How much water should we drink a day ?
Whether you’ve had fatigue or even dry skin, you’ve probably been told to drink more water as a cure. But this advice comes from decades-old guidance… and may have no scientific basis.
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”.
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.
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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.
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.
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, even 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.
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 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.
“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.”
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. (Source BBC)
