Comparison of basal whole-body protein kinetics and muscle protein synthesis between young and older adults

Significant research has been dedicated to counteracting age-related muscle loss, but underlying mechanisms have not been clearly established. Previous research examining differences in basal protein kinetics between young and older individuals has been limited by a lack of evaluation of protein breakdown and net balance. The aim of this study was to more comprehensively examine differences in basal protein kinetics between younger and older males and females. Basal whole-body protein kinetics and muscle fractional synthetic rate (FSR) from 91 younger (18-38 years; 52% female) and 66 older (51-81 years; 53% female) healthy adults were determined using stable isotope tracer techniques (L-[ring-² H₅ ]phenylalanine and L-[ring-² H₂ ]tyrosine). There were no group × sex interaction effects (p > .05). Older individuals had greater whole-body protein synthesis (mean difference old-young (Δ) ± SE: 28.54 ± 8.15 mg/kg LBM/hr; p = .001) and breakdown (Δ: 15.44 ± 7.33 mg/kgLBM/hr; p = .038), but a less negative net balance (Mean ± SD: Young: -31.22 ± 7.42 mg/kg LBM/hr; Old: -18.11 ± 21.60 mg/kg LBM/hr; p < .001) compared to young individuals. Basal FSR was not significantly different between young and older (Δ: 0.007 ± 0.003%/hr; p = .052). Across the age range, females had greater whole-body protein turnover (PSΔ: 19.10 ± 7.00 mg/kgLBM/hr; PBΔ: 19.22 ± 6.31 mg/kgLBM/hr; p < .01) compared to males. Results demonstrate a difference in basal whole-body protein kinetics between young and older adults, with older adults having a higher protein turnover rate and a less negative net balance. Across the age range, females were also found to have a higher turnover rate compared to males. Differences may represent a shift in older physiology toward mechanisms that increase the efficiency of amino acid reutilization, especially in women.