By Christie Aschwanden, Slate
Coffee is one of those things that make curmudgeons like Andy Rooney throw up their hands. They used to tell us coffee is bad for us, he complains. Now they say it's good. Why should we believe any of it?
Rooney's complaint about the back and forth nature of coffee studies is no exaggeration. An analysis published last year concluded that coffee consumption may increase the risk of lung cancer. Yet a study published in June found that heavy coffee drinking was associated with a 60 percent decrease in the risk of advanced prostate cancer, and another out this month shows that coffee drinkers are less likely than abstainers to harbor antibiotic-resistant bacteria in their nasal passages.
In the last decade alone, scientists have published hundreds of papers attributing both harms and health benefits to coffee. What gives?
The Starbucks generation isn't the first to find itself bombarded with conflicting stories about coffee's wholesomeness. While epidemiological studies of coffee are a modern invention, the angel/devil messages they've turned up are as old as the brew itself. Coffee, with its dark color, strong flavor, and mind-altering properties, has always aroused awe and suspicion in cultures that encountered it, says Mark Pendergrast, author of Uncommon Grounds: The History of Coffee and How It Changed the World.
According to Pendergrast's book, coffee has stimulated intellectual and often irreverent pursuits among users throughout the ages, often sparking backlash. One governor of Mecca banned the drink after discovering satirical musings about him coming from local coffeehouses. In 1674, a group of London women grew angry with their husbands for spending so much time at coffeehouses (often in an attempt to sober up after the pub), and published a pamphlet warning that the beverage would make them impotent. The men fought back with a competing pamphlet claiming that coffee actually added a "spiritualescency to the Sperme." In 1679, French doctors blasted coffee, because it "disaccustom[ed] people from the enjoyment of wine."
Pendergrast traces modern health concerns about coffee to 19th-century inventor C. W. Post, maker of the cereal drink and coffee substitute Postum. Ads for Postum warned of coffee's health dangers, which purportedly included "coffee heart," "coffee neuralgia," and "brain ***." Post's claims never had any science behind them, but in the 1960s and early '70s epidemiological studies linked coffee to breast lumps, heart disease, and bladder cancer. Those early studies didn't hold up either, but they did spur a resurgence of anxiety about America's favorite morning drug, now phrased in the contemporary language of public health.
Some of the current research does imply that coffee is bad for you. An analysis of more than two dozen studies concluded that coffee drinkers had about a 20 percent increased risk of developing urinary tract cancer, and a 2010 meta-analysis found that heavy coffee drinking may raise lung cancer risk. Women who drink coffee tend to have lower bone density and an increased risk of fractures compared with nondrinkers, and numerous studies show that coffee increases blood pressure.
And then there are the studies suggesting that coffee is good for you. Coffee drinkers are less likely to get liver cancer, endometrial cancer, and prostate cancer than abstainers, and coffee has also been associated with a reduced risk of Alzheimer's and Parkinson's diseases.
Trying to assess coffee through the lens of these studies is an exercise in frustration. Is a potential drop in Alzheimer's risk worth a possible increase in urinary tract cancer risk? Framing the question this way quickly leads to an Andy Rooney-style mind overload: Is coffee good for us or bad for us? Who knows?
It's easy to wonder whether the scientific ambivalence about coffee simply mirrors our own tortured attitudes about the drink. Drinking coffee is pleasurable and mind-altering. (If it weren't a potent drug, we wouldn't be drinking it in the first place.) Anything that gives us a buzz must be a little naughty, and yet that shame at needing our daily fix only adds to coffee's allure. It may seem odd to suggest that peer-reviewed science would be in thrall to our moral qualms, but consider the similar tangles of data over the health risks and benefits of drinking wine, eating chocolate, or even smoking marijuana. The scientists are telling us what we want to hear: Pleasure and pain go hand in hand.
In fact, there's every reason to view the research on coffee with skepticism. Nearly all the studies on coffee and human health suffer from a common problem, says Melissa Wellons, a University of Alabama physician who co-authored a recent review article on the benefits and risks of caffeinated beverages: They're observational. Such studies take groups of people with and without a condition and look for differences between them. That's one way to find associations, but it can't establish mechanisms or causality. For instance, the prostate cancer study that made headlines in May enrolled 47,911 men and surveyed them about their lifestyles and dietary habits over a 20-year stretch. During that time, 642 of the participants developed advanced prostate cancer. When researchers sorted the surveys, they found there were fewer heavy coffee drinkers in the group with advanced prostate cancers than in the group without. Indeed, men who drank six or more cups of coffee per day were 60 percent less likely to be in the group with the deadly cancer. That could mean that drinking huge amounts of coffee will cut your risk of having advanced prostate cancer by 60 percent. Or it could mean that having advanced prostate cancer decreased your likelihood of being a heavy coffee drinker by 60 percent. It's hard to know, a priori, which explanation is correct.
If you divide the observational studies into two piles—"coffee is good for you" and "coffee is bad for you"—the bulk of the flimsy evidence falls on the good-for-you side. More to the point, there just isn't any persuasive evidence that coffee is harmful, says Wellons, even though people have been drinking it for centuries. That's not to say coffee can't produce a minor array of inconveniences. Caffeine at doses of 65 mg or more can help relieve headaches, but habitual caffeine use can spur chronic headaches, too. Coffee makes you more alert, but it can also provoke insomnia, anxiety, and even tremors at high doses. Regular drinkers seem to have a decreased risk of developing diabetes, but for anyone who already has the disease, a hefty cup of coffee can make things worse. Up to three cups of coffee per day may slightly lower the risk of heart attack, but drinking a large dose might also trigger a heart attack if you're already at risk.
Rest assured, we'll see more conflicting research in years to come. There's nothing especially remarkable about coffee that makes it difficult to study. Scientists could get more convincing answers if they ran intervention trials—by assigning a bunch of people to either drink coffee or abstain from it for years or decades, and then checking which group got more cancer and other diseases. But such studies take lots of money and time, and they're unlikely to happen because they're so expensive. (Industry has little incentive to bankroll this research, and the government seems to have more pressing priorities.) Instead, we're bound to get more headline-grabbing observational data that hint at what we already think we know: Coffee is scary and wonderful.