A study in nearly 70,000 women found that those who drank at least a cup of coffee with lunch were one-third less likely to develop type 2 diabetes over several years than non-coffee drinkers. This was true for decaf and caffeinated coffee, with or without sugar. But drinking coffee at any other time of the day didn't influence diabetes risk at all.

"Our findings strongly suggest that only coffee taken with lunch may reduce diabetes risk," Dr. Daniela S. Sartorelli of the University of Sao Paulo in Ribeirao Preto, Brazil, and her colleagues wrote in the American Journal of Clinical Nutrition.

Over a dozen studies have linked coffee drinking to a lower risk of type 2 diabetes -- the type closely linked to obesity. But the mechanism behind the relationship hasn't been established and no studies have looked at whether the timing of coffee drinking influences this effect.

To investigate, Sartorelli's team looked at 69,532 French women participating in a large European nutrition study. The women ranged in age from 41 to 72 when they were enrolled in the study, and were followed for 11 years, on average. During that time, 1,415 of them developed type 2 diabetes. Overall, those who drank at least three cups of coffee daily were 27 percent less likely to become diabetic.

But when the researchers looked at the timing of coffee consumption, they found that only lunchtime coffee drinking reduced type 2 diabetes risk; women who drank more than a cup with lunch every day were 33 percent less likely to develop diabetes.

The lunchtime effect was seen only for black coffee, not for coffee with milk added, but because the number of study participants who drank coffee with milk at lunch was small, the significance of this finding isn't clear, the researchers say. Lunchtime coffee benefits could have something to do with timing, or they might be related to the types of food that people eat at lunch, Sartorelli and her team suggest.