Inclusion Criteria:

• Adults aged 18 to 65 years;
• Formal clinical indication for RYGB or sleeve gastrectomy after multidisciplinary assessment;
• Bariatric surgery scheduled and no previous bariatric procedure;
• Able to understand the study and provide written informed consent.

Exclusion Criteria:

• Moderate or severe hepatic impairment, liver enzymes greater than 3 times the upper limit of normal, or Child-Pugh class B or C.
• Estimated glomerular filtration rate below 30 mL/min/1.73 m².
• Concomitant use of drugs that substantially affect rosuvastatin pharmacokinetics and cannot be temporarily discontinued, including cyclosporine, gemfibrozil, or potent OATP1B1/BCRP or CYP2C9 inhibitors.
• Systemic antibiotic use within 3 months or probiotic, prebiotic, or synbiotic use within 4 weeks before sampling.
• Continuous current rosuvastatin or other statin therapy unless medically discontinued with an adequate washout period under physician supervision.
• Clinically relevant baseline hypotension, significant hyperkalemia, renal impairment, or another condition that makes a losartan test dose unsafe.
• Severe postoperative complication preventing oral dosing, including active gastrointestinal leak, intractable vomiting, or severe anastomotic stenosis.
• Pregnancy. Participants of childbearing potential undergo routine preoperative beta-hCG testing.