A 2-Week Ketogenic Diet in Combination With Letrozole to Modulate PI3K Signaling in ER+ Breast Cancer

Inclusion Criteria: * All participants must provide written informed consent. * Patients must have histologically confirmed primary invasive mammary carcinoma * The tumor must be estrogen receptor positive * The tumor must be HER2 negative (negative IHC or FISH) * The primary tumor size must be at least 2 mm in size. * Patients must be post-menopausal defined by any of the following: * Subjects at least 55 years of age. * Subjects younger than 55 years of age and amenorrheic for at least 12 months or serum follicle-stimulating hormone (FSH) levels and estradiol levels in the post-menopausal range by local lab criteria * Subjects with history of bilateral oophorectomy or prior radiation castration with amenorrhea for at least 6 months. * Patients must have clinical stage I, II, or III invasive mammary carcinoma planning to undergo surgical treatment with either segmental resection or total mastectomy. * Patients must have BMI \>= 30. * A core biopsy from the time of diagnosis must be available. * Mammogram or ultrasound required prior to screening * Patients must have adequate organ function based on the following laboratory parameters: * Serum creatinine \<= 1.5x ULN * SGOT, SGPT \<= 4x ULN (unless known steatohepatitis) * Serum albumin \>= 2.0 g/dL * Total serum bilirubin \<= 1.5x ULN (or \<= 3x ULN if known Gilbert's syndrome) Exclusion Criteria: Patients with locally advanced disease who are candidates for other preoperative (chemo)therapy at the time of initial evaluation. This includes patients with inflammatory breast cancer. * Evidence of distant metastatic disease (stage IV). * Serious medical illness that in the judgment of the treating physician places the patient at high risk of operative mortality. * Serious medical illness that in the judgment of the treating physician would preclude the use of a ketogenic diet. * Severe uncontrolled malabsorption condition or disease (e.g. grade II/III diarrhea, severe malnutrition, short gut syndrome). * Diabetes mellitus requiring insulin therapy. * Dementia, altered mental status, or any psychiatric condition that would prohibit the understanding or rendering of informed consent. * Participation in any other neoadjuvant therapeutic clinical trial. * Concurrent anti-cancer therapy other than endocrine therapy (e.g. chemotherapy, radiotherapy, immunotherapy, or any other biologic therapy). * Concurrent treatment with an investigational agent. * Use of an investigational drug within 30 days or 5 half-lives, whichever is longer, preceding the first day of dietary intervention.