The coffee-drinker's last excuse seems finally to have vanished. For years beleaguered office workers, long-distance lorry drivers and students pulling all-nighters have been able to rebut fears about the health risks of caffeine consumption with a simple truth: drinking coffee helps you stay alert and get the job done.
Except that now, it seems, it does not. According to a study published this week, coffee-drinking in British workplaces is undermining concentration and damaging productivity. The survey found 76% of workers drinking three or more cups of coffee, tea or cola every day - enough caffeine to act, in the words of researcher Dr David Lewis, "as a powerful diuretic [causing] people to visit the toilet more frequently, which can lead to power-zapping dehydration."
You'd be entitled to feel confused: few substances in recent years have been subject to so many entirely contradictory clinical studies as caffeine - not even wine and beer, the other once frowned-upon stimulants now periodically presented as elixirs of good health.
Depending on whom you believe, coffee-drinking either raises blood pressure, increases the chances of miscarriage and the risk of developing Alzheimer's, arthritis and infertility - or is a powerful protection against the worst effects of bronchial asthma, colon cancer, bladder cancer, hayfever, Parkinson's, heart disease, depression, anxiety, gallstones, dermatitis, chronic angina and hypotension.
A web of sponsors on both sides of the debate means that few studies are entirely free of commercial interests: this week's study, in one of the more clunkingly obvious examples, was funded by Volvic, manufacturers of an ideal water-based alternative to latte-slurping.
Besides dehydration, coffee blocks the receptors of adenosine, a chemical in the brain that normally counters the effects of adrenaline - which explains why it does enhance the sense of alertness. But related effects of this process, some researchers believe, can be calamitous: "The daily blood pressure elevations caused by caffeine consumption," says Dr James Lane of Duke University in North Carolina, "could contribute to an increased risk of coronary heart disease in the general population that consumes caffeine."
A group of Swedish researchers reported in December that women who drank more than four cups of coffee per day in early pregnancy doubled the chances of spontaneous abortion, while caffeine intake is also thought to affect birthweight - though the precise mechanisms by which it contributes to any of these problems remain mysterious.
An unresolved argument rages, too, over its addictiveness: even a single cup of instant coffee daily is sufficient to induce withdrawal symptoms when it is taken away; on the other hand, studies into the parts of the brain which are most affected by cocaine, nicotine and morphine addictions have repeatedly failed to reveal any such effect from coffee drinking.
But the issue reaches far beyond coffee. Caffeine makes a regular appearance in the ingredients lists of numerous common painkillers. But since most researchers deny that it has any painkilling function, and argue that one of the key symptoms of caffeine withdrawal is headaches, it doesn't take a particularly paranoid conspiracy theorist to suggest that something vaguely suspicious might be going on. And non-coffee-drinkers are not, of course, immune: there is around 50mg in a cup of tea, and up to 20mg in a cup of hot chocolate.
And yet for every study demonstrating a health risk, there is another suggesting benefits. Take coffee and cancer: a year ago, a study published in the Journal of Epidemiology and Public Health found that smokers reduced by half the extra risk they run of contracting bladder cancer if they drink coffee as well.
"For centuries people have been investigating exactly the same thing. Around 2.5bn cups are drunk per day in the world, so if this was an issue, it would be a problem of epidemic proportions," says Dr Euan Paul, scientific adviser to the industry-backed coffee science information centre. "Recent papers are in complete contradiction to the Volvic findings."
The debate is complicated by the fact that coffee remains, relatively speaking, a drink of the wealthy (and, in the case of the high-end joints, the profligate). Since environmental and dietary factors are likely to mean they contract fewer serious illnesses in any case, epidemiological studies concluding that coffee enhances health are at risk of drawing unsubstantiable conclusions.
"It's also hard to separate out the effect of coffee from caffeine," says Dr Mary Berrington of the cancer research campaign. "Something else in the coffee may be protective, such as flavinoids. But in the case of cancer, most of the epidemiology studies suggest there is no great association with coffee, either positive or negative."
The only point of agreement among these studies is that the most serious negative effects of coffee-drinking, if they exist at all, only begin to kick in after at least three cups and possibly more, while few would dispute that enough coffee drunk all at once would kill. Everything in moderation: it's a boring motto to live by, admittedly, but in this case, very probably true.
