This prospective, single-center observational cohort study aims to explore the relationship between skeletal muscle quality, fat distribution, and metabolic health in Chinese patients with obesity, and to evaluate how bariatric surgery influences these parameters.

A total of 120 participants will be enrolled, including 60 patients undergoing bariatric surgery and 60 age- and sex-matched healthy controls. The study involves cross-sectional comparisons of ectopic fat and muscle composition, as well as longitudinal follow-up of surgical patients at multiple time points up to 5 years postoperatively.

MRI will be used to quantify regional fat and muscle composition, while metabolic parameters, inflammatory markers, and gut microbiota profiles will also be assessed. Primary outcomes include skeletal muscle mass and fat infiltration, visceral and subcutaneous fat volumes, and changes in insulin resistance.

This study seeks to clarify the mechanisms by which bariatric surgery improves metabolic function and to identify early changes in muscle-fat composition that may predict long-term metabolic outcomes.

Inclusion Criteria (For Surgical Group):

• Aged 18 to 65 years (inclusive)
• Meets clinical criteria for sleeve gastrectomy or Roux-en-Y gastric bypass surgery
• Willing and able to comply with study procedures and follow-up visits Capable of understanding and signing the informed consent form

Inclusion Criteria (For Healthy Control Group):

• Aged 18 to 65 years (inclusive)
• Body mass index (BMI) between 18.0 and 24.0 kg/m²
• No history of metabolic disorders (e.g., hypertension, type 2 diabetes, fatty liver disease, or dyslipidemia)
• No history of gastrointestinal surgery
• Able to undergo MRI and willing to provide baseline clinical data and biological samples
• Able to understand and sign informed consent

Exclusion Criteria :

• Type 1 diabetes mellitus or latent autoimmune diabetes in adults (LADA)
• History of pancreatitis, gastrointestinal or pancreatic surgery
• Severe complications of type 2 diabetes or major organ dysfunction that may affect surgical safety
• Active alcohol or drug dependence, severe psychiatric illness, cognitive impairment, or suicidal ideation
• Pregnant or breastfeeding women, or women planning pregnancy during the follow-up period
• Contraindications to MRI examination, including:

Cardiac pacemaker Artificial heart valves Ferromagnetic vascular clips Metallic foreign bodies in the eye

∙Considered unsuitable for the study by investigators based on risk-benefit assessment

This study will enroll adult participants aged 18 to 65 years, including two groups: (1) obese patients undergoing bariatric surgery (either sleeve gastrectomy or Roux-en-Y gastric bypass), and (2) healthy individuals with normal BMI (18.0-24.0 kg/m²) and no metabolic comorbidities. The surgical group consists of patients eligible for metabolic surgery based on national clinical guidelines. The healthy control group is matched by age and sex and will undergo only baseline assessments. All participants must be able to provide informed consent and comply with study procedures. The study population is recruited from Xiangya Third Hospital, Central South University.

De-identified individual participant data (IPD) related to primary and secondary outcomes (e.g., MRI measurements, metabolic indicators, and inflammatory markers) may be shared with qualified researchers upon reasonable request. Data will be made available after publication of the main results and approval by the principal investigator.

Beginning 6 months after publication of primary results; available for up to 3 years.

Requests must be accompanied by a research proposal and data use agreement. Access will be granted to researchers whose proposed use of the data has been approved by the study team.