Metabolomic signatures associated with muscle dysfunction during the SIRIUS-21 space analog mission

Introduction There is growing interest in exploring outer space to expand scientific knowledge and access valuable material resources. However, progress is limited by the harsh space environment, including microgravity, ionizing radiation, isolation and confinement (IC), disrupted circadian rhythms, and nutritional challenges. These stressors contribute to health problems across multiple organ systems, particularly the musculoskeletal, immune, renal/urinary, and cardiovascular systems. Although we have extensive insight into skeletal muscle pathophysiological changes induced by these spaceflight-related stressors, the molecular mechanisms underlying functional alterations caused by prolonged IC in skeletal muscle remain incompletely understood. Methods During the 8-month SIRIUS-21 space analog mission, five participants (3 males, 2 females) underwent plasma and saliva sampling and electromyography (EMG) assessment at five time points: baseline (Ingress), early, mid, and late isolation and confinement (IC), and 14 days post-mission (Egress). Metabolites were analyzed by LC-MS/MS. Longitudinal metabolite changes were assessed using repeated-measures ANOVA with Tukey HSD post hoc testing, and direct metabolite-EMG relationships were explored using Spearman rank correlations. Results Metabolomic profiling identified 45 skeletal muscle-related metabolites with nominally significant longitudinal changes (p < 0.0001–0.05): 8 were detected in both plasma and saliva, 26 only in plasma, and 11 only in saliva. Direct exploratory Spearman correlations with EMG metrics were identified for a subset of metabolites, including pipecolic acid, phenylacetic acid, L-phenylalanine, L-acetylcarnitine, L-glutamine, and pyruvaldehyde. EMG findings showed a progressive decline in muscle activation and fatigue resistance during prolonged IC, with partial recovery after egress. Exploratory sex-related analyses were interpreted descriptively because the cohort included only two females and three males. Conclusion Prolonged IC was accompanied by coordinated changes in muscle function and circulating or salivary metabolites. The identified metabolites represent candidate metabolic signatures associated with muscle dysfunction and require validation in larger independent cohorts before diagnostic or therapeutic utility can be established.