Fast-Her: Fasting Effects on Breast Cancer Treatment

We hypothesize that promoting a fasting state will strengthen the anti-cancer effects of PI3K inhibitors in metastatic breast cancer (MBC) treatment. The primary objective of this study is to assess acceptability of prolonged fasting in this population.

Inclusion Criteria: * Age ≥18 years * Histologically confirmed metastatic breast cancer * Menopausal or medically ovarian suppressed * Currently receiving capivasertib therapy (2 tablets twice daily, 4 days per week) * Stable on current capivasertib regimen for at least 2 weeks * an ECOG performance status of 0-1 * BMI ≥25kg/m2 * Able to provide informed consent * Willing to comply with study procedures including CGM wear, food tracking by mCC app and dietary modifications * Access to smartphone or tablet for mobile application use Exclusion Criteria: * Type 1 diabetes mellitus * Uncontrolled type 2 diabetes (HbA1c \>9.0%) * History of severe hypoglycemia * Eating disorders or contraindications to fasting * Pregnancy or breastfeeding * Significant gastrointestinal disorders affecting food absorption * Unable to fast for medical reasons * Concurrent participation in other dietary intervention studies * A history of significantly abnormal lab results within 4 weeks of consent date, such as hematologic (Hgb \< 10.0, platelets \< 100), hepatic (LFTs \> 2X nl), renal (Cr \> 1.5)