This research will examine the feasibility of conducting a strict whole-food, plant-based dietary intervention in women with stable metastatic breast cancer currently undergoing conventional treatments. In addition, this research will provide preliminary data on dietary intakes and the effect of plant-based nutrition on numerous outcomes reflecting cancer prognosis and overall health using advanced imaging, various blood biomarkers linked to cancer progression, and numerous symptom questionnaires.

Inclusion Criteria:

• Women with a confirmed diagnosis of metastatic breast cancer with a stable treatment regimen, demonstrated by no changes in primary cancer therapy 6 weeks prior to consent and no anticipated changes in primary cancer therapy in the 4 weeks following consent.
• Systemic therapy may consist of any conventional treatment including anti-hormonal, cytotoxic, targeted monoclonal antibody or small molecule kinase inhibitors or any combination of the above. Women who have previously declined conventional cancer therapy are also eligible provided they meet all other eligibility criteria.
• Expected to survive for at least 6 months.
• Eligibility classification for enrollment into the study - T: any; N: any; M:1.
• Any ER/PR/HER2 status is eligible.
• Age > 18 years.
• Must be willing to adopt a strict, whole-foods, plant-based diet.
• Participant must be willing and able to comply with the protocol for the duration of the study including scheduled testing and weekly office visits.
• Able to speak and read English fluently.

Exclusion Criteria:

• Inability to tolerate a normal diet.
• Current use of insulin or sulfonylureas.
• Active malabsorption syndrome at time of consent (ie. Crohn's disease, major bowel resection leading to permanent malabsorption).
• Current eating disorder.
• Uncontrolled diarrhea.
• Plant-based food allergies or intolerances.
• Recent consumption (in the past 6 months) of a vegan diet.
• GFR < 30 on 2 or more lab tests in the past 90 days.
• Serum potassium > 5.3 on 2 or more lab tests in the past 90 days.
• Major surgery within 2 months of starting study program.
• Psychiatric disorder that prohibits giving informed consent.
• Current smoking.
• Current high risk alcohol use (> 7 drinks per week).
• Current illicit substance use.
• Current warfarin use.

Only women will be enrolled due to low numbers of male breast cancer patients and inability to create subgroup analyses.

Data will be made available for collaboration or other purposes with appropriate data use agreements in place.