The hydration myth: Why you don't need to drink eight glasses of water a day

Do you really need to drink a gallon of water a day? Photo / Getty Images

Archie Harrison Mountbatten-Windsor knows all about hydration.

Last year, Prince Harry told Oprah Winfrey that the toddler's favourite word was "hydrate". Yes, Archie! Not quite knee-height, and he already knows that hitting the H2O is not just a medical imperative; it's hip, too.

Hydration, it seems, is the new religion, evangelised about by everyone from the NHS (which recommends we all drink six to eight glasses a day: about three and a half pints, or two litres) to reality star Khloe Kardashian (whose "water journey" involves a gallon-sized bottle - that's eight pints, or 4.5 litres - with motivational messages in place of the measurement marks, from "remember your goal" through "don't give up" to "you did it" when she's chugged the lot).

And in summertime, the cult of hydration reaches fever pitch. As I type, mothers across the Britain are screaming "Wait! You forgot your water bottle!" as their children lope off to school. I am no exception. At home, I badger my kids to drain their glasses. At work, I place a carafe of water on my desk like a personal challenge.

Water is the stuff of life and dehydration is, unequivocally, dangerous. But what are the proven benefits of hitting Kardashian's gallon target, or even the NHS's more moderate eight-glass mark? In 2002, The American Journal of Physiology published a study of the eight-glass rule. Its author - Heinz Valtin, an expert in renal physiology - concluded: "There is no scientific evidence that we need to drink that much."

"I'm not telling people not to drink more water - I don't want to yuck anyone's yum - but to spend any time or money worrying about it is a waste," says Aaron Carroll, a distinguished professor of paediatrics at Indiana University School of Medicine and co-author of Don't Swallow Your Gum! Myths, Half-Truths, and Outright Lies About Your Body and Health.

"This idea you have to consciously force yourself to consume a certain amount of pure, unadulterated water on top of your normal diet, well ... there's no rational or scientific basis for that at all."

The myth, he suggests, may have its roots in a recommendation made by the Food and Nutrition Board of the United States National Research Council, back in 1945. "A suitable allowance of water for adults is 2.5 litres daily in most instances," they wrote. But while we may have clung obsessively to that line, we have forgotten the part that came next: "Most of this quantity is in prepared foods."

The water in your food counts towards your daily hydration. A 2016 randomised controlled trial of 72 men even concluded that the hydrating effects of water, lager, coffee and tea were nearly identical. In the developed world, it's hard to avoid water. Elsewhere, there are people living in genuine hardship, yet surviving, he points out. Meanwhile: "The vast majority of people who are obsessing about this are living through the greatest abundance in human history. Yet we are more worried than ever that our bodies are going to collapse."

Clinical dehydration, he points out diplomatically, is unlikely to result from a few hours idling in a British park. "When you're dehydrated, it usually means you've lost between 5 and 10 per cent of your body weight in water. That's an enormous amount of water. We're not talking, 'Oh dear, I've forgotten to drink a glass of water since breakfast.' You will know about it. You'll be in the hospital ... And you have to get that way through serious deprivation or really bad vomiting and diarrhoea."

Okay, so my children and I are unlikely to shrivel up if we venture out for a few hours without our water bottles. But what about the less life-or-death claims made for drinking lots of water - the ones that are less about self-preservation and more about self-optimisation? Is there scientific evidence that drinking lots more water will clear my mind and my skin, for example?

Carroll shakes his head: "I liken it to a tank of gas in a car. If you run out of gas, you're in trouble. But the car operates just fine within a certain range. And once that tank is full, if you add more gas, it's just spilling out the sides. The car doesn't work any better. It can't go any further or faster, it's just spilling out. That's your body. All you're doing is creating more trips to the bathroom."

"It's amazing the NHS still perpetuates these myths," agrees Dr Margaret McCartney. The Scottish GP, writer and broadcaster is particularly, well ... peed off, with the oft-touted idea that urine colour is an accurate indicator of adequate hydration. "NHS organisations have pushed this through colour charts, suggesting that unless you've got really clear urine you're somehow dehydrated," she says. "It's absolute rubbish. In healthy people, yellow urine is actually a sign that your kidneys are working appropriately to preserve your fluids. If your urine's very clear, it means you're drinking fluids your body doesn't need."

All bodies are different, requiring different amounts of water in different contexts and environmental conditions, says McCartney - and we should trust them. "We've got this amazing physiology that will tell us when we need to drink, in order to keep our fluids exquisitely well-balanced. It's a really trustworthy, well-attuned system. If you're fit and healthy, have free access to fluids and you're able to drink to thirst and comfort, then there shouldn't be any need for you to be measuring your fluid intake at all."

If the NHS does not always articulate this fully, celebrity culture can actively obscure it. Last year, the actress Katie Holmes shared the secret to her glowing complexion: "I drink a lot of water and try to eat a lot of vegetables to keep my skin as healthy as possible." She was not the first celebrity to suggest that drinking water (not a genetic lottery, fabulous wealth and a huge amount of effort) was behind her youthful glow. Her comment was notable, however, because she was acting as a spokesman for a "healthy hydration" campaign sponsored by ... Evian.

Kids will drink when they're thirsty, experts say - just make sure it's water rather than sugary drinks they're reaching for. Photo / Getty Images

After the bottled water companies came the wellness industry. Now we can "optimise" our hydration with filters, "activated" or "enhanced" waters, or even bottles containing crystals that will "charge" our water with healing vibes (Victoria Beckham posted hers on Instagram last month). A basic necessity - freely available for anyone with access to a tap or working well - has been turned into a mystical (and, more importantly, lucrative) self-improvement tool."

What makes me really sad is there's lots of influencers who want you to drink more, with no science behind it," says McCartney. "Making people worry and not trust their body instead of cherishing it and trusting it."

Dr Heather Sequeira, a psychologist specialising in obsessive compulsive disorder and trauma, agrees. "I have noticed in my clinical practice that some people are feeling less able to rely on their own internal cues and feeling increasingly anxious about their own bodies and their own body signals," she says. "They're turning to Google for advice rather than tuning in to their own bodies ... trusting influencers rather than their own bodies."

Drinking water is a good example of this. "Some very health conscious people are feeling anxious and 'unsafe' unless they are carrying around a water bottle from which they are constantly sipping."It is habit that can slide from "health consciousness" to "health anxiety", says Sequeira. "We might associate sipping water with 'doing the right thing' and therefore being 'safe', but the end result is actually that we feel increasingly anxious, obsessively monitoring what we have drunk. We are increasingly focusing in on the amount and, more worryingly, focusing on the 'micro-sensations' in our throat and mouth, rather than tuning in to appropriate thirst signals that our body is sending up."

Still, there are some groups who might benefit from giving their water intake a little extra thought. "Older people - say, from 65 onwards - typically drink less than younger age groups and as a consequence are more likely to become dehydrated," says James Goodwin, director of science and research impact at the Brain Health Network, and author of Supercharge Your Brain.

Goodwin, former chief scientist at the charity Age UK, says that as we age "our physiological mechanisms of body water regulation become blunted". For example: "Secretion of the anti-diuretic hormone that helps our bodies to conserve water declines progressively, meaning that more water is excreted from the body, leading to dehydration if it is not replaced."

Meanwhile, "increased water release from the kidneys into the bladder triggers a more frequent urge to urinate". Add potential mobility problems to the mix and, Goodwin says, you have a situation that "disincentivises older people from drinking adequately, with consequential dehydration and general and brain health effects".

So older people might do well to be a little more diligent about their water consumption. How about children? I put the question to Aaron Carroll who, as a paediatrician, is well placed to deal with my parental neuroses.

My worry, I say, is that my eight-year-old might get too engrossed in play to listen to her body's cues and drink when thirsty. "Your kids are not more distracted than squirrels or dogs, or any animal on Earth," he says. He hasn't met my children, but I take his point. "I promise you: eventually they'll drink."

Water bottles have their place, though. "If not presented with the option of water, kids tend to gravitate towards beverages with sugar," says Carroll. "If you're giving them a water bottle so that a healthier option is always available to them, then fantastic. If you're giving it to them because you think they're likely to get dehydrated without it, it's not going to happen. They will tell everyone they're thirsty long before a problem arises."

Water is vital to our wellness, just not in the ways or quantities that the wellness industry so often suggests. To twist a line from Coleridge, there's water, water, everywhere - but you don't have to drink every drop.