Patients who underwent major gastrointestinal surgery is potentially at risk of malnutrition due to reduced oral intake, increased nutritional need, reduced gastrointestinal absorption function, and/or metabolic changes after surgery. The postoperative malnutrition is associated with low quality of life and seems to be related long-term nutritional status. This study is a multicenter, open-labeled prospective randomized clinical trial to examine the effect of postoperative oral nutritional supplements (ONS) after major gastrointestinal surgery by comparing the change of body weight and other nutritional parameters between the experiment group that is supplied with ONS and the control group without ONS.

Inclusion Criteria:

• Male or female who are 20 or more years old and not more than 80 years old
• Discharge from a hospital is planned within 2 weeks after major gastrointestinal surgery (except when ileostomy is performed)
• oral intake is possible at the time of discharge
• no preoperative chemotherapy or preoperative radiotherapy
• voluntarily agreed with the informed consent of this clinical trial

Exclusion Criteria:

• Intravenous or other specific nutritional treatment is needed
• BMI >25 and postoperative weight loss is not > 5% of preoperative body weight at the time of discharge
• Allergy to milk, whey, bean, salmon, or the investigational product
• Residual of cancer in the abdominal cavity postoperatively if it is cancer case
• Presence of synchronous other cancers that needs treatment.
• When investigator judged that the patient is not eligible to the trial