Recent research has shown that moderate coffee consumption may be associated with a reduced risk for type 2 diabetes and Alzheimer's disease. Since reactive oxygen species (ROS) are believed to be involved in both of these diseases, it was theorized that antioxidants in coffee might contribute to the risk reduction.
Scientists examined whether or not dark roast coffee has a stronger antioxidant effect than light roast. They found that dark roast was the most effective.
According to the study, as reported by Green Med Info:
"Furthermore, administration of the [dark roast] led to a significant body weight reduction in pre-obese subjects."