The goal of this randomized control trial is to compare the effect of a liquid formula rich in nutrients and low in calories (supplement) with the progressive diet with conventional foods that we usually recommend in patients after surgery. The main question it aims to answer is if the addition of a high-protein, volume-controlled nutritional supplement as a fundamental part of the diet during the first two months after surgery will preserve the patient's nutritional status and prevent the appearance of nutritional deficits and associated complications.

Participants will be evaluated at the beginning, at 1 month and at the end of the study (2m) and the following determinations will be made:

• Analytical determination
• Nitrogen balance by determining urea N2 in 24-hour urine
• Anthropometric determinations
• Body composition determined by impedanciometry
• Resting energy expenditure and nutrient oxidation measured by indirect calorimetry.
• Energy, protein and hydration intake.
• Gastrointestinal tolerance through self-registration of symptoms. Researchers will compare the effect of a complete hypocaloric liquid formula group with the usual progressive diet group to see if they achieve the protein intake goal established for patients after obesity surgery, preservation of nutritional status and lean mass during the rapid phase of weight loss and the gastrointestinal tolerance during the first two months after surgery.

Inclusion Criteria:

Patients of both sexes between 18 and 65 years of age undergoing bariatric surgery in our center With the ability to understand the objectives and proposals of the study That they agree to participate in the study.

Exclusion Criteria:

Patients who have had perioperative complications and who require an extended hospital stay Patients who after the intervention require some type of artificial nutritional support Patients who have undergone a surgical technique other than BPG.