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).