This study is testing the effects of a ketogenic diet on patients with early stage breast cancer. Participants will keep to a ketogenic diet for 3 weeks prior to surgery.

After screening for eligibility participants will meet with a dietitian who will provide education on a ketogenic diet (KD) along with additional educational material. Patients will adhere to a ketogenic diet for 3 weeks. Serum glucose and ketones will be measured by patients daily to assess adherence to KD. Patients will also keep a daily diary that will note food intake along with physical activity.

Inclusion Criteria:

• Be willing and able to provide consent for the study
• Age ≥ 18 on the day consent is obtained
• Eastern Cooperative Oncology Group (ECOG) score ≤ 2
• Patients must be postmenopausal the time of study entry
• Stage I or II breast cancer using Tumor, Nodes and Metastasis (TNM) stage groups per American Joint Committee on Cancer (AJCC) 8th edition
• Plan for upfront breast surgery with mastectomy or partial mastectomy and does not require pre-operative systemic therapy per standard of care
• Must have tissue available from initial breast biopsy performed per standard of care
• have the ability in the opinion of the investigator to adhere to a ketogenic diet and comply with study requirements such as ability to participate in telemedicine, utilize apps on a smart device, measure glucose/ketones at home, accurately log food intake and exercise, prepare food that is in line with ketogenic diet

Exclusion Criteria:

• Clinical indication for neoadjuvant therapy

• Partial mastectomy or mastectomy planned for less than 3 weeks from study entry

• Inability to comply with ketogenic diet

• Consistent use of ketogenic diet within past 3 months

• Participation in another diet program during study period

• Nonepithelial breast malignancy such as sarcoma or lymphoma

• Body Mass Index < 18.5

• Comorbidities and/or active or ongoing illness that in the opinion of the investigator will limit patient's ability to safely participate in study

• Type I diabetes

• Use of insulin or other oral hypoglycemic drugs for diabetes

• Has had any of the following within the past 6 months

  • Myocardial infarction or unstable angina
  • Ventricular arrythmia
  • Acute decompensated heart failure
  • Cerebrovascular accident
  • Hypertensive emergency

• Uncontrolled hypertension despite antihypertensive use

• End-stage renal, liver, or cardiac disease

• genetic disorders that affect lipid metabolism (example: pyruvate carboxylase deficiency, porphyria, primary carnitine deficiency, carnitine palmitoyltransferase I or II deficiency, carnitine translocase deficiency, beta-oxidation defects, among others)

• G6PD (glucose-6-phosphate dehydrogenase) deficiency

• Malabsorption syndromes (such as but not limited to inflammatory bowel disease, history of gastric sleeve, prior bowel resection)

• History of recurrent kidney stones or predisposition to kidney stones

• Triglycerides ≥ 500

• Has known psychiatric or substance use disorders that would interfere with participation in the study