This study aims to compare the effects of time-restricted eating (TRE, 16/8) in adults with obesity who have and have not undergone bariatric surgery (BS). The study will include adults aged 18-65 years: those with obesity who have not undergone BS, and those who have experienced insufficient weight loss (IWL) or weight regain (WR) following BS. Both groups will follow a 16:8 time-restricted eating protocol for 6 weeks, with an 8-hour eating window recommended between 10:00 AM and 6:00 PM, though participants may adjust to other 16-hour fasting windows. Participants will be evaluated before and after the intervention in terms of anthropometric measurements, biochemical parameters, and dietary habits. The study is being conducted at a private hospital in Bursa, Turkey, and is planned to be completed with a total of 40 participants (20 per group).

Obesity remains a major global health concern, with bariatric surgery (BS) being the most effective treatment for severe obesity. However, some patients experience insufficient weight loss (IWL) or weight regain (WR) approximately 18 months after surgery, which is associated with recurrence of obesity-related comorbidities and reduced quality of life.

Time-restricted eating (TRE), a form of intermittent fasting, has emerged as a promising dietary approach for obesity management. The 16/8 TRE model, which limits food intake to an 8-hour window, has shown beneficial effects on body weight, insulin resistance, glycemic control, and lipid profiles in non-surgical individuals with obesity. However, evidence regarding its efficacy and safety in post-bariatric surgery patients remains limited.

This prospective controlled study will enroll 40 adults with obesity (20 per group): Group 1 - adults with obesity who have not undergone BS; Group 2 - adults with IWL or WR following BS. All participants will follow a 16:8 TRE protocol for 6 weeks, with nutritional principles aligned with ASMBS guidelines. Follow-up visits will be conducted face-to-face at baseline, day 15, day 30, and day 45, with additional weekly contact via phone or WhatsApp. Anthropometric measurements will be assessed at baseline and at week 6. Dietary intake will be evaluated using 3-day food records and food frequency questionnaires, analyzed with the BEBİS-9 nutrition software.

Inclusion Criteria:

• Adults aged 18-65 years with obesity who have not undergone bariatric surgery
• Adults aged 18-65 years with obesity who have undergone bariatric surgery and experienced insufficient weight loss (IWL) within 18 months post-surgery, -defined as %EWL <50% at 18 months
• Adults aged 18-65 years with obesity who have undergone bariatric surgery and experienced weight regain (WR), defined as >25% increase in %EWL from the lowest recorded weight

Exclusion Criteria:

• Pregnant or breastfeeding individuals
• Acute illness, active infection, or comorbid conditions limiting treatment efficacy and safety, including: cancer, type 1 diabetes, renal or hepatic failure, recent stroke or myocardial infarction, nephrolithiasis, drug or alcohol abuse, eating disorders, severe depression or other psychiatric disorders, inflammatory bowel disease (IBD), neoplasia, arrhythmic heart disease, heart failure, respiratory failure, or corticosteroid therapy
• Professional athletes

Individual participant data will not be shared due to participant confidentiality and institutional data privacy regulations.