A healthy diet has been defined in various ways. The investigators wish to compare the effects of two diets on ER positive breast cancer tissues. The diets will be administered between the time of diagnosis and the time of surgical removal. Both diets have been proposed as healthy, but differ in their composition. One constitutes a low fat diet with extra fiber, fruits and vegetable servings, while the other comprises a diet low in carbohydrate content, aiming to lower insulin secretion. Patients will be randomized to each group, with 20 assigned to the low fat arm and 45 to the low carbohydrate arm.