The Role of Comorbidities on Aging-Induced Changes in Muscle Health and Amino Acid Kinetics

Abstract

ObjectivesAging and comorbidities are known to influence amino acid metabolism, contributing to reduced muscle and cognitive dysfunction in older adults. Whether these aging-induced manifestations remain after correction for the presence of comorbidities is unclear. MethodsOlder adults were selected from the MEDIT database. The subjects were divided into two age groups: 264 young old (age 50–70 y: n = 149 with at least 1 comorbidity based on Charlson comorbidity index (CCI > 0) and n = 115 with CCI = 0), and 196 middle old (age 70–85 y: n = 119 with CCI > 0 and n = 77 with CCI = 0). Body composition was assessed by DXA, respiratory and skeletal muscle strength by mouth pressures and dynamometry, and cognition and mood by assessments and questionnaires. Postabsorptive kinetics of multiple amino acids were measured by pulse stable isotope administration and subsequent blood sampling. Plasma amino acid concentrations and enrichments were analyzed by LC-MS/MS to measure whole-body protein production (WBP) and conversion rates. Statistics are done with ANCOVA with sex and BMI and age group as independents. Results expressed as means [95% CI]. ResultsThe middle old had lower lean mass on whole body level and in the extremities (p < 0.0001), and lower respiratory (p = 0.05) and handgrip (p = 0.005) strength compared to the young old, independent of muscle health lowering effects of CCI. Although no differences were observed in cognition and mood between the groups, an age x CCI interaction was found for depression, anxiety (p = 0.03), and lean mass extremity (p = 0.04). The middle old had lower values for WBP of tau-methylhistidine (p = 0.01), hydroxyproline (P = 0.008), and taurine (p = 0.004), and plasma glutamine concentration (P = 0.016). In addition, the middle old showed higher glutamine to glutamate conversion (p = 0.02). Age x CCI interaction was observed for plasma concentration of tyrosine (p = 0.04). No differences were observed in plasma concentrations and WBP of phenylalanine and the branched-chain amino acids (leucine, valine, and isoleucine) between the groups. ConclusionsMuscle loss and weakness and specific disturbances in amino acid kinetics due to aging exist even after correction for the presence of comorbidities. Funding SourcesNone