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.