At some point today the answer will be yes. The average adult drinks about 1.7 litres of fluids a day: water, tea, coffee, soda, milk, fruit juice and more. But our ideas of what we should be drinking are clouded by urban myths, wishful thinking and dubious health claims. Time to mix in some fact

You can’t not drink water – it is life’s most essential nutrient.

But that does n’t necessarily mean more is better

It’s not called Adam’s ale for nothing. Water was presumably what our early ancestors drank, to the exclusion of everything else. If you stopped drinking it now you would be dead within a week. It is the only nutrient whose absence is lethal in so short a time. But how much you should drink is surprisingly contentious. It is common to hear eight glasses a day – about 2 litres – even if you don’t feel thirsty. In 2002, physiologist Heinz Valtin of Dartmouth Medical School in New Hampshire tried to track down the source of this advice. The closest he came was a 1974 book that casually advised six to eight drinks a day – not just water but also soft drinks, coffee, tea, milk and even beer.

As for its scientific validity, Valtin found none. As the Food and Nutrition Board of the US National Academies of Science, Engineering and Medicine advises: “The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide”. The only exception is some elderly people, whose feedback mechanisms go awry, meaning they can become dehydrated without thirst.

Generally, there is little to gain by doing more than just quenching your thirst. Water does not remove toxins from the skin, visibly improve your complexion or cure constipation. There is some support for the idea that drinking Coldwater makes you burn calories, and water with a meal does reduce overall calorie intake, perhaps because it helps fill you up or displaces calories from sugary drinks. But the overall influence of water on weight is far from clear. There is a sliver of evidence that being well hydrated can protect against health problems including colorectal and bladder cancer, heart disease, hypertension, urinary tract infections and kidney stones. Good hydration makes it easier for the kidneys to extract waste, reducing wear and tear on them. Dehydration headaches do exist and water can cure them (although there are hundreds of other reasons why your head might ache), and drinking lots when you have a cold may loosen mucus, easing the symptoms. Water may not be a cure-all, but the downsides of overdoing it are mild. Besides rare deaths through over hydration among marathon runners and ecstasy users, the worst of it is that many people who regularly push the fluids too hard appear to be mildly hyponatremic – they have too little sodium in their blood. This is not a major problem, but has been associated with mild cognitive impairment and an increased risk of falling in older people.

Overall, though, “there are few negative effects ofwater intake and the evidence of positive effects is quite clear”, according to a recent review. Perhaps the most implausible claim of all has the strongest support: water can improve focus, at least among children. Several studies have found that having children aged 7 to 9 drink water improves their attention and, in some cases, recall. Perhaps children of this age are more prone to dehydration, which can cause a decline in alertness, concentration and working memory.

Tap or bottled?

For some, tap water is too clean, laced with chlorine-containing compounds used to sterilize it. For others it’s not clean enough, teeming with nasty pathogens and traces of chemicals. Then there’s the fluoride often added to it for dental health: decried as a Communist plot in 1950s America, it remains controversial in some quarters today.

Whether for those reasons or simple taste, many people prefer to buy bottled water. Either way, that could be a waste of money, in most parts of the west at least. Around 25 per cent of bottled water sold in the US is simply tap water from municipal sources. A large proportion of bottled water is chlorinated just like tap water – for good reason. Water chlorination is impressively effective at preventing serious diseases such as dysentery, cholera and typhus.

Evidence that chlorination can produce carcinogenic byproducts or compounds that reduce male fertility is “inadequate”, according to the world Health Organization, and the risk is extremely small compared with that from poorly sterilized water. Tap water does contain traces of pharmaceuticals, toiletries and cosmetics, but the US environmental protection agency says “There are no known human health effects from such low-level exposures in drinking water”. As for fluoridation, there is no evidence that this causes any health problems except where accidents lead to over-fluoridation, which can cause vomiting and diarrhea.

There is stronger evidence that the minerals in some bottled waters, especially sodium, can be harmful. And while the health benefits of mineral-rich waters have long been touted, the enormous variation between brands makes this impossible to test. As for taste, that is also impossible to test objectively. But if it is an issue, just chill your tap water: it makes bad flavours much less noticeable.

Soda, squash and juice

We love to spice up plain old water by adding something sweet – with potentially disastrous consequences

Sugary drinks rot your teeth, and them or you drink, them or they will not. Fizzy pop is generally assumed to be the worst. That is not because of dissolvedco2 – it is a myth that sparkling mineral water is any worse for your teeth than the plain variety – but because of the combination of sugar and common flavorings such as phosphoric acid. Their high sugar content means squashes and sodas deliver a huge calorie hit without filling you up: one standard can of a drink like cola provides more than the recommended daily amount of “free” or added sugar. That piles in excess energy that we store as fat. Those who regularly imbibe sugary drinks are more

Likely to be overweight, regardless of income or ethnicity, and consuming a can of sweetened fizz or the equivalent a day increases the risk of type 2 diabetes by a quarter. Overall, this form of liquid sustenance has little to recommend it.

Diet sodas

So, if the main problem with sugary drinks is sugar, eliminate that and you eliminate the problem, right? Not so fast. Some studies indicate that diet sodas help with weight loss, but others find a seemingly paradoxical association with weight gain. Mice consuming artificial sweeteners can even develop glucose intolerance, a precursor to type 2 diabetes. It is tricky to pin down cause and effect in human studies, says Vasanti Malik, a nutrition scientist at Harvard University: people who are already overweight may be consuming diet drinks in an effort to lose weight, skewing the stats. And the animal studies have been criticized as unrealistic, with mice or rats in some experiments consuming quantities of sweeteners equivalent to us gobbling a few hundred tablets a day.

But there are plenty of reasons why low-calorie sweeteners might not always have their intended effect. One is psychology: you had a diet cola this afternoon, so you can have an ice cream this evening. Alternatively it could be that the intenseness of the artificial stuff, which can be 200 times as sweet as sugar, drives us to prefer sweet things, says Malik. Or perhaps sweeteners disrupt our gut bacteria, or our normal hormonal response to sugar intake. “As a result, the body doesn’t respond as well when real sugar is consumed,” says Susan Swithers at Purdue University in West Lafayette, Indiana, leading to weight gain.

The latest review concluded last year that choosing diet drinks over normal sugary drinks can contribute to weight loss. But the uncertainty should give us pause for thought, says Swithers. “The reality is that no one should be drinking a sweetened beverage every day, whether it’s regular soda or ‘diet’ soda,” she says. “It’s like candy in a can either way.”

Fruit juices

Pure fruit juice feels like a healthy alternative. It’s 100 per cent fruit, after all, and contains good stuff that fizzy drinks don’t, such as vitamins, minerals and antioxidants. The UK National Health Service says one small 150 milliliter glass of pure fruit juice counts towards your five-a-day.

But only one. Fruit juice is missing a lot that fruit has: the juice of one orange contains 0.4 grams of fibre, compared with 1.7 grams in an actual orange. And it is as sickly sweet as sweetened drinks. The World Health Organization recommends that the natural sugar in fruit juice should be lumped together with that added to food and sweetened drinks as free sugar, and advises strict limits on how much we should consume. Orange juice and Coca-Cola contain roughly the same amount, and some juices even more (see “Sugar to go”, below). That suggests pure fruit juices should carry the same health warnings as added-sugar drinks.

In truth, we don’t know whether fruit juices are better or worse for you than soda, says epidemiologist Nita Forouhi of the University of Cambridge: other lifestyle factors such as income, diet, smoking and exercise that may differ between habitual juice drinkers and habitual soda drinkers make it hard to draw watertight conclusions. A review by Forouhi’s group and others in 2015 did conclude that added-sugar drinks, artificially sweetened drinks and fruit juices were all potentially associated with type 2 diabetes, but differing study designs mean the evidence for artificially sweetened drinks and fruit juices might be “subject to bias”. In other words, the jury’s still out.

Sports drinks

Sports drinks’ main claim is that they improve athletic performance and recovery by replacing fluid, energy and electrolytes – sodium, potassium and chloride – lost during exercise. A review published in 2000 concluded that sports drinks probably do improve performance compared with drinking water. In 2006 the European Food Safety Authority agreed.

But most sports drinks also come with a stonking sugar content, and more recent studies have questioned earlier conclusions. An analysis published in the BMJ in 2012 found a “striking lack of evidence” for any claim related to sports drinks. They may help elite athletes, but are unlikely to do anything for ordinary people. In the meantime, there’s another competitor: low-fat chocolate milk. Its 4:1 mixture of carbohydrates and protein appears to be ideal for muscle recovery after a workout, and it is cheaper than most alternatives, too. “The research has been positive – most studies have found it to be just as effective or superior to an over-the counter recovery beverage,” says nutrition and exercise scientist Kelly Pritchett of Central

Milk

It’s a plentiful source of calcium – but many health claims for milk and its substitutes don’t stand up to scrutiny

Milk is a richly nutritious mixture of water, proteins, minerals, vitamins, sugars, saturated fat and cholesterol. All mammals make it, but humans are the only ones to drink it beyond their early years. Should we? Breast milk – or synthetic versions of it – provides the “perfect balance of nutrients” for babies in their first year, says Andy Bernstein, a pediatrician at Northwestern University in Evanston, Illinois. After that, full-fat cow’s milk is recommended as a good source of fat for brain development, dropping to 1 or 2 per cent fat milk from age 2. But although programmes in the US and UK that gave milk to children in schools were associated with huge health benefits, it is not clear why.

“We don’t know if there is something specific or special about milk, or if it is just the fact that these children are getting more calories, protein, nutrients in general, ”says Andrea Wiley of the University of Indiana at Bloomington. A recent study of children in Kenya found that supplemental milk helped those with stunted growth catch up in height, but provided no benefits over a non-milk nutritional supplement for children developing normally. For adults, the benefits seem even more dubious. There is no conclusive evidence, for example, that getting extra calcium from milk is vital for maintaining healthy bones or avoiding fractures. Other foods besides milk – “beans and greens”, largely – are also rich in calcium, and most researchers now argue that a generally healthy diet and plenty of weight-bearing physical activity is what keeps bones healthy.

And we should perhaps be careful not to overdo the white stuff. A Swedish study published in 2014 found that drinking three glasses of milk a day over an average of 20 years increased overall mortality compared with drinking just one –while showing that consuming fermented milk products such as yogurt and cheese reduced both fracture risk and overall mortality. The authors of that study recommend caution in interpreting the results, though, as there were a number of potentially confounding factors they could not control for. The fermentation finding is not fully understood either, says Amy Lanou of the University of North Carolina at Asheville, although it might have something to do with a reduction in the milk sugar lactose during fermentation. “If some of these effects are mediated by milk sugar, that may be a reason,” says Lanou.

Soya milk

If cow’s milk isn’t necessarily all that healthy, what about its most common substitute? Soya milk has a bit less fat than cow’s milk, but often comes presweetened, counting towards your intake of free sugar. Its reputation for reducing harmful LDL cholesterol is overblown, too – even if you drank about eight glasses per day that would only equate to a 3 per cent drop in LDL. Other supposed health benefits – preventing breast and prostate cancer, reducing risk of osteoporosis and hot flushes associated with the menopause – are ascribed to soya milk’s high levels of compounds known as phytoestrogens. These can mimic the effect of the hormone estrogen or, in some instances, block it. But none of these effects has been convincingly demonstrated in trials, while a few studies have suggested consumption of soya milk may actually increase breast cancer risk.

Organic or non-organic?

Organic risk at the levels present in mass-produced milk.

Coffee

Many of us seek liquid stimulation, and caffeine is our drug of choice. It’s probably not as bad a habit as we think

Coffee is no good for you – that’s the received wisdom, at least. It is full of caffeine that’s addictive and can make you bounce off the walls, give you headaches and disrupt sleep. Excessive consumption has been linked to heart disease and cancer. And although coffee increases alertness and focus, the effects are short-lived. Users quickly become tolerant: people who regularly drink coffee are no more alert on average than those who don’t. For regulars, the morning brew merely reverses the fatiguing effects of caffeine withdrawal, bringing them back to a baseline level of alertness.

Sounds like one to avoid, then. But Kirsty Pourshahidi of the Northern Ireland Centre for Food and Health in Coleraine, UK, thinks that’s over brewed. “Having looked into it, I don’t feel so bad having three or four cups of coffee a day,” she says.

Pourshahidi has just carried out a review of the evidence, in work partly funded by the Italian coffee company Illycaffи. For a start, she finds few grounds to suppose that imbibing a moderate amount of caffeine is harmful. For an addictive substance, caffeine is surprisingly easy to kick, too: simply getting people to gradually cut their intake over four weeks is an effective strategy.

Beyond caffeine, coffee contains high levels of compounds called chlorogenic acids, known to slow the body’s absorption of glucose. How this works isn’t clear, but it backs up the observation that coffee drinkers have a lower risk of type 2 diabetes. On the other hand, two oily compounds in coffee, cafestol and kahweol, do seem to increase “bad” cholesterol that clogs blood vessels – but most coffee we drink, including instant, doesn’t contain much of either, says Pourshahidi. Espresso machines almost entirely get rid of them and French presses don’t do a bad job

Either. The thing to avoid is the boiled, unfiltered coffee popular in Turkey, Norway and Sweden. Studies on coffee consumption and cancer typically find no correlation or a mildly beneficial effect, except among peoplewhogulpdown40ormorecups a day and those who drink Turkish style coffee. Even where coffee drinkers

Appear to be at greater risk than nondrinkers, the studies generally fail to show a proportional relationship between the amount consumed and risk, suggesting some other factor is involved – perhaps that people who drink coffee also drink more alcohol or smoke more, says Pourshahidi. Last June, the world Health Organization changed its stance on coffee from “possibly carcinogenic” to “no conclusive evidence”. The sole caveat was that any hot drink – above 70°C – increases the risk of esophageal cancer. So enjoy the odd coffee, but do yourself a favour: let it cool.

Tea

Tea drinkers are often bathed in smug satisfaction: unlike coffee drinkers, their beverage of choice full of life giving, leafy goodness. Much of the buzz centres on flavonols, with a particular focus on green tea and its most abundant flavonol, epigallocatechin-3- gallate (EGCG). It boasts antioxidant and anticancer effects, at least when added to cells in a dish.

Time to pour some cold water. Although some studies have found that drinking green tea (and to a lesser extent, black tea) lowers the risk of breast, gut and lung cancers, a 2009 review of 51 studies involving a total of 1.6 million people concluded that the evidence was highly contradictory. It is a similar equivocal story for other supposed benefits. Extracts of both green and black tea reduce blood sugar levels in diabetic rats and mice, and boost glucose metabolism in healthy human volunteers. Tea and its extracts may also reduce cholesterol and blood pressure in people at risk of cardiovascular disease, and animal studies suggest that catechins, compounds found in black tea, can inhibit enzymes that digest fat and starch and perhaps boost metabolism. Some or all of that might explain a small correlation between tea consumption and weight loss in overweight or obese people. So far so good, but the bad news is that the amount of weight lost was so small as to be irrelevant to health, and probably outweighed by other lifestyle choices. Still, a nice cuppa is unlikely to do you much harm. One woman did lose all her teeth at 47 due to a fluoride overdose from tea, but she had been brewing up 100 to 150 teabags daily for 17 years. For most of us, tea’s fluoride content and anti-bacterial properties actually protect our gnashers. A study of tea’s potential as a mouthwash found that green tea killed just as many bacteria as a standard chlorhexidinebased version, and would probably work out cheaper. Black tea similarly fights cavities and stimulates the mouth’s own antibacterial enzymes.

Super - fluids

Many implausible drinks are touted as having health-giving properties. Few of them actually do

Coconut water

Being potassium-rich, coconut water supposedly enhances your ability to absorb water during prolonged exercise. If that were true, though, it would also increase your risk of over hydration. In fact, studies show it is no better or worse at hydrating than a much cheaper beverage: water. As yet there is no scientific verdict on more recently trending hyper hydrating waters – including watermelon water, as endorsed by singer Beyoncé, and birch sap water, as endorsed by Nordic folklore.

Beetroot juice

Rich in nitrates that can relax blood vessels and improve blood circulation, there is some scientific support to the idea beetroot juice is good for you. But drink it in moderation: its sugar content is on a par with orange and other common fruit juices (see “Soda, squash and juice”, page 34). Too much nitrate has also been tentatively linked with an increased risk of stomach cancer.

Wheatgrass smoothies

Wheatgrass contains a smorgasbord of vitamins and minerals, as well as chlorophyll, claimed by some to boost the production of red blood cells. But studies show it is unlikely to benefit you much more than munching green veg such as broccoli and spinach.

Kefir

A fermented milk drink akin to yoghurt, kefir is prized for its supposed beneficial effects on microbes in your gut. Studies in mice suggest there might be a link – although it is too early to say whether there is an effect in humans, or how big it is.

Urine

Lost in the desert, you are far from any source of fresh water and your bottle is empty. What do you do? You know the drill: unzip your pants. And not just there, if some have their way. From acne to anaemia via obesity and various cancers, many are the ills that urine has been said to alleviate – seeing as it contains vitamins, minerals, proteins, enzymes, hormones, antibodies and amino acids your body has discarded.

For Joel Topf, a nephrologist at Oakland University in Michigan, though, that’s a clue to how useful the active ingredients really are. “The chemicals are not necessarily toxic, but they aren’t something that the body wanted to hold on to the first time,” he says. Not only that, but they are at concentrations far too low to be useful. So, urine is disgusting and unhelpful, but harmless, right? Well, maybe not.

One component of urine rarely mentioned by those who promote drinking it is phosphorus, a possible cardiac toxin. The myth that urine is perfectly sterile is just that, too – a myth. Drinking it could bring you down with all sorts of nasties. Let’s seek advice from the real survival experts. When it comes to preserving precious bodily fluids, the US army’s 1999 survival field manual puts urine firmly alongside blood, seawater and fish juices in its “DO NOT DRINK” category. It’s not a cultural thing that we don’t like drinking urine, says Topf – it’s evolutionary. “Urine is waste, not medicine. Stop drinking your urine.”

Vinegar

Cleopatra supposedly dissolved pearls in vinegar to make Mark Antony a love potion. Some 2000 years later, people are still banging on about vinegar’s power to cure erectile dysfunction – and lower your blood pressure, cholesterol and weight to boot. On the cardiovascular front, they might be on to something. A small scale study conducted in 2010 at Arizona State University showed that both diabetic and non-diabetic volunteers had more stable blood sugar and insulin after a meal of complex carbohydrates if they first had a drink of diluted vinegar. Other studies have shown similar, if small, effects.

Acetic acid – the source of vinegar’s characteristic mouth-puckering bite – is generally thought to be responsible, although no one can pinpoint how. Vinegar also contains a teeming collection of amino acids and polyphenolic compounds that might play a part. The jury is still out on whether all this actually makes you lose weight – although one South Korean study on the effects of drinking pomegranate vinegar did show that, regardless of whether participants lost weight, they did lose a particularly dangerous type of body fat.

Any benefits must be set against the deleterious effects of acetic acid on tooth enamel. And would-be Mark Antonys should note: for pearls to improve your sex life, they are best served whole.

Alcoholic drinks

“Wine is the most healthful and hygienic of beverages.” So said that master of liquid health and hygiene, Louis Pasteur. With more than 100 studies confirming a link between moderate alcohol consumption – one or two drinks a day – and a decreased risk of heart attack or stroke, it is tempting to raise a glass to him and to that.

Stay that hand, though. A confounding factor in most such studies is that people who drink in moderation also tend to share characteristics that lower heart-disease risk: they exercise more regularly, have a healthier weight, sleep better and are more affluent than those who drink to excess or never drink (perhaps because they quit due to health problems).

Overall, there is no consensus on who, if anyone, might benefit from moderate alcohol consumption, and by how much. Older people do seem to benefit more, but that could be because their overall risk of heart disease is higher. There is similarly no undisputed evidence that red wine arrests cognitive decline, as has been suggested. Set these small and disputed benefits alongside the 3.3 million deaths that the World Health Organization attributes directly to alcohol consumption each year, and the fact it is classified alongside asbestos as a class 1 carcinogen, and things start to look that much less positive. But let’s not go overboard. There’s a lot to be said for alcohol’s role as a social lubricant, and few studies connect moderate consumption with any significant increased mortality risk. One drink also makes you better at creative problem-solving. As with so much, the difficulty is knowing when to stop.