A randomized clinical trial in two groups of 55 patients with esophageal, gastric, or pancreatic cancer undergoing surgical treatment with or without neoadjuvant therapy. The intervention group will receive nutritional therapy with preoperative oral nutritional immunonutrient supplements for at least 15 days before surgery, and postoperative nutritional supplements with an immunonutrient formulation to be maintained for at least 3 months following surgery. The control group will consist of standard clinical practice following the center's established ERAS (Enhanced Recovery After Surgery / RICA) protocol, whereby only patients at risk of malnutrition through nutritional screening receive standard preoperative supplements, and postoperative supplementation is administered during hospital admission in cases of decreased oral intake. Postoperative complications will be assessed using the Clavien-Dindo classification, and muscle mass will be measured via abdominal CT scans at the L3 level. All patients will undergo ultrasound for the measurement of the rectus femoris/quadriceps femoris and abdominal fat mass, as well as bioelectrical impedance analysis (BIA). Serum and plasma aliquots will be sent to the center's biobank for the determination of circulating muscle-related biomarkers.
