Most studies over the past decade have painted tea as a therapeutic beverage and coffee as its dastardly counterpart–a brew that challenges weak hearts and joints. However, such black-and-white characterizations appear to have overstated coffee's dark side. New data now indicate that drinking java–lots of it, and especially the caffeinated form–can curb type II diabetes.
A related study by the same research group also probed dietary iron's role in diabetes. It revealed that one form of the mineral appears to aggravate risk of developing the disease: the heme iron in red meat, a type that is especially easy for the body to pick up and use.
Together, these studies reinforce that for the estimated 16 million people at high risk of insulin-resistant diabetes–a potentially life-threatening metabolic disorder currently afflicting some 8 percent of U.S. adults–nutritional guidelines should probably extend beyond limiting one's intake of sugar, fat, and calories.
The new coffee findings "are intriguing" but emerge only in heavy consumers, notes Frank B. Hu of the Harvard School of Public Health. Risk reductions linked with downing fewer than 4 cups per day were only about 2 to 7 percent. However, among adults drinking 4 to 5 cups daily, the chance of developing diabetes fell by 30 percent. And for men who routinely rev up their metabolism with 6 or more cups of coffee per day, the risk of type-II diabetes–the type formerly known as adult-onset diabetes–plummeted another 20 percent. Women take note: Drinking more than 5 cups offered females no additional benefits.
The new findings are consistent with reports from a Dutch group last year (See Diets to Ward Off Diabetes). Those researchers correlated diabetes incidence with coffee consumption over a decade in a group of 17,000 adults. Hu observes that his group's analysis looked at a far larger population–some 42,000 men and 84,000 women–over 15 to 18 years and "was also more rigorously conducted." For instance, the Dutch study couldn't separate the possible effects of regular coffee and decaf, nor did it account for other sources of caffeine, such as regular tea, diet colas, and chocolate.
Among the biggest risk factors for diabetes are obesity, a sugar-rich diet, smoking, lack of regular exercise, and a family history of the disease. These also proved good predictors in the populations that Hu's team followed, female nurses and a group of male health professionals. The latter included veterinarians, dentists, optometrists, pharmacists, and podiatrists.
The Harvard researchers used statistical techniques that first accounted for these known risk factors before attempting to tease out any effect of coffee or caffeine. In fact, Hu observes, most of the caffeine consumed by the health professionals his team followed traced to coffee. It overwhelmed other sources, except in coffee abstainers or decaf-only drinkers.
With the abundance of data, several subtle trends emerged. For instance, drinking 4 or more cups of decaf per day cut a man's risk of developing diabetes by 25 percent and a woman's by 15 percent. There was also a small but statistically significant reduction in risk attributable to caffeine alone. However, in the populations studied, colas and chocolate proved very small caffeine contributors to the daily diet.
Although many teas also contain caffeine, they tend to offer only about 45 milligrams per 8-ounce cup–about the same as a can of cola but just a fraction of the 140 mg provided in a typical cup of coffee. Interestingly, "we saw no effect whatsoever from drinking tea," even in the highest consumption groups, Hu reports. Many herbal teas offer no caffeine at all. It may be, Hu concedes, that most people in the highest tea-drinking group don't down enough caffeine to match that in cups of coffee. Then again, he told Science News Online, "it's possible that there may be unique combinations of compounds in coffee [not present in tea] that played a role." Many studies have highlighted a host of potentially healthful chemicals in tea. It may be, Hu says, that pigments and antioxidants in tea offer their benefits through pathways that don't help the body control blood-sugar concentrations.
In any case, Hu doesn't advocate changing one's beverage choices on the basis of this study alone. Even if coffee's diabetes-fighting benefits withstand the test of time, a host of other studies has linked heavy consumption of the caffeinated brew to heart risks, rheumatoid arthritis, and other ailments.
Frank B. Hu
Nutrition and Epidemiology
Harvard School of Public Health
665 Huntington Avenue
Boston, MA 02115
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