A randomized controlled trial on 40 bariatric surgery candidates who will be randomized into two arms (app vs. control). All the participants will receive the standard care of 3-6 preparation to bariatric surgery meetings with a dietitian, and only those participants who will assigned to the experimental group will get access to our study's smartphone app site during this period.

Evaluations will be performed at baseline, at the end of the intervention phase (after 3-6 meetings as needed) and at 1 and 2 years post-surgery (data will be collected at 1 and 2 years post-surgery by phone calls).

Data which will be collected for this study include:demographics and medical data, anthropometrics including body composition analysis, handgrip Strength, physical activity, hydration state, bariatric surgery knowledge questionnaire, quality of life by visual analogue scale, compliance to dietary and lifestyle BS recommendations, readiness for surgery, the subject's experience regarding the smartphone app, surgical data and adherence to post-surgical follow-up by the bariatric team.

A randomized controlled trial on 40 bariatric surgery candidates who will be recruited while attending to the bariatric clinics and by advertisement about the study. Patients will be randomized into two arms (app vs. control) using an online software. All the participants will receive the standard care of 3-6 preparation to bariatric surgery meetings with a dietitian, and only those participants who will be assigned to the experimental group will get access to our study's smartphone app site during this period. The study specific smartphone app site will be based on current literature plus investigators experience and will include a wide knowledge on types of surgeries, dietary guidelines pre- and post-surgery, fluids type and amount, knowledge on supplementation use pre and post-surgery, general dietary knowledge, psychological information, physical activity, recommendation and tips for long-term surgery success, emphasis will be made on long-term follow-up meetings with the bariatric team and practical information on the preparation to the hospitalization process. The smartphone app site will contain video-based modules and encouraging messages. In addition, patients will be encouraged to use the platform to ask questions and receive answers regarding any aspect of the surgery.

Evaluations will be performed at baseline, at the end of the intervention phase (after 3-6 meetings as needed) and at 1 and 2 years post-surgery (data will be collected at 1 and 2 years post-surgery by phone calls).

Data which will be collected for this study include:demographics and medical data, anthropometrics including body composition analysis, handgrip Strength, physical activity, hydration state, bariatric surgery knowledge questionnaire, quality of life by visual analogue scale, compliance to dietary and lifestyle BS recommendations, readiness for surgery, the subject's experience regarding the smartphone app, surgical data and adherence to post-surgical follow-up by the bariatric team.

Inclusion Criteria:

• Pre-surgery: 18 years old or older, BMI ≥ 40 kg/m² or BMI≥35 kg/m² with comorbidities.
• Planned surgery types: LSG, RYGB and OAGB
• Reading and speaking Hebrew.

Exclusion Criteria:

• Previous bariatric surgery.
• Planned surgery types: LAGB or BPD-DS.
• Patients who underwent more than 1 preparation to bariatric surgery meeting with a dietitian.
• Patients with an active psychiatric disorder (uncontrolled) or have other contraindications to bariatric surgery such as addiction to alcohol or drugs.
• Diabetic patients taking Insulin.
• Patients who do not own a smartphone.