Terry Graham of the University of Guelph says caffeine in the coffee can alter the body's sugar response.

Graham and graduate students Lesley Moisey and Stia Kacker used two types of cereal -- one with low-levels of sugar and one with moderate levels -- and examined the difference in response when healthy male subjects who drank caffeinated and decaffeinated coffee one hour before eating breakfast.

The study, published in the American Journal of Clinical Nutrition, showed blood sugar levels in subjects who ate the low-sugar cereal jumped 250 percent higher when they drank caffeinated coffee than when they drank decaffeinated.

"Caffeine interferes with our body's response to insulin,"Graham says. "It makes us resistant to insulin which in turn makes our blood-sugar levels go higher."

Several spikes in blood sugar a day can have adverse health effects and those at risk for type 2 diabetes should be careful and consider drinking decaffeinated coffee, Graham says.