When sugar levels in the blood get too high, insulin is released, which brings the levels back down. With insulin resistance, also known as decreased insulin sensitivity, sugar levels need to get much higher before insulin release is triggered. Over time, this resistance can cause problems and lead to diabetes.

Dr. Robert Ross of Queens University in Kingston, Ontario, stated that, "Through mechanisms that have yet to be firmly established, caffeine attenuates any of the beneficial effects of exercise or weight loss on insulin resistance."

While the clinical implications remain unclear, Ross added, the findings are a "red flag" for doctors and are particularly important for obese patients and those with diabetes.

Ross and his team evaluated sugar metabolism in 23 men before and after a three-month exercise program. Before and during the exercise program, the men were given caffeine or inactive "placebo." The subjects included eight sedentary lean men, seven obese men with type 2 diabetes, and eight obese men without diabetes.

Before the exercise program, caffeine reduced insulin sensitivity by 33 percent in the lean and obese men and 37 percent in the men with diabetes compared to placebo. After the exercise program, insulin sensitivity fell 23 percent after caffeine intake in the lean men, 26 percent in the obese men, and 36 percent in the diabetic men. Comparison of the two study phases, showed that exercise did not improve insulin resistance related to caffeine intake.