Effects of Preoperative Parenteral Nutrition With Omega-3 Fish Oil Lipid Emulsion on Postoperative Outcomes in Patients Undergoing Major Abdominal Surgery

The goal of this study is to explore if preoperative ω-3 fish oil lipid emulsion parenteral nutrition (PN) can improve postoperative outcomes in patients undergoing major abdominal surgery. The main questions it aims to answer are: * Does preoperative PN containing ω-3 fish oil lipid emulsion improve postoperative nutritional status (prealbumin, albumin) in patients undergoing major abdominal surgery? * Does preoperative PN containing ω-3 fish oil lipid emulsion affect other postoperative clinical outcomes (length of hospital stay, inflammatory status, postoperative complication rates, BMI changes, liver function changes, time to first flatus and defecation) in these patients? Researchers will compare the ω-3 fish oil lipid emulsion PN group (preoperative ω-3 PN) to the control group (preoperative normal diet) to see if there are differences in postoperative nutritional status, safety, and other clinical outcomes. Participants will: * In the intervention group, receive preoperative parenteral nutrition containing ω-3 fish oil lipid emulsion. * In the control group, maintain a normal preoperative diet. * Undergo blood tests and clinical assessments at baseline (before PN use) and on postoperative days 1, 3, and 7 (or at discharge, whichever comes first).

Inclusion Criteria: 1. 18 - 80 years old; 2. NRS2002 score ≥ 3 points; 3. Diagnosed as malnourished or at high risk of malnutrition according to the GLIM standard; 4. Patients undergoing elective major abdominal surgery with an estimated operation time of ≤ 4 hours; 5. Voluntary participants in this clinical study, with patients or their families signing the informed consent form. Exclusion Criteria: 1. Patients with severe hyperlipidemia (such as hypertriglyceridemia), severe coagulation disorders or acute thrombo/thrombotic diseases (including fat embolism), intrahepatic cholestasis, liver failure, renal failure that cannot undergo hemofiltration/dialysis, acute myocardial infarction or stroke; 2. Patients with moderate/severe diseases (sepsis, mechanical intestinal obstruction); 3. Have the following contraindications for intravenous infusion: hemodynamic instability (such as collapse or shock), metabolic disorders (severe trauma, severe sepsis, acidosis), insufficient cellular oxygen supply, acute pulmonary edema, compensatory heart failure, or electrolyte imbalance (water intoxication, hypotonic dehydration, hypokalemia); 4. Pregnant or lactating; 5. Have immune system diseases or are undergoing immunomodulatory therapy or have undergone splenectomy; 6. Use glucocorticoid anti-inflammatory drugs, thyroid drugs; 7. Allergic to ω-3 fat emulsions or allergic to other drug components used in the study (such as eggs, fish, soy protein, peanut protein or other components in the prescription); 8. Have mental disorders or are unable to cooperate with treatment and follow-up; 9. Participate in other clinical trials; 10. The investigator deems it inappropriate for the patient to participate in this study for other reasons.