Researchers suspect that substances other than caffeine explain why coffee is associated with lowered risk of diabetes. In fact, caffeine in short-term studies decreases insulin sensitivity, which should theoretically worsen the condition, not protect against it. But coffee is rich in many other substances that are biologically active, some of which are only beginning to be investigated.
"Caffeine has received most of the research attention, but it is only one of hundreds of substances found in coffee," says coffee chemist Tomas de Paulis, a researcher at Vanderbilt University's Institute for Coffee Studies, which receives funding from coffee manufacturers.
He and his colleagues are investigating substances in coffee called quinides, which increase the capacity of the liver to use glucose. That, in theory, should improve blood sugar control in diabetics. Unlike caffeine, these substances may be unique to coffee, created during the process of roasting coffee beans.
Coffee, like tea, is also turning out to be a plentiful source of antioxidants, which may protect against the damage caused by unstable free-radical oxygen molecules. In an analysis published early this year that looked at the diets of 2,672 Norwegians -- among the world's most avid coffee drinkers -- coffee was found to be the biggest contributor of antioxidants on the menu.
The antioxidants in coffee may also explain preliminary findings that suggest that coffee drinking may lower the risk of oral cancer and heart disease. In addition, coffee is a good source of the mineral magnesium, which could partly explain why it seems to protect against diabetes. Diabetics often have abnormally low levels of the mineral.
Guilt by association
Why, then, has coffee had a bad reputation for so long? One reason is guilt by association. In early studies, coffee drinkers often tended to be smokers and alcohol drinkers, says Leitzmann, who is now a researcher at the National Cancer Institute. As a result, some early studies blamed higher cancer rates on coffee.
"We now know that those risks are really associated with smoking or excessive alcohol consumption, not drinking coffee," Leitzmann says.
In other instances, lack of an understanding of biological mechanisms led to unwarranted concerns. Early studies showed that pregnant women who reported drinking more coffee were more likely to miscarry, for example. Coffee was blamed for miscarriages. Researchers now know that in early pregnancy, many women find the taste and smell of coffee unpleasant, as a result of hormonal changes. The healthier the pregnancy, the stronger those signals, and the less coffee women tend to drink.
"When the placenta is in place and the pregnancy is a healthy one, most women get a strong pregnancy signal, so they drink less coffee. Women who don't have as good an implantation and are therefore at increased risk of miscarriage have less of a pregnancy signal, and so they tend to go on drinking their usual amount of coffee," Leviton explains. "Drinking coffee doesn't cause the miscarriages. It's just a marker for how weak the pregnancy signal is."
Many of the early worries over coffee surfaced in very small studies that lacked statistical legitimacy and may have been biased by a tendency to assume there were dangers to drinking coffee, Leviton says.
The latest evidence comes from very large studies that involve tens of thousands of people and whose findings are generally considered to be more reliable. A few small studies suggested increased risk of some cancers, for example, but when scientists in 2000 pooled the available data from many studies, they found no statistical link between the disease and coffee drinking.
A few worries persist. Caffeine can aggravate arrhythmias, or irregular heartbeats, so cardiologists sometimes advise people with such conditions to switch to decaf. Insomnia sufferers are also typically advised to give up caffeinated coffee, especially late in the day and evening. Because caffeine can make its way into breast milk, the American Academy of Pediatrics recommends that nursing mothers drink decaf coffee or other non-caffeinated beverages.
Caffeine in coffee also raises blood pressure temporarily, so people with hypertension might do well to avoid it. But several extensive studies have shown that coffee drinkers are no more likely than nondrinkers to suffer chronic high blood pressure -- evidence that coffee doesn't cause hypertension.
"The real news is that drinking coffee poses no danger to most people," Leviton says. "If coffee turns out to have benefits, as some of the new evidence suggests, all the better."
For now, the findings are preliminary enough that even the researchers who have turned up benefits say it's too early to recommend that people who don't drink coffee should start.
"We need to know more about the mechanisms at work in conditions like Parkinson's or diabetes before we can feel comfortable making recommendations," says Peter Martin, who directs the Institute for Coffee Studies at Vanderbilt.
Ultimately, Martin says, the latest research is more likely to yield new drugs based on compounds found in coffee than recommendations for people to drink the beverage. Several pharmaceutical companies are already looking at developing drugs based on components found in coffee beans.
Still, for the more than 166 million Americans who drink coffee -- up more than 5 million from 2002, according to figures released by the National Coffee Assn. -- the new findings offer one more reason to love a good strong cup of java.