David Katz, MD
Photo: Mackenzie Stroh

Q. I've seen news stories that say coffee and tea are good for diabetics and, more recently, one that says caffeine is bad. Which is it? —Charlotte Ferrill, Mooresville, Indiana

A. Getting a straightforward answer from research is tough. Even findings we once thought were beyond question—margarine is good, eggs are bad, beta-carotene supplements are healthy—later proved to be exactly wrong. Because so much depends on how the research is done, it helps to understand the methods scientists use.

Observational studies (in which researchers compare healthy and unhealthy people and their lifestyles to see if any trends emerge) on coffee suggest that people who drink four cups or more daily are about a third less likely to develop diabetes. Similar findings have been seen with tea and decaffeinated coffee. But a recent prospective study (investigators monitor people's habits and watch to see who gets sick, a method that produces more reliable results than observational studies) suggests that all these beverages may have no real effect.

Scientists have looked at caffeine alone—in pills that delivered a dose of about two cups of coffee—and the results have been less muddy: A study employing gold-standard methodology (blinded, randomized, and placebo controlled—neither the researchers nor the subjects know who's getting real pills) found that caffeine actually reduced insulin sensitivity, which can increase your risk for diabetes. The hitch with these findings is that coffee has antioxidants—chlorogenic acid in particular—that might counteract or even overpower the negative effects of caffeine on diabetics. As more studies are done on coffee and tea, we'll be able to figure out whether they're healthy for diabetics. For now, here's the bottom line: The beverages appear to do no harm to blood sugar, and they may even help control it.