Sex and Age-Related Changes in Lean Mass of C57BL/6 Mice: A New Method for Assessing Body Composition

More than 40% of adults aged 65 and older have one or more physical disabilities that hinder their ability to perform essential daily tasks, significantly impacting their independence and quality of life. This study examines age-related changes in body composition in C57BL/6 mice to explore the relationship between lean mass, total mass, and age-related weakness. Using the EchoMRI-500 body composition analyzer, we measured lean mass and correlated it with independently measured total mass across different age groups (young, middle aged, and old) and sexes (male and female). Our results show that both lean and total mass increase progressively with age in both sexes. However, the lean-to-total mass ratio declines with age, reflecting the reduction in lean mass observed in humans. Additionally, male mice were consistently more muscular than female mice in old age. Specifically, the lean-to-total mass ratios were 0.525 for male mice and 0.203 for female mice. These data provide a simple yet accurate approach for estimating lean mass from total mass, offering a valuable tool for studying age-related weakness.