UC Berkeley

BACKGROUND: D3-creatine (D3Cr) dilution provides an accurate estimate of total body skeletal muscle mass, yet few studies have examined its relationship with function and performance across the lifespan, particularly before age 70. We modelled the association of age with D₃Cr muscle mass across adulthood and compared it with age-related differences in muscle function and performance. METHODS: Adults aged 30-69 years (n = 69; 33 men) and 70+ (n = 826; 344 men) from the Study of Muscle, Mobility and Ageing completed assessments of D3Cr muscle mass, magnetic resonance imaging (MRI) thigh muscle volume, 1-RM leg strength and leg extension power, 4 m walking speed and cardiorespiratory fitness (VO2 peak). Regression models estimated annualised percent differences with age for each outcome. RESULTS: In men, progressively lower D₃Cr muscle mass with advancing age (-0.5%/year in young adults to -1.4%/year in oldest-old) paralleled the pattern observed for leg strength and walking speed (P > .05). Larger age-associated differences were observed for leg power and VO₂ peak, and smaller for MRI thigh muscle volume. In women, D₃Cr muscle mass was already lower in young adulthood (-0.6%/year) and remained relatively stable thereafter, a pattern similar to MRI thigh muscle volume. However, age-related differences in strength, power and VO₂ peak in women generally exceeded those in D₃Cr muscle mass, while walking speed aligned more closely. CONCLUSION: Age-associated differences in muscle mass tracked closely with strength (men) and functional capacity. These results indicate that the role of low muscle mass in functional decline has been underestimated, emphasising the importance of strategies to preserve/enhance muscle mass throughout adulthood.