Global skeletal muscle metabolomics reveals mechanisms behind higher response to resistance training in older adults

Resistance training (RT) is a highly effective intervention for combating frailty by improving muscle mass, strength, and function in aging. Older adults often show heterogeneous muscle-related responses to RT. The purpose of this study was to discover how responsiveness to RT manifests in muscle-specific metabolomic responses in a cohort of older adults.

This study is a secondary analysis on the vastus lateralis muscle biopsies collected from a completed RT and whey protein supplementation trial. We utilised data from a total of 50 participants who performed unilateral knee extensions twice weekly for 10 weeks. One leg completed 1 set, and the other completed 4 sets of 8–15 repetitions. We analysed the 4-set condition, previously shown to induce greater muscle hypertrophy. Response variability was assessed using MRI-measured muscle cross-sectional area (CSA) changes. Responders were defined as those with hypertrophy exceeding the 1.7% method error. Quadriceps CSA in the lower responders (LowR) increased from 53.6 ± 12.1 cm² to 55.4 ± 12.8 cm² after 10 weeks of RT (3.3 ± 1.7%, P < 0.001), and in higher responders (HighR) from 53.7 ± 12.5 cm² to 59.2 ± 13.6 cm² (10.3 ± 2.0%, P < 0.001).

Muscle biopsies were taken from the vastus lateralis before and after RT. We performed untargeted LC-MS metabolomics to investigate changes in muscle metabolic regulation. Partial least squares discriminant analysis (PLS-DA) using polar extracts achieved a 75% average correct classification rate for predicting HighR and LowR, validated using 1,000 bootstraps. We then performed N-way ANOVA on log-transformed metabolic features to test for differences before and after RT in HighR (n=25, mean age 67±4 years) vs. LowR (n=25, mean age 69±5 years).

There were no significant differences in the baseline metabolomic profile. HighR participants showed greater relative levels of amino acids (e.g., isoleucine, leucine, valine, phenylalanine, lysine, glutamine, methionine, tyrosine, citrulline, tryptophan, kynurenine, indole) and gut-related metabolites (choline, indole, kynurenic acid, adrenaline, isoprenaline) (FDR < 0.05). Several gut-derived metabolites were significantly elevated in HighR, including indole metabolites, 4-hydroxyhippurate, proline, and stachydrine (FDR < 0.05). Pathway enrichment using Mummichog revealed significant enrichment of tyrosine, aspartate, and tryptophan metabolism (P-Fisher < 0.05).

Our findings identify branched-chain amino acid catabolism, tryptophan metabolism (indole and kynurenine pathways), the TCA cycle, gut-derived metabolites, carnosine, and acylcarnitine metabolism as prominent pathways disrupted in LowR. Metabolomics offers potential to improve intervention strategies to reduce sarcopenia and frailty in aging.

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