Drinking coffee at breakfast dramatically increases blood sugar levels, a group of University of Guelph researchers have found.

Blood glucose levels in people who ate low glycemic index (low-sugar) cereal spiked by 250 per cent if they drank caffeinated coffee before or with breakfast, compared to decaf coffee.

Caffeine makes the body resistant to insulin, which in turn increases blood sugar, the study's lead researcher Terry Graham told Canada AM from Kitchener, Ont.

Graham says this shouldn't pose a problem for healthy people because their bodies can handle increases in blood sugar.

"Coffee is a very healthy beverage to have in your lifestyle, as long as you have a healthy lifestyle in terms of exercise, diet, weight control," said Graham, professor of human health and nutritional sciences at the University of Guelph.

But obese people and those with Type 2 diabetes should avoid caffeine before or with meals since their bodies already have trouble producing insulin.

"While coffee has many positive aspects, caffeine itself creates an insulin resistance, in other words it impairs the ability of insulin to get its job done in terms of managing blood sugar levels," said Graham.

"You are challenging a body that isn't able to cope with blood sugar changes as well as it should."

The team concluded that some people may be better off choosing a sweeter cereal and drinking decaf in the morning instead of eating a low-sugar cereal and consuming caffeine with it.

The study monitored the blood sugar levels of 10 male university students. The subjects drank two mugs of either regular or decaf coffee and ate breakfast cereal. One day they consumed the low-sugar cereal Kellogg's All-Bran and the next day they ate the moderately sugary cereal Kellogg's Crispix.

Researchers found that caffeine provoked higher blood sugar and insulin levels even when the lower glycemic index cereal was consumed.

The study's findings were published in May's edition of the American Journal of Clinical Nutrition.