In September 2013, the First Lady of the United States, Michelle Obama, lent her name to a major health campaign. That campaign was a drive to make Americans drink more water. ‘I’ve come to realise that, if we were going to take just one step to make ourselves and our families healthier, probably the single best thing we could do is to simply drink more water,’ she said.
There is a longstanding belief that many of us are ‘chronically dehydrated’. We don’t drink enough water, and the resulting dehydration, of which we are largely unaware, causes a variety of ills, including cognitive difficulties and poor memory. Several news agencies, including CBS, reported in 2013 that up to 75 per cent of Americans suffered from this problem.
The answer is, apparently, that we should all drink more water. Mrs Obama suggests that we (well, Americans, but I’m sure she’d say Brits too) drink one more glass of water a day; CBS and others say, much more specifically, that ‘experts’ tell us to drink eight glasses of water a day. This claim has been doing the rounds for ages; it has almost achieved the status of received wisdom. Drinking eight glasses of water a day is good for you.
But where did that come from? It’s surprisingly hard to find out. Certainly, the claim that three-quarters of Americans are under-watered seems to be false. The admirable mythbusting website Snopes has looked into it, and the only place they can find it is in a 1992 book called Your Body’s Many Cries for Water, by Fereydoon Batmanghelidj. He says that people ‘need to learn they’re not sick, only thirsty’, and that drinking more water would cure or prevent ‘arthritis, angina, migraines, hypertension and asthma’. It appears to be based on no empirical research.
The ‘eight glasses’ (if you’re wondering, each glass is supposed to be eight fluid ounces, or a bit less than half a pint, so the advice is sometimes known as ‘8 x 8’) claim is hard to source as well. There’s a review of the literature in the American Journal of Physiology from 2002 which says ‘No scientific studies were found in support of 8 x 8.’ The final line of its abstract admits that it’s impossible to prove a negative, and so rather plaintively ‘invites communications from readers who are aware of pertinent publications’. Dr Heinz Valtin, the author of the study, later said in an interview that ‘I searched for ten months with the help of a professional librarian. There wasn’t a single paper that gave any scientific support to this recommendation.’ It seems pretty clear there’s a shortage of evidence.
Dr Valtin’s best guess for where the ‘eight glasses’ myth comes from is a 1940s recommendation by the Food and Nutrition Board of the Institute of Medicine, which in a roundabout way suggested that adults ought to take in about two litres of water a day. But, Dr Valtin points out, the very next sentence in the advice is ‘and much of this can be gained from the solid food that we eat’. White bread is about 30 per cent water. Fruits and vegetables are much more. We’re taking in water all the time. And, of course, we drink tea and coffee and juice and soft drinks, all of which are 90-plus per cent water, and do a perfectly good job of keeping us hydrated.
That last sentence may surprise you if you’ve read that caffeinated drinks dehydrate you. Caffeine is a diuretic, but only, according to Dr Valtin, in large doses. A 2003 review of the literature in the Journal of Human Nutrition and Dietetics found that ‘Doses of caffeine equivalent to the amount normally found in standard servings of tea, coffee and carbonated soft drinks appear to have no diuretic action.’ Even mildly alcoholic drinks, such as beer, can hydrate you if drunk in moderation.
So how do you know whether you’re getting enough water? Well, the simplest way to tell is if you feel thirsty. ‘The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide,’ says the US Food and Nutrition Board. If that’s not certain enough for you, check your urine colour. Pale yellow is hydrated, darker yellow might indicate dehydration. There are Dulux-style charts available online if you want more detail.
Let’s drop the eight glasses of water thing, then. It’s a misunderstood solution to a made-up problem. Instead, let’s get back to Mrs Obama’s claim in the first paragraph. Would we all really be better if we drank an extra glass of water a day? Probably not, unless you’re feeling thirsty all the time, in which case you ought to have worked it out on your own. Perhaps she means we ought to replace a glass of Coke with a glass of water, and that would be a good thing if you’re struggling with your weight and want to get your calorie intake down. But simply drinking a glass of water on top of your daily routine will do no good at all — and if you end up buying bottled water, you’re just adding to an environmental problem and costing yourself money. The most important message to take away here is that your body is actually quite good at knowing how much water it needs, which shouldn’t be a surprise. None of your ancestors died of dehydration before they managed to have children. Evolution is a marvellous thing.
Tom Chivers is Senior Writer for BuzzFeed UK