The prevalence of obesity and Type 2 diabetes has increased dramatically worldwide. Research on Glycemic Index (GI) diets has been largely developed, however the combination with metformin in humans was not investigated yet. We hypothesized that the low-GI diet in combination with metformin would elicit greater improvements on glucose metabolism, lipid profile and body composition on overweight or obese type 1 subjects. This study aims to evaluate the metabolic effects of the adoption of dietary advice to follow caloric restriction high/low GI diets, and low-GI diet associated with metformin on glucose metabolism, lipid profile and body composition on overweight or obese type 1 subjects.