HMB Plus Vitamin D to Preserve Muscle in Older Adults Starting Semaglutide

This randomized, double-blind, placebo-controlled trial will evaluate whether daily calcium beta-hydroxy-beta-methylbutyrate (calcium-HMB) plus vitamin D3 preserves muscle mass, physical function, and resting metabolism in adults aged 65 to 85 years with type 2 diabetes who are newly starting semaglutide as part of usual clinical care. Participants will receive calcium-HMB plus vitamin D3 or matching placebo for approximately 6 months. The primary outcome is change in appendicular lean mass measured by dual-energy X-ray absorptiometry from baseline to month 6. Secondary assessments include body composition, muscle strength, physical performance, gait, physical activity, resting metabolic rate, metabolic measures, and whole-body quantitative computed tomography. An optional mechanistic substudy of up to 36 participants will include D3-creatine dilution, nonradioactive stable isotope infusions, repeated blood sampling, a standardized meal, breath sampling, and vastus lateralis muscle biopsies at baseline and month 6.

Inclusion Criteria: * Age 65 to 85 years, inclusive. * Type 2 diabetes mellitus, defined by HbA1c at or above 6.5% or current use of antihyperglycemic medication. * Body mass index at or above 27 kg/m2. * Newly initiating semaglutide as standard clinical care under the direction of a treating clinician. * Able to walk 400 meters without assistance; use of a cane is permitted. * Able to understand and provide legally effective informed consent. * English speaking for the initial study because the consent documents, questionnaires, study-product instructions, safety instructions, and procedure-specific materials are available only in English. Exclusion Criteria: * Estimated glomerular filtration rate below 45 mL/min/1.73 m2. * Use of HMB, creatine, or high-dose whey protein supplements within the previous 3 months. * Weight change greater than 5% during the previous 3 months. * Recent fracture within the previous 6 months. * Neuromuscular disease, including Parkinson disease, or another condition that would interfere with study outcomes or safe participation. * Chronic corticosteroid use. * Personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2. * History of pancreatitis. * Smoking more than 7 cigarettes per day. * Previous malabsorptive or restrictive intestinal surgery or a malabsorptive syndrome. * Pregnancy or breastfeeding. * Recent history of neoplasia within the previous 5 years, as defined by the protocol. * Other active inflammatory conditions or neuromuscular disorders that could interfere with study outcomes or safety. * Serum 25-hydroxyvitamin D below 15 ng/mL. * Any medical, functional, laboratory, medication-related, or safety condition that the investigator determines makes study participation unsafe or prevents completion of essential procedures.