This is a phase II, Single-Arm Clinical Study evaluating the efficacy and safety of a fasting-mimicking diet (FMD) combined with neoadjuvant chemotherapy in patients with ER-positive, HER2-negative breast cancer.

Participants will be assigned to Intervention group: 4-day FMD cycles (827± 100kcal/day on Day 1, 637± 100kcal/day on Days 2-4) synchronized with T-EC chemotherapy.

The primary endpoint is pathological complete response (pCR) rate. Secondary endpoints include metabolic changes, immune markers, quality of life, and safety (CTCAE v5.0). A total patients will be enrolled to detect a 15% pCR improvement (25% vs. 10%, α=0.05, power=80%).

Inclusion Criteria:

• Histologically confirmed ER-positive , HER2-negative invasive ductal carcinoma meeting at least one of the following:1)T1c-T2 N1 with histological grade 2 (ER 1-10%) or grade 3. 2)N2-N3 or grade 3 and/or Ki67 ≥40%. 3)T3-T4
• Age 18-75 years at enrollment
• Body mass index (BMI) ≥18.5 kg/m²
• Adequate hematological function:1）White blood cell count>3.0×10⁹/L.2)Absolute neutrophil count ≥1.5×10⁹/L.3)Platelets ≥100×10⁹/L.
• Adequate organ function:1）Total bilirubin ≤1.5 × upper limit of normal (ULN).2）ALT/AST ≤2.5 × ULN. 3）Alkaline phosphatase ≤5 × ULN.4）Creatinine clearance ≥50 mL/min-1)
• ECOG performance status 0-2
• Willing and able to provide written informed consent

Exclusion Criteria:

• Prior history of any malignancy (including contralateral breast cancer)
• Received any prior neoadjuvant chemotherapy cycle for current diagnosis
• Uncontrolled endocrine disorders:1)Diabetes mellitus requiring insulin or oral hypoglycemics.2)Hyper/hypothyroidism requiring medication
• Active autoimmune disease requiring systemic immunosuppressants
• Current use of antipsychotic medications
• Known hypersensitivity to FMD components (e.g., soy protein, milk protein, nuts)
• Pregnancy or lactation.