BBC – Future – How much water should you drink daily?

At the beginning of the 19th century, people had to be close to death before deigning to drink water. According to Vincent Priessnitz, founder of hydropathy, in other words "the cure for water", only those "reduced to the last stage of poverty quench their thirst with water".

Many people, he added, have never drunk more than half a liter of pure water in one sitting.

How Times Have Changed In the UK, adults today consume more water than in recent years, while in the United States, sales in the United States have been declining. bottled water have recently surpassed soda sales. We have been bombarded with messages telling us that drinking liters of water every day is the secret to good health, energy and beautiful skin, and that it will make us lose weight and avoid cancer. .

Commuters are encouraged to bring bottled water to the London Underground, students are advised to bring water to their yards and few office meetings can begin without a giant pitcher filled with water in the middle. of the desk.

The rule of "8×8" feeds this appetite for water: the unofficial board recommends that we drink eight glasses of water of 240 ml per day, a little less than two liters, in addition to any other drink .

This "rule", however, is not supported by scientific discoveries – neither the official UK or EU guidelines say we should drink as much.

Where does it come from? Most likely, it seems, as a result of a misinterpretation of two guiding elements – both dating back several decades.

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In 1945, the US Food and Nutrition Board of the National Research Council advised adults to consume one milliliter of fluid per recommended calorie of food, which equates to two liters for women on a 2000 calorie diet and two and a half times for men eating 2500 calories. Not only water, which included most drinks, as well as fruits and vegetables, which can contain up to 98% water.

In 1974, meanwhile, the book Nutrition for Good Health, written by nutritionists Margaret McWilliams and Frederick Stare, recommended the average adult to consume six to eight glasses of water a day. But, the authors write, this can include fruits and vegetables, caffeine and soft drinks, or even beer.

In thirst we have confidence

The water is of course important. Representing about two-thirds of our body weight, water transports nutrients and waste around our body, regulates our temperature, acts as a lubricant and energy absorber in our joints and plays a role in most of the chemical reactions that occur inside of us.

We are constantly losing water through perspiration, urination and breathing. Ensuring enough water is a good balance and is essential to avoid dehydration. The symptoms of dehydration can become detectable when we lose between 1 and 2% of our body water and we continue to deteriorate until we find liquid reserves. In rare cases, such dehydration can be fatal.

Experts widely agree that we do not need more liquid than the amount reported by our body,

Years of unfounded claims around the 8×8 rule have led us to believe that feeling thirsty means we are already dangerously dehydrated. But experts generally agree that we do not need more fluid than the amount reported by our body.

"Hydration control is one of the most sophisticated things we have developed during evolution, even since our ancestors crawled out of the sea to land. We use a lot of sophisticated techniques to maintain proper hydration, "said Irwin Rosenburg, senior scientist at the Neuroscience and Aging Laboratory at Tufts University in Massachusetts.

In a healthy body, the brain senses the dehydration of the body and triggers the thirst to stimulate alcohol consumption. It also releases a hormone that signals the kidneys to retain water by concentrating the urine.

"If you listen to your body, it will tell you when you're thirsty," says Courtney Kipps, sports physician and leading clinical researcher in Sports Medicine, Exercise, and Health and Medicine. UCL, and medical director of Blenheim and London Triathlons.

"The myth that it is too late when one is thirsty is based on the assumption that thirst is an imperfect marker of a fluid deficit, but why everything else in the body should be perfect and the thirst to be imperfect "It worked very well for thousands of years of human evolution."

Although water is the healthiest option as it does not contain calories, other drinks also hydrate us, including tea and coffee. Although caffeine has a slight diuretic effect, research indicates that tea and coffee still contribute to hydration, as do alcoholic beverages.

Drink healthy

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

Nevertheless, research suggests that there are significant benefits to avoiding even the early stages of mild dehydration. A number of studies have shown, for example, that drinking enough to prevent mild dehydration helps us support brain function and our ability to perform simple tasks, such as problem solving.

Some studies suggest that fluid intake can help manage weight. Brenda Davy, a professor of human nutrition, diet and exercise at the Virginia Polytechnic Institute and the State University, has conducted some studies on fluid intake and weight.

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

The two groups were also asked to aim for 10,000 steps a day, and those who drank glasses of water adhered better. Davy assumes that this is due to the fact that a slight dehydration of about 1 to 2% is quite common and that many people do not realize that this is happening – and even that moderate level can affect our mood and our level of energy.

However, Barbara Rolls, professor of intensive care medicine at University College London, says that any weight loss associated with drinking water probably comes from the water used as a substitute for sugary drinks.

"The idea that refueling water before a meal will melt the pounds is not well established and that water consumed alone gets emptied very quickly from the stomach." . But if you consume more water in the foods you eat, like soup, this can help you satiate yourself because the water is tied to food and stays longer in the stomach. , she says.

Another presumed health benefit of drinking more water is improving skin tone and hydrated skin. But there is a lack of evidence to suggest a credible scientific mechanism behind this. (Learn more about drinking more water is good for the skin).

Too much of a good thing?

Those of us who aim eight glasses of water a day do not hurt us. But the belief that we need to drink more water than what our bodies report can sometimes lead to dangerous waters.

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

In the past decade, at least 15 athletes have died of excess hydration at sporting events.

Over the past decade or so, Kipps has been aware of at least 15 cases of athletes dead from excessive hydration at sporting events. She suspects these cases of being partly because we have become suspicious of our own mechanism of thirst and think that we need to drink more than our body demands to avoid dehydration.

"Nurses and doctors in hospitals will see severely dehydrated patients who suffer from serious health problems or who can not drink for days, but these cases are very different from the dehydration that worries people during marathons," says -she.

Johanna Pakenham ran the 2018 London Marathon, the hottest ever recorded. But she does not remember anymore because she drank so much during the race that she developed excessive hydration called hyponatremia. She was taken to the hospital later in the day.

"My friend and partner thought I was dehydrated and they gave me a big glass of water. I had a heart attack and my heart stopped. I was flown to the hospital and lost consciousness from Sunday night to the following Tuesday, "she says.

Pakenham, who plans to run the marathon again this year, said the only health tip offered by the friends and the marathon posters was to drink a lot of water.

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

"All that would have been necessary for everything to go well is to take a few electrolyte tablets, which increase the level of sodium in the blood. I ran some marathons before and I did not know it, "she says.

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

How many?

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

"The maximum sweat in the desert can sweat is two liters per hour, but it's really difficult," says Hugh Montgomery, director of research at the Institute for Sport, Exercise and Health in London.

By carrying about 500 ml of water for a 20-minute ride on the London Underground, you're never going to get hot enough to sweat at this rate – Hugh Montgomery

"The idea of ​​carrying around 500 ml of water for a 20-minute ride on the London Underground – you'll never heat enough for you to sweat at that speed, even if you're sweating."

The UK NHS recommends that people who feel more comfortable deviating from official recommendations than thirst, drink between six and eight glasses of fluid a day, including less fat milk and no sugar drinks, including tea and coffee.

It is also important to remember that our thirst mechanisms lose in sensitivity once we are over 60 years old.

"As we age, our natural mechanism of thirst becomes less sensitive and we are more prone to dehydration than younger people. As we grow older, we may need to be more aware of our fluid habits to stay hydrated, "says Davy.

Most experts agree that our fluid requirements vary according to age, size, gender, environment and level of physical activity. 39, a person.

"One of the mistakes of the 8×8 rule is its over-simplification of how we, as organizations, react to the environment we are in," Rosenburg says. "We should think of fluid needs in the same way as energy needs, where we talk about the temperature in which we are and the level of physical activity practiced."

Most experts tend to think that we should not worry about drinking an arbitrary amount of water a day: our bodies signal us when we are thirsty, just as they do when we are hungry or tired. The only health benefit of drinking more than what you need, it seems, will be the extra calories you spend running in the bathroom more often.

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