Body Composition Changes After Initiation of GLP-1 Receptor Agonists in Adults With Obesity or Type 2 Diabetes

Researchers want to understand what happens to muscle and fat in the body after adults with obesity and/or type 2 diabetes start taking a GLP-1 receptor agonist medication (GLP-1RA), a type of drug already approved and commonly prescribed for weight loss and blood sugar control. This is not a drug trial. No experimental medication will be given. Instead, the study will follow people who are already starting this treatment or having their dose adjusted, as part of their regular medical care, and it will not change or interfere with any decision made by the participant's own doctor. These medications help people lose weight, but not all weight loss is the same. Some of it can come from muscle instead of fat, and losing muscle can affect strength and daily function over time. This study will use a scan called DXA (dual-energy X-ray absorptiometry.) to measure how much muscle and fat change over about one year, along with tests of grip strength, walking speed, and everyday physical performance. About 168 adults will take part at two centers in Mexico: the Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán in Mexico City and the Instituto de Diabetes, Obesidad y Nutrición in Cuernavaca. After an initial visit, participants will have a phone check-in at 12 weeks and follow-up visits at 24 and 52 weeks, including blood tests, body measurements, and short questionnaires about diet, physical activity, and muscle health. Some participants may also choose to give an extra blood sample for optional genetic testing; this is voluntary and does not affect participation in the main study. There is no placebo, randomization, cost, or payment involved. All treatment decisions remain with the participant's own physician. By observing these changes under real-world conditions, the study aims to help doctors better protect muscle while patients lose fat during treatment with these medications.

Inclusion Criteria: 1. Agrees to participate in the study by signing informed consent 2. Age 18 years or older 3. Diagnosis of obesity and/or type 2 diabetes mellitus, documented in the clinical record 4. Receiving, on indication of the treating physician and as part of routine clinical care, a therapy with activity on the GLP-1 receptor, including a) GLP-1 receptor monoagonists, b) dual GIP/GLP-1 coagonists, or c) other multiple coagonists with activity on the GLP-1 receptor holding current health authorization for clinical use in Mexico 5. Meeting one of the following conditions at the time of recruitment: a) starting an eligible therapy during the recruitment period, or b) being in an active dose titration phase, with an upward dose escalation documented by the treating physician, even if treatment started four or more weeks earlier, provided the participant has not yet reached a stable maintenance dose 6. Willingness and ability to attend follow up evaluations at 24 weeks (plus or minus 3 weeks) and 52 weeks (plus or minus 6 weeks) Exclusion Criteria: 1. Diagnosis of type 1 diabetes or other specific forms of diabetes 2. Pregnancy or breastfeeding at the time of recruitment 3. Conditions that limit adequate measurement of body composition by DXA, including body weight above the equipment's technical limit of 159 kg, the presence of metallic devices or materials that interfere with the measurement, or having undergone a radiologic contrast study within the 7 days before the evaluation 4. Acute illness at the time of recruitment that could interfere with the study evaluations or with longitudinal follow up, including hospitalization within the 4 weeks before the baseline visit, active pneumonia, acute COVID-19 infection, diabetic ketoacidosis, or major surgery within the 6 weeks before recruitment 5. Chronic diseases or conditions associated with severe alterations in body composition, such as active cancer, active systemic inflammatory disease, advanced heart failure, or kidney disease on replacement therapy 6. Current or recent use, within 3 months, of systemic corticosteroids at chronic pharmacologic doses or medications with a direct modulating effect on muscle mass 7. Current or recent use, within 3 months before the index date, of weight loss medications other than GLP-1RA, such as phentermine, topiramate, or naltrexone/bupropion 8. Simultaneous participation in a double blind randomized clinical trial