From the wikipedia entry on metformin (I underlined parts I want to remember):
Metformin treatment of people at risk for type 2 diabetes may decrease their chances of developing the disease, although intensive physical exercise and dieting work significantly better for this purpose. In a large U.S. study known as the Diabetes Prevention Program, participants were divided into groups and given either placebo, metformin, or lifestyle intervention, and followed for an average of three years. The intensive program of lifestyle modifications included a 16-lesson training on dieting and exercise followed by monthly individualized sessions with the goals to decrease the body weight by 7% and engage in a physical activity for at least 150 minutes per week. The incidence of diabetes was 58% lower in the lifestyle group and 31% lower in those given metformin. Among younger people with a higher body mass index, lifestyle modification was no more effective than metformin, and for older individuals with a lower body mass index, metformin was no better than placebo in preventing diabetes. After ten years, the incidence of diabetes was 34% lower in the group of participants given diet and exercise and 18% lower in those given metformin. It is unclear whether metformin slowed down the progression of prediabetes to diabetes (true preventive effect), or the decrease of diabetes in the treated population was simply due to its glucose-lowering action (treatment effect).
here is a link to the study mentioned above: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1370926/
looking through the study, the medication intervention is less that stellar as the participants must have been eating pretty high carb (relative to what I eat) since they were encouraged to follow the 'Food pyramid'. What that means to me is that the medication intervention potentially could have seen even greater success if they had suggested lower carb:
"The standard lifestyle recommendations for the medication groups were provided in the form of written information and in an annual 20-to-30-minute individual session that emphasized the importance of a healthy lifestyle. Participants were encouraged to follow the Food Guide Pyramid 14 and the equivalent of a National Cholesterol Education Program Step 1 diet, 15 to reduce their weight, and to increase their physical activity."
the recommendations for the intensive lifestyle modification group don't mention low-carb, although it was low- calories and emphasized walking: