Objectives: Muscle mass decline is common among adults, and in certain groups, it may occur earlier. This study evaluated the change in skeletal muscle mass (SMM) and relative skeletal muscle mass (RSMM) in three population groups (rural, rural-to-urban migrant, and urban). Materials and methods: Secondary analysis of the PERU MIGRANT study, a cohort with 8 years of follow-up. SMM was assessed using the Lee equation, and RSMM was determined by dividing SMM by body weight, whereas the exposure was the population group. The change in MME and MMER in the three groups of interest was assessed using linear mixed models, and coefficients (β) and 95% confidence intervals (95% CI) were reported. Results: A total of 989 subjects (rural: 201, migrant: 589, and urban: 199) were analyzed, with a mean age 48.0 (SD: 12.0) and 522 (52.8%) women. At baseline of the cohort, SMM was higher in urban dwell...