwin Rosenburg, senior scientist at the Neuroscience and Ageing Laboratory at Tufts University in Massachusetts.

“水合作用是人類從海洋進化到陸地后演變出的一項復雜功能。人體有很多復雜的方法可以保持水分充足。”馬薩諸塞州塔夫茨大學(Tufts University)神經科學與衰老實驗室(Neuroscience and Aging Laboratory)資深科學家羅森伯格(Irwin Rosenburg)表示。

In a healthy body, the brain detects when the body is becoming dehydrated and initiates thirst to stimulate drinking. It also releases a hormone which signals to the kidneys to conserve water by concentrating the urine.

健康狀況正常時,大腦會檢測身體什么時候開始脫水,并通過口渴的感覺來提醒人們喝水,同時還會釋放一種激素,告訴腎臟儲蓄尿液來保持水分。

“If you listen to your body, it’ll tell you when it's thirsty,” says Courtney Kipps, consultant sports physician and principal clinical teaching fellow of Sports Medicine, Exercise and Health and UCL, and medical director of Blenheim and London Triathlons.

“如果你仔細聆聽,身體會告訴你它什么時候渴了。”運動醫學顧問醫師基普斯(Courtney Kipps)說。他是倫敦大學學院運動醫學、運動與健康臨床教學主任、布倫海姆和倫敦鐵人三項運動(Blenheim and London Triathlons)醫學主任。

“The myth that it’s too late when you’re thirsty is based on the supposition that thirst is an imperfect marker of a fluid deficit, but why should everything else in the body be perfect and thirst be imperfect? It’s worked very well for thousands of years of human evolution.”

“當你感到口渴時,一切都太遲了,此論斷基于這樣一種假設,即表明體液不足,口渴并非一個良好指標。為什么人體中的其他一切指標都是準確的,而只有口渴有問題呢?在人類幾千年的進化過程中,口渴機制一直運行良好。”

While water is the healthiest option since it has no calories, other drinks also hydrate us, including tea and coffee. Although caffeine has a mild diuretic effect, research indicates that tea and coffee still contribute to hydration – and so do alcoholic drinks.

水不含卡路里,是最健康的選擇,但其他飲料也能為我們的身體補充水分,包括茶和咖啡。雖然咖啡因有輕微的利尿作用,但研究表明,茶和咖啡仍然有助于水合作用,酒精飲料也是如此。

Drinking to good health

喝出健康

There’s little evidence suggesting that drinking more water than our body signals for offers any benefits beyond the point of avoiding dehydration.

除了避免脫水,幾乎沒有證據表明超過身體所需喝水還有其他好處。

Still, research suggests there are some important benefits to avoiding even the early stages of mild dehydration. A number of studies have found, for example, that drinking enough to avoid mild dehydration helps support brain function and our ability to do simple tasks, such as problem-solving.

不過,研究表明,在輕度脫水的早期階段就開始喝水確實有作用。例如,許多研究發現,通過喝水避免輕度脫水有助于增強大腦功能,提高完成簡單任務(比如解決問題)的能力。

Some studies suggest fluid consumption can help manage weight. Brenda Davy, a professor of human nutrition, food and exercise at Virginia Polytechnic Institute and State University, has carried out a few studies looking at fluid consumption and weight.

一些研究表明,攝入液體有助于控制體重。弗吉尼亞理工學院(Virginia Polytechnic Institute)和弗吉尼亞州立大學(State University)的人類營養、食品和運動教授戴維(Brenda Davy)進行了有關液體消耗和體重的研究。

In one study, she randomly assigned subjects to one of two groups. Both groups were asked to follow a healthy diet for three months, but only one was told to drink a 500ml glass of water half an hour before eating each meal. The group who drank the water lost more weight than the other group.

在一項研究中,她將受試者隨機分為兩組。兩組都被要求遵循健康飲食三個月,但只有一組需要在飯前半小時喝500毫升的水。結果顯示,喝水的那組比另一組減重更多。

Both groups were also told to aim for 10,000 steps a day, and those who drank the glasses of water better adhered to this. Davy guesses this is because mild dehydration of around 1-2% is quite common, and many people may not realise when this happens – and even this mild level can affect our mood and energy levels.

兩組受試者每天都要走一萬步,但喝水的一組堅持得更久。戴維猜測,這是因為1-2%左右的輕度脫水很常見,很多人可能甚至都沒有意識到,但即使是這種輕度脫水也會影響我們的情緒和能量水平。

But Barbara Rolls, a professor of intensive care medicine at University College London, says that any weight loss associated with drinking water is more likely to come from water being used as a substitute for sugary drinks.

但是倫敦大學學院重癥監護醫學教授羅爾斯(Barbara Rolls)說,喝水能減肥可能是因為水替代了含糖飲料。

“The notion that filling up on water before a meal will melt the pounds away is not well established, and water consumed on its own empties out of the stomach really quickly. But if you consume more water through the food you eat, such as soup, this can help fill you up as the water is bound to the food and stays in the stomach for longer,” she says.

“飯前喝水能減肥的觀點并不對,因為你喝下去的水很快就會從胃里排出。但是,如果你通過食物攝取更多的水,比如說湯,就會產生飽腹感,因為水與食物結合在一起,在胃里停留的時間更長。”她說。

Another alleged health benefit of drinking more water is improved skin complexion and better moisturised skin. But there is a lack of evidence to suggest a credible scientific mechanism behind this.

據說多喝水對健康的另一個好處是能改善膚色和保濕肌膚,但這一說法缺乏科學證據。

Too much of a good thing?

物極必反

Those of us aiming for eight glasses of water per day aren’t doing ourselves any harm. But the belief we need to drink more water than our bodies signal for can sometimes become dangerous.

每天喝8杯水并不會危害健康。但是,有時候“覺得自己一定要喝超過身體所需的水”這種想法會帶來危害。

Too much fluid consumption can become serious when it causes a dilution of sodium in blood. This creates a swelling of the brain and lungs, as fluid shifts to try to balance out blood sodium levels.

攝入過多液體會稀釋血液中的鈉,可能會導致腦部和肺的腫脹,這時情況就危險了。因為體液為了平衡血液中的鈉含量會自行轉移。

Over the last decade or so, Kipps has been aware of at least 15 cases of athletes who’ve died from over-hydration during sporting events. She suspects these cases are partly because we’ve become distrustful of our own thirst mechanism and that we think we need to drink more than our bodies are calling for to avoid dehydration.

在過去10年左右的時間里,基普斯發現有至少15名運動員在體育賽事期間死于過度飲水。她認為部分原因是我們不信任口渴反應,認為需要喝比身體所需更多的水,以避免脫水。

“Nurses and doctors in hospitals will see severely dehydrated patients who have serious medical conditions or who haven’t been able to drink for days, but these cases are very different from the dehydration that people worry about during marathons,” she says.

她說:“醫院的護士和醫生會碰到情況十分嚴重的重度脫水病人,或者已經好幾天無法喝水了,但這些情況和人們在馬拉松比賽中擔心的脫水非常不同。”

Johanna Pakenham ran the 2018 London Marathon, the hottest on record. But she can’t remember most of it because she drank so much water during the race that she developed over-hydration, known as hyponatremia. She was rushed to hospital later that day.

帕克納姆(Johanna Pakenham)參加了2018年的倫敦馬拉松,這是有記錄以來天氣最熱的一次,但她已不記得當時的情況,因為在比賽中喝了太多的水,帕克納姆出現了水合過度,又被稱為“低鈉血癥”。當天晚些時候,她被緊急送往醫院。

“My friend and partner thought I was dehydrated and they gave me a big glass of water. I had a massive fit and my heart stopped. I was airlifted to hospital and unconscious from the Sunday evening until the following Tuesday,” she says.

“我的朋友和伴侶以為我脫水了,就給我喝了一大杯水。隨后,我開始全身痙攣,心臟停止跳動。后來,我被救護直升機送到醫院,從周日晚上一直昏迷到下周二,”她說。

Pakenham, who plans to run the marathon again this year, says the only health advice offered by friends and marathon posters was to drink lots of water.

帕克納姆今年還打算參加馬拉松比賽,她說朋友們和馬拉松海報提供的唯一健康建議就是多喝水。

“All it would’ve taken for me to be okay was having a few electrolyte tablets, which increase the sodium levels in your blood. I’ve ran a few marathons before and I didn’t know that,” she says.

“我只要吃幾片電解質片就好了,能提高血液中的鈉含量。我以前跑過幾次馬拉松,但我當時不知道。”她說。

“I really want people to know that something so simple can be so deadly.”

“我真的很想告訴人們,如此簡單的事情也可能致命。”

How much?

喝多少為好?

The idea that we must be constantly hydrated means many people carry water with them wherever they go, and drink more than their bodies require.

“我們必須經常補水”這種想法意味著很多人無論走到哪里都要隨身攜帶水,并且比身體所需喝的多。

“The maximum a person in the hottest possible heat in the middle of the desert might sweat is two litres in an hour, but that’s really hard,” says Hugh Montgomery, director of research at the Institute for Sport, Exercise and Health in London.

倫敦運動、鍛煉與健康研究所(Institute for Sport, Exercise and Health)的研究主管蒙哥馬利(Hugh Montgomery)表示:“一個人在沙漠高溫環境下每小時排汗量為2升,但這種情況真的很難碰到。”

“The idea of carrying around 500ml of water for a 20-minute journey on the London Underground – you’re never going to get hot enough to sweat at that rate, even if you’re dripping with sweat.”

“你帶500毫升的水坐20分鐘倫敦地鐵,怎么樣也不會像在沙漠里那樣出汗。”

For those who feel more comfortable going off official guidelines rather than thirst, the UK’s NHS advises drinking between six to eight glasses of fluid a day, including lower fat milk and sugar-free drinks, including tea and coffee.

英國國民保健系統(NHS)建議,對于那些不按官方指引喝水也不會覺得口渴的人,每天喝6到8杯液體即可,包括低脂牛奶、無糖飲料、茶和咖啡。

It’s also important to remember that our thirst mechanisms lose sensitivity once we’re over 60.

還要記住一點,人到了60歲,口渴機制就會失去敏感度。

“As we age, our natural thirst mechanism becomes less sensitive and we become more prone to dehydration than younger people. As we age, we may need to be more attentive to our fluid consumption habits to stay hydrated,” says Davy.

“隨著年齡增長,我們天生的口渴機制會變得遲鈍,比年輕人更容易脫水。所以,人老了后需要更注意自己的飲水習慣,以保持水分,”戴維說。

Most experts agree that our fluid requirements vary depending on a person’s age, body size, gender, environment and level of physical activity.

大多數專家都認為,我們對液體的需求取決于年齡、體型、性別、環境和運動水平。

“One of fallacies of the 8x8 rule is its stark over-simplification of how we as organisms respond to the environment we’re in,” says Rosenburg. “We ought to think of fluid requirement in the same way as energy requirement, where we talk about the temperature we’re in and level of physical activity were engaged in.”

羅森伯格說:“8乘8法則的一個錯誤之處在于,它把人類作為有機體對所處環境的反應完全簡單化了。我們應該用能量需求的方式來計算應攝入的液體量,包括我們所處的環境溫度和體能活動的水平。”

Most experts tend to agree we don’t need to be concerned about drinking an arbitrary amount of water per day: our bodies signal to us when we’re thirsty, much like they do when we’re hungry or tired. The only health benefit of drinking more than you need, it seems, will be the extra calories you expend by running to the loo more often.

大多數專家認為,我們其實不需要擔心每天喝多少水。當我們口渴時,身體會發出信號,就像肚子餓了或身體累了一樣。喝過量水對健康的唯一好處可就是跑廁所消耗的額外熱量。

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