Author(s): DESORMEAUX, G., BRUNETTA, H., BARBEAU, P.A., COATES, A., PIGNANELLI, C., RAHMAN, F., BURR, J., HOLLOWAY, G., Institution: UNIVERSITY OF GUELPH, Country: CANADA, Abstract-ID: 1557

INTRODUCTION:
Excessive training, or overtraining, has been proposed to impair skeletal muscle mitochondrial function, challenging the prevailing assumption that exercise is uniformly beneficial. However, prior evaluations of mitochondrial bioenergetics after overtraining have not fully considered potential alterations in exercise induced mitochondrial biogenesis, leaving the metabolic consequences of overtraining unresolved.
METHODS:
To address this gap, we investigated skeletal muscle insulin signaling and mitochondrial bioenergetics following a 3 week overtraining protocol in nine highly trained endurance athletes who increased their training volume to ~150% of habitual levels. Muscle biopsies were analyzed using whole muscle quantitative proteomics, permeabilized fiber mitochondrial respiration, H₂O₂ emission assays, and western blotting of oxidative stress markers. Intramuscular insulin signaling was assessed following an oral glucose challenge, and systemic glucose homeostasis and insulin levels were measured separately using an oral glucose tolerance test with serial blood sampling.
RESULTS:
Proteomics analysis revealed an upregulation of proteins related to fatty acid metabolism and mitochondrial content induced by overtraining. Functionally, both maximal complex-I and maximal complex I/II-linked oxidative phosphorylation, as well as H2O2 emission were increased in permeabilized muscle fibres. However, when normalized to mitochondrial content, these effects were largely attributed to increased mitochondrial abundance rather than altered intrinsic function. Despite elevated submaximal mitochondrial H2O2 emission and oxidative stress following excessive training, insulin signaling within skeletal muscle (i.e. Akt phosphorylation) during an oral glucose challenge was improved. At the systemic level, overtraining did not alter blood glucose tolerance or circulating insulin concentrations during an oral glucose tolerance test suggesting excessive exercise induces neither systemic nor skeletal muscle insulin resistance. In a secondary exploratory analysis, participants were stratified based on validated criteria into overreached versus non-overreached phenotypes. This approach revealed a unique proteome signature in individuals who were overreached, marked by smaller increases in proteins involved in cytoskeleton organization, glycogen metabolism, and protein translation. However, despite such classification, we did not observe reductions in either mitochondrial bioenergetics or insulin signaling within skeletal muscle.
CONCLUSION:
Altogether, these findings demonstrate that short-term overtraining in highly trained endurance athletes induces mitochondrial biogenesis without impairing mitochondrial function or skeletal muscle insulin signaling. In addition, a distinct proteome signature is found between individuals who were overreached and those who were not, suggesting a fundamental molecular basis for exercise training adaptation.