This is a prospective, randomized, two-armed study to assess the efficacy and safety of isocaloric dietary restriction in the treatment of obese without diabetic for a period of 4 weeks.

This is a randomized, open-label, two arms trial. Based on inclusion and exclusion criteria, 72 eligible patients, who are 18-45 years old, with BMI between 35 to 45kg/m2 are assigned to one of two intervention groups: isocaloric dietary restriction; bariatric surgery. Both groups were given a dietary intervention for a period of 4 weeks (in rigorous supervision confined in inpatient department). The main purpose of this study is to clarify the efficacy of isocaloric dietary restriction, compared to bariatric surgery, in losing weight and improving metabolic factors. This is an investigator-initiated study performed by department of endocrinology and metabolic disease, Ruijin hospital affiliated with Shanghai Jiaotong University School of Medicine.

Inclusion Criteria:

• Age between 18 to 45 years old
• 35 ≤ BMI ≤ 45 kg/m2

Exclusion Criteria:

• Obesity induced by other endocrinologic disorders
• Severe gastrointestinal diseases or contraindication for surgery
• History or newly known of diabetes
• Use of hypoglycemic medications, antibiotic in the preceding 2 months, or yogurt within 5 days
• Weight change more than 5% of body weight within the past 3 months
• Severe or unstable cardiovascular, liver or renal diseases or known cancer
• History of drug or alcohol abuse or other substance abuse
• Any mental disorders or current use of antidepressantsPregnancy or lactation or consideration of pregnancy
• Allergies to specific food ingredients in the intervention