Based on existing literature, we posit that a leucine-restricted diet is safe and well-tolerated in patients with advanced gastric cancer receiving combined chemotherapy and immunotherapy. Patients adhering to this dietary regimen exhibit a significant reduction in serum leucine concentrations, with no notable impact on the serum levels of other amino acids. Furthermore, leucine restriction promotes the activation of immune cells within the tumor microenvironment. When applied in conjunction with chemotherapy and immunotherapy for advanced gastric cancer, this approach demonstrates synergistic anti-tumor efficacy. It is expected to enhance tumor response rates , improve the 1-year survival rate, prolong overall survival (OS), and ultimately optimize patient prognosis.

This study employs a self-controlled design to comprehensively evaluate the safety and efficacy of a leucine-restricted diet combined with chemotherapy and immunotherapy in patients with advanced gastric cancer, with follow-up assessments of survival status to determine the 1-year survival rate and Overall Survival (OS). For patients eligible for R0 resection, anti-tumor efficacy will be pathologically assessed using the Tumor Regression Grade system, alongside statistical analysis of tumor response rates following combined therapy. Furthermore, the study will investigate mechanistic changes by comparing the tumor immune microenvironment and serum amino acid profiles (including leucine) before and after dietary intervention, while monitoring alterations in patient body composition, ultimately aiming to validate the hypothesis that leucine restriction enhances therapeutic outcomes in this setting.

Inclusion Criteria:

• Diagnosis and Treatment Plan: Patients with advanced gastric cancer with distant metastasis, confirmed by imaging modalities (such as CT or PET-CT) and clinical pathological data, who are indicated for combined chemotherapy and immunotherapy.
• Demographics: Aged 18 to 70 years, regardless of gender.
• Dietary Capability: Capable of oral intake or receiving liquid diet via nasogastric tube.
• Consent: Willing to participate in this study and have signed the Informed Consent Form (ICF).
• Exclusion of Other Malignancies: No concurrent primary malignant tumors other than gastric cancer.

Exclusion Criteria:

• Cognitive or Psychiatric Impairment: Cognitive dysfunction or psychiatric disorders that 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：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 efficacy evaluation.
• Treatment Tolerance: Inability to tolerate combined chemotherapy and immunotherapy, or occurrence of severe gastrointestinal adverse events following such treatment.