Duke University researchers gave 10 diabetic patients caffeine pills equivalent to four cups of coffee a day. They found the caffeine increased the patients' blood sugar levels by eight percent over the day.
One possible explanation is that caffeine may interfere with the transfer of glucose from blood into body cells. Another possibility is that caffeine may stimulate the liver to release glucose when it's not needed.
The researchers plan to do another study with more patients.
The findings are published in the February issue of Diabetes Care.