Serine/Glycine-Restricted Diet With Chemoradiotherapy for Rectal Cancer

Inclusion Criteria: * Written informed consent obtained prior to any study-related procedures. * Male or female, aged 18-80 years. * Histologically confirmed locally advanced rectal cancer staged as cT3, cT4, or node-positive by pelvic MRI. * ECOG performance status score of 0 or 1. * NRS-2002 score \< 3 (no significant nutritional risk). * BMI ≥ 18.5 kg/m² (may be adjusted per actual conditions). * Capable of oral intake or via feeding tube and able to tolerate enteral nutrition. * Adequate organ function as defined by the following laboratory criteria: ANC ≥ 1.5×10⁹/L (without G-CSF support within 14 days); Platelet count ≥ 100×10⁹/L; Hemoglobin ≥ 9 g/dL (without transfusion or erythropoietin use within 7 days); Serum albumin ≥ 3.0 g/dL; Total bilirubin ≤ 1.5× ULN; AST and ALT ≤ 2.5× ULN; Creatinine clearance ≥ 50 mL/min (Cockcroft-Gault formula) or serum creatinine ≤ 1.5× ULN; INR ≤ 1.5× ULN, PT and APTT ≤ 1.5× ULN; Urine protein \< 2+ (if ≥2+, 24-hour urine protein \< 2.0 g required for enrollment); Cardiac enzymes within normal range (isolated laboratory abnormalities without clinical significance permitted). * Female patients of childbearing potential must have a negative serum pregnancy test within 3 days prior to study treatment initiation and agree to use highly effective contraception from signing informed consent through at least 6 months after the last dose of study treatment. * All patients (male or female) at risk of pregnancy must use contraceptive methods with a failure rate \< 1% per year throughout the treatment period and up to 120 days after the last dose of study treatment (or 180 days after the last dose of chemotherapy). Exclusion Criteria: * Stage I or Stage IV rectal cancer. * Cognitive impairment or psychiatric disorders that prevent comprehension of the study content. * Central nervous system or meningeal metastases. * Clinically symptomatic moderate to severe ascites (requiring therapeutic paracentesis within 2 weeks before study treatment start; patients with small asymptomatic ascites may be enrolled). * Uncontrolled or moderate to severe pleural effusion and pericardial effusion. * Severe diarrhea, intractable vomiting, severe malabsorption syndrome, paralytic or mechanical intestinal obstruction; tracheoesophageal fistula, gastrointestinal perforation or fistula, or intra-abdominal abscess; gastrointestinal bleeding (CTCAE grade ≥ 3 within 6 months or grade ≥ 2 within 3 months before study treatment start, e.g., abnormal vaginal bleeding, hematemesis). * Any other condition that may affect the study results or lead to forced discontinuation (e.g., alcohol abuse, drug abuse, serious concurrent diseases, severe laboratory abnormalities, family or social factors) as judged by the investigator. * Known allergy to any active ingredient or excipient of the study drugs or nutritional powder. * Poorly controlled diabetes mellitus. * Severe cardiovascular disease, including cerebrovascular accident (CVA), transient ischemic attack (TIA), myocardial infarction, or major vascular disease within 6 months prior to enrollment; uncontrolled symptomatic cardiac disease such as unstable angina, NYHA class II or higher heart failure, LVEF \< 50% on echocardiography, or severe arrhythmia not controlled by medication. * Pregnancy or lactation. * Any other condition that the investigator considers unsuitable for enrollment.