The Mars500 study provided a unique long‑term human dataset to investigate how chronic high salt intake influences renal and hepatic metabolism. In the present analysis, urinary metabolite profiles were quantified by a fully validated LC‑MS/MS panel that included amino acids, organic acids, acylglycines and bile acids. The panel covered all metabolites measured in the urine samples of the Mars105 and Mars520 participants. The data revealed a clear activation of the hepatic urea cycle in response to increased sodium intake. This was evidenced by a significant reduction of the arginine/ornithine ratio, a marker of urea cycle activity, when sodium excretion rose. The decrease in this ratio was not accompanied by a proportional increase in urinary urea, indicating that the enhanced urea production served primarily to facilitate sodium‑driven water reabsorption rather than to excrete excess nitrogen. In addition, the excretion of α‑aminoadipate and glutamate, both intermediates of lysine catabolism and a nitrogen donor for the urea cycle, was elevated, further supporting increased protein catabolism from muscle as the nitrogen source for urea synthesis.

Metabolic shifts consistent with a switch to fatty‑acid oxidation were also observed. Urinary acylglycines derived from the catabolism of the branched‑chain amino acids leucine, isoleucine and valine were markedly reduced under high salt conditions, suggesting a decreased reliance on these amino acids for energy. Correspondingly, ketone bodies were increased: 3‑hydroxybutyrate and α‑hydroxybutyrate were found in higher concentrations, indicating enhanced hepatic ketogenesis. No significant changes were detected in markers of gluconeogenesis, supporting the hypothesis that high salt intake suppresses glucose production and promotes lipid oxidation to meet the energetic demands of osmolyte synthesis.

Bile acid metabolism also responded to sodium loading. Both unconjugated and sulfated bile acids were elevated in the urine, implying increased hepatic synthesis and altered renal handling. The rise in bile acids, particularly taurine‑conjugated lithocholic acid, may activate the farnesoid‑X receptor (FXR) and the G‑protein‑coupled bile acid receptor TGR‑5, pathways that have been linked to the regulation of aquaporin‑2 expression and water reabsorption. The simultaneous increase in hydrophobic bile acids and the observed changes in urinary organic acids suggest a coordinated hepatic‑renal response to maintain fluid balance under chronic salt loading.

The project was funded by the German Federal Ministry of Education and Research (BMBF) under grant number 50WB2024. It involved collaboration between the Children’s Clinic Erlangen, where the LC‑MS/MS assays were developed and performed, and the bedrest study team in Toulouse, who provided complementary data for validation. The research team, led by Dr. J. Titze, coordinated the analytical workflow, data integration, and pathway analysis. Over the course of the project, more than 20 peer‑reviewed papers were published and presented at international conferences, and a comprehensive manuscript detailing the metabolomic findings is currently in preparation. The final report, submitted in 2024, summarizes the mechanistic insights into sodium‑driven water conservation, the role of the urea cycle, and the interplay between bile acid signaling and renal water handling in humans.