Caffeine significantly reduced insulin sensitivity in a small study, and was not corrected by the usually beneficial effects of weight loss or exercise.
The results contradict previous research showing that coffee or other caffeinated beverages could help reduce risk of diabetes.
Canadian researchers investigated the effect of caffeine ingestion on eight sedentary lean men and seven obese men with diabetes. They also included eight obese men without type 2 diabetes.
The men took either 5mg per kg of body weight daily or a placebo prior to and during a three-month exercise programme.
Before the exercise programme, caffeine reduced insulin sensitivity by 33 per cent in both the lean and obese men and 37 per cent in the men with diabetes compared to placebo. After the exercise programme, insulin sensitivity fell 23 per cent after caffeine intake in the lean men, 26 per cent in the obese men, and 36 per cent in the diabetic men.
The researchers concluded that "exercise was not associated with a significant increase in insulin sensitivity in either the caffeine or placebo trials, independent of group".
The findings, published in the March issue of Diabetes Care (vol 28, issue 3, pp 566-572), seem to contradict recent reports that coffee intake may cut the risk of diabetes. However, coffee contains several other substances that may affect sugar metabolism, such as antioxidants, potassium and magnesium, suggested the researchers.