A study published in the February, 2008 issue of Diabetes Care indicates that consuming caffeine may make it difficult to control blood sugar levels. It was a small study, involving 10 subjects with type-2 diabetes. The subjects managed their diabetes with diet and took no drugs.
The subjects were able to choose their food for lunch and dinner, but were given a nutrient drink for breakfast. They were all fitted with a monitor that continually measured their blood sugar over a three day period. On one day they were given caffeine capsules (equivalent to four cups of coffee) and on the other days they were given a placebo.
The caffeine increased the amount of glucose in the blood immediately after meal by an average of 9% after breakfast, 15% after lunch and 26% after dinner. Overall, the average amount of sugar in the blood went up about 8% on the day of caffeine consumption.