This trial is a multi-center, single-arm, prospective, observational cohort study of patients undergoing primary bariatric surgical procedures in Korea. A total of 100 patients will be recruited over a 1-year period from 2016 to 2017, and will be followed for a mean follow-up period of 1 year after surgery. The eligible subjects who have given their consent to participate will undergo one of two surgical procedures; sleeve gastrectomy and Roux-en Y gastric bypass.

The primary objective of this clinical trial is to measure the changes in body weight (kg) and waist circumference (cm) of the enrolled patients at postoperative 1 year. Secondary outcomes are improvement or remission rates of obesity-related co-morbidity (hypertension, type 2 diabetes, dyslipidemia, and sleep apnea), and the change in quality of life. Additionally, postoperative incidences of morbidity and mortality, micronutrient status, body composition, and cost-effectiveness of surgical intervention will be examined.

Inclusion Criteria:

1. body mass index (BMI) ≥ 35 kg/m2 or BMI ≥ 30 kg/m2 with co-morbidities such as hypertension, glucose intolerance, dyslipidemia, and sleep apnea
2. being suitable for general anesthesia
3. being able to give informed consent and committed to follow-up
4. Female patients should have a negative urine pregnancy test at screening and agree to use reliable method of contraception for 1 year.

Exclusion Criteria:

1. prior bariatric surgery of any kind
2. being unable to understand the risks, benefits and compliance requirements of this trial
3. pregnancy or planning on being pregnancy
4. non-Korean speaker
5. American society of anesthesiology (ASA) class IV or higher
6. malignancy within 5 years.