A prospective randomized clinical trial of all the patients undergoing Roux - en-Y gastric bypass will be performed. Patients will be randomly assigned into 2 groups: those patients receiving a preoperative balanced energy high-protein formula (group 1) and those receiving preoperative Immunonutrition (group 2). Changes between groups in acute inflamatory markers, pain, postoperative septic complications and lenght of stay, among others will be investigated.

A prospective randomized clinical trial of all the patients undergoing Roux - en-Y gastric bypass will be performed. Patients will be randomly assigned into 2 groups: those patients receiving a preoperative balanced energy high-protein formula (group 1) and those receiving preoperative Immunonutrition (group 2). Changes between groups in acute inflamatory markers (CRP, fibrinogen, leucocites), pain (VAS scale), postoperative septic complications .(anastomotic leak and organ/space SSI)and lenght of stay, among others will be investigated.

Inclusion Criteria:

1. Patients undergoing a laparoscopic gastric bypass (BPG) as a bariatric procedure
2. Men and Women older than 18 years.
3. Willing to participate in the study and giving their written consent

Exclusion Criteria:

1. Patients younger than 18 years.
2. Patients undergoing other bariatric surgery techniques.
3. Patients undergoing any other surgical procedure added to the bariatric technique.
4. Patients with medical or surgical pathologies that at the discretion of the investigator do not allow their participation in the study.
5. Inability to understand the nature and purpose of the study and / or to accept written participation in the study.
6. Impossibility to comply with pre-established clinical follow-up.