Study after study has shown antidiabetic effects of coffee. The March 10 Journal of the American Medical Association carries the latest epidemiological evidence—two European studies showing that people who drink 6 to 10 cups of coffee, primarily caffeinated, per day tend to develop type 2 diabetes at lower rates than individuals do who drink 2 or fewer cups a day.

For several years, scientists have been asking what constituent of java works to control blood sugar. Gradually, chlorogenic acids, a relatively minor family of chemicals in coffee beans, have emerged as prime candidates.

Much attention focused on caffeine, which has turned out to have a detrimental effect. Terry E. Graham of the University of Guelph in Ontario and his coworkers recently tested the effect of pure caffeine, caffeinated coffee, and decaf on blood sugar in lean and obese people with and without type 2 diabetes. The amount of caffeine in a mug or two of strong coffee was sufficient to disrupt control of blood sugar for several hours in any of those 67 individuals, says Graham. A paper detailing the 56 nondiabetic volunteers is due out soon in the American Journal of Clinical Nutrition.

Giving the volunteers caffeine in plain water followed an hour later by a slug of sugar water induced the highest blood sugar concentrations. The same amount of caffeine delivered in 2 cups of coffee before the sugar jolt raised blood sugar concentrations about 75 percent as much as the pure caffeine did. However, when the researchers gave people two cups of decaffeinated coffee and then the sugar, their blood sugar concentrations were even lower than when they drank plain water before the sugar.

That result suggests that the decaf—and, therefore, some coffee component other than caffeine—has an antidiabetic effect, says Graham.

Last year, Linda M. Morgan and her colleagues at the University of Surrey in England tested nine healthy volunteers. Each made three morning visits to Morgan's lab after fasting overnight. In the lab, they each downed 25 grams of sugar in 2 cups of a beverage. On one morning, they took the sugar in regular coffee; another morning, in decaf; and a third morning, in water. After each sugary drink, they submitted to tests of how much ingested glucose entered their blood during the next 3 hours.

Both coffee types enabled the volunteers to control blood glucose significantly better than they did after drinking the glucose-containing water, the scientists reported in the October 2003 American Journal of Clinical Nutrition. Once in the blood, however, glucose that was drunk with caffeinated coffee tended to stay there, as it would in a person with diabetes. The finding is consistent with other studies showing that caffeine can impair insulin's responsiveness to blood sugar.

By keeping the concentrations of chlorogenic acids the same in the decaf and caffeinated coffees, the researchers made sure that those compounds weren't the source of the effect. Earlier studies by the group had shown that low concentrations of chlorogenic acids naturally present in apples attenuated the release of glucose into the blood after volunteers ate the fruit.

Michael N. Clifford, the research team's food chemist, hypothesizes that chlorogenic acids, which are present in far greater concentrations in coffee than in fruit, reduce the efficiency of molecular-scale pumps that move glucose across the walls of cells lining the digestive tract. These acids would thereby tend to keep sugar in the gut and out of the bloodstream, reducing the chances of the high spikes of blood sugar that exacerbate diabetes.

Jane Shearer of Vanderbilt University in Nashville and her colleagues have studied the effects of pure chlorogenic acids, isolated from decaf, on enzymes that regulate the liver's release of glucose. Ordinarily, between meals, the liver sends glucose into the blood to keep it available to tissues. In people with diabetes, the liver inappropriately sends out glucose even after a meal has already boosted blood concentrations of the sugar.

The researchers showed in rats that chlorogenic acids disrupt the liver enzymes' action, bogging down glucose's release into the blood. This helps prevent blood sugar spikes after meals, the researchers reported in the November 2003 Journal of Nutrition.

Tea too?