BBC – Future – How much water should you drink per day?


By the beginning of the 19th century, people had to be near death before they deigned to drink water. Only those "reduced to the last stage of poverty satisfy their thirst for water," according to Vincent Priessnitz, the founder of hydropathy, also known as "water healing."

Many people, he added, had never drunk more than half a liter of pure water at one go.

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

Passengers are encouraged to bring bottled water to the London Underground, students are advised to bring water to classes and some office meetings can begin without a giant jug of water in the middle of the table.

Feeding this appetite for water is the "8×8 rule": the unofficial advice recommending that we drink eight glasses of 240ml of water a day, totaling just under two liters, on top of any other drinks.

This "rule", however, is not backed by scientific breakthroughs – nor the official UK or EU guidelines say that we should be drinking as much.

Where did you come from? Most likely, from misinterpretations of two orientations – both from decades ago.

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In 1945, the American Council on Food and Nutrition of the National Research Council advised adults to consume one milliliter of liquid for each recommended calorie of food, which equals two liters for women on a 2,000 calorie diet and two and a half years for men eating 2,500 calories. Not just water, which included most types of drinks – as well as fruits and vegetables, which can contain up to 98% water.

In 1974 meanwhile, Nutrition for Good Health, co-authored with nutritionists Margaret McWilliams and Frederick Stare, recommended that the average adult consume six to eight glasses of water per day. But, the authors wrote, this may include fruits and vegetables, caffeinated and soft drinks, even beer.

At headquarters we trust

Water is obviously important. With about two thirds of our body weight, water carries nutrients and waste around the body, regulates temperature, acts as a lubricant and cushions our joints and plays a role in most of the chemical reactions that occur within us.

We are constantly losing water through sweat, urination and breathing. Ensuring that we have enough water is a good balance and crucial to avoiding dehydration. The symptoms of dehydration can become detectable when we lose 1 to 2% of our body's water and continue to deteriorate until our fluids return. In rare cases, this dehydration can be fatal.

Experts largely agree that we do not need more fluid than the amount our bodies signal when it signals

Years of unfounded claims surrounding the 8×8 rule have led us to believe that thirst means we are already dangerously dehydrated. But experts largely agree that we do not need more fluids than the amount our bodies signal when they signal it.

"The control of hydration is one of the most sophisticated things we have developed in evolution, even since the ancestors left the sea to land. We have a number of sophisticated techniques that we use to maintain proper hydration, "says Irwin Rosenburg, senior scientist at the Neuroscience and Aging Laboratory at Tufts University in Massachusetts.

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

"If you listen to your body, it will tell you when you are thirsty," says Courtney Kipps, a consultant in sports medicine and a leading specialist in clinical teaching of Sports Medicine, Exercise and Health and UCL and medical director of Blenheim and London Triathlons.

"The myth that it's too late when you're thirsty is based on the assumption that thirst is an imperfect marker of a water deficit, but why should the rest of the body be perfect and thirst imperfect? It has worked very well for thousands of years of human evolution. "

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

Drinking for good health

There is little evidence to suggest that drinking more water than our body signals offers benefits beyond the point of avoiding dehydration.

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

Some studies suggest that fluid consumption can help manage weight. Brenda Davy, professor of human nutrition, food and exercise at Virginia Polytechnic Institute and State University, conducted some studies that analyzed the consumption and weight of fluids.

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 advised to drink a glass of 500ml water before eating each meal. The group that drank water lost more weight than the other group.

Both groups were also instructed to look for 10,000 steps a day, and those who drank the most water glasses joined. Davy guesses that because dehydration is around 1 to 2% is quite common, and many people may not realize when it happens – and even this light level can affect our mood and energy levels.

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

How is dehydration?

Dehydration means you are losing more fluids than you get. According to the NHS, the symptoms of dehydration include dark yellow urine; feeling tired, dizzy or dizzy; have dry mouth, lips and eyes; and urinating less than four times a day. But the most common symptom? Just feeling thirsty.

"The notion that filling the water before a meal melts the pounds is not well established, and the water consumed by itself is rapidly draining out of the stomach. But if you consume more water through the food you eat, such as soup, this can help fill you up because water is attached to food and stays in the stomach for longer, "she says.

Another claimed health benefit of drinking more water is improving the appearance of the skin and better hydration of the skin. But there is a lack of evidence to suggest a reliable scientific mechanism behind it. (Read more about drinking extra water is good for your skin).

Too much of a good thing?

Those of us who aim for eight glasses of water a day are doing no harm to ourselves. But the belief that we need to drink more water than our bodies indicate can lead to dangerous water.

Excessive fluid intake can become severe when it causes a dilution of sodium in your blood. This creates a swelling in the brain and lungs as the fluid moves to try to balance blood sodium levels.

Over the last decade, at least 15 athletes have died of excessive hydration during sporting events

Over the last decade, Kipps learned of at least 15 cases of athletes who died of excessive hydration during sporting events. She suspects these cases are partly because we become distrustful of our own thirst mechanism and we think we need to drink more than our bodies are asking to avoid dehydration.

"Nurses and hospital doctors will see severely dehydrated patients who have severe medical conditions or who could not 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 ever recorded. But she does not remember anymore because she drank so much water during the race that developed excessive hydration, known as hyponatremia. She was taken to the hospital later that day.

"My friend and partner thought I was dehydrated and they gave me a big glass of water. I had a huge attack and my heart stopped. I was taken by helicopter to the hospital and unaware of Sunday night until the following Tuesday, "she says.

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

I really want people to know that something so simple can be so deadly – Johanna Pakenham

"All I took to get well was to have some electrolytes, which raise blood sodium levels. I've done some marathons before and I did not know that, "she says.

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

How many?

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

"The most that a person in the hottest possible heat in the middle of the desert can sweat is two liters at a time, but that's really hard," says Hugh Montgomery, research director at the Institute for Sport, Exercise and Health in London .

Carrying about 500ml of water for a 20-minute trip on the London Underground, you'll never get warm enough to sweat at that rate – Hugh Montgomery

"The idea of ​​taking about 500 ml of water for a 20-minute trip on the London Underground – you'll never get warm enough to sweat at this rate, even if you're sweating."

For those who feel more comfortable getting out of the official guidelines than from headquarters, the NHS UK recommends drinking six to eight glasses of liquid per day, including low-fat milk and unsweetened drinks including tea and coffee.

It is also important to remember that our thirst mechanisms lose sensitivity when we reach age 60.

"As we get older, our natural thirst mechanism becomes less sensitive and we become more prone to dehydration than the younger ones. As we get older, we may need to be more aware of our fluid consumption habits to stay hydrated, "says Davy.

Most experts agree that our fluid needs vary based on a person's age, body size, sex, environment, and physical activity level.

"One of the fallacies of the 8×8 rule is its great simplification of how we, as organisms, respond to the environment in which we are inserted," says Rosenburg. "We should think of the fluid requirement in the same way as the energy requirement, where we talk about the temperature we are in and the level of physical activity in which we are engaged."

Most experts tend to agree that we do not have to worry about drinking an arbitrary amount of water each day: our bodies signal us when we are thirsty, much like when we are hungry or tired. The only health benefit of drinking more than you need is, it seems, the extra calories you spend running to the bathroom more often.

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