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How much water should you drink a day?

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.


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.


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.


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.


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


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


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


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.


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.


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.


“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.


“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.”


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.


Drinking to good health


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


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.


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.


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.


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.


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.


Too much of a good thing?


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.


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.


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.


“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.


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.


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.


“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.


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


How much?


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.


“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.”


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.


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


“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.


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


“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.”


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.




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