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 overbrewed. “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 Illycaffè. 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 people who gulp down 40 or more cups a day and those who drink Turkish-style coffee. Even where coffee drinkers appear to be at greater risk than non-drinkers, 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 oesophageal cancer.
So enjoy the odd coffee, but do yourself a favour: let it cool.
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 chlorhexidine-based version, and would probably work out cheaper. Black tea similarly fights cavities and stimulates the mouth’s own antibacterial enzymes.
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 doesn’t remove toxins from the skin, visibly improve your complexion or cure constipation. There is some support for the idea that drinking cold water 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 of water 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 sterilise 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 typhoid. 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 sterilised 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 diarrhoea.
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.
Shared from the New Scientist