Safety and Efficacy of Leucine-Restricted Diet Combined With Neoadjuvant Chemotherapy and Immunotherapy in Gastric Cancer

Inclusion Criteria: * Pathological Confirmation: Histologically confirmed locally advanced gastric cancer. * Demographics: Aged 18 to 70 years, inclusive, regardless of gender. * Dietary Capability: Capable of oral intake or receiving a liquid diet via nasogastric tube. * Consent: Willing to participate in the study and have signed the Written Informed Consent Form (ICF). * Staging and Treatment Indication: No evidence of distant metastasis on imaging examinations (such as CT or PET-CT), with a clinical stage of locally advanced gastric cancer, indicating the need for neoadjuvant chemo-immunotherapy prior to surgery. * Exclusion of Other Malignancies: No concurrent primary malignant tumors other than gastric cancer. Exclusion Criteria: * Cognitive or Psychiatric Impairment: Cognitive dysfunction or psychiatric disorders severe enough to prevent the patient from understanding the study content or providing informed consent. * Diabetes Mellitus: Diagnosis of Type 1 or Type 2 diabetes mellitus. * Gastrointestinal Conditions: Presence of severe diarrhea, intractable vomiting, severe malabsorption syndrome, paralytic or mechanical intestinal obstruction, or active gastrointestinal bleeding. * Allergy/Hypersensitivity: Known hypersensitivity or allergy to any of the main components of the leucine-deficient nutritional powder. * Concomitant Supplements: Current use of other nutritional supplements that may potentially confound the study results or affect the evaluation of efficacy. * Treatment Tolerance: Inability to tolerate neoadjuvant chemo-immunotherapy, or occurrence of severe gastrointestinal adverse events following such treatment. * Pathological Diagnosis: Postoperative pathological diagnosis confirming non-primary gastric cancer (e.g., metastatic tumors from other origins).