Metabolic Adaptations to Caloric Restriction: Time- and Group-Dependent Metabolomic Signatures from the CALERIE™ Trial

Background : Caloric restriction (CR) improves markers of biological aging, yet long-term effects on the human metabolome remain unclear. Objective: This study examined the effects of CR (2 years) in healthy adults without obesity on circulating metabolites linked to aging and metabolic adaptations. Methods: Untargeted metabolomics was performed using fasted plasma samples collected at baseline, 12, and 24 months (BL, 12M, 24M) from CALERIE™ participants randomized to CR or ad libitum (AL) control. A total of 864 known metabolites were identified and grouped into nine biologically coherent super pathways to support pathway‑level interpretation (amino acid, peptide, carbohydrate, energy, lipid, nucleotide, cofactors and vitamins, xenobiotics, and partially characterized molecules). Principal component analysis (PCA) summarized metabolite variation, and linear mixed models assessed intervention effects on each PC in group-by-time interactions. Results: Three principal components showed significant group‑by‑time interactions: PC2 (carbohydrate), PC5 (partially characterized molecules), and PC4 (lipid). Carbohydrate (PC2) and partially characterized metabolites (PC5) decreased from baseline to 12M in both groups; from 12M to 24M, levels stabilized in CR but increased in AL for PC2, while PC5 continued to decline in AL and increased in CR. Lipid metabolites (PC4) decreased in CR and increased in AL at 12M, with the pattern reversing from 12M to 24M. Key contributors included malto‑saccharides and related carbohydrate intermediates for PC2, glutamine degradants and lactone sulfates for PC5, and sphingolipids for PC4. Conclusion: Calorie restriction produced distinct, time‑dependent shifts in carbohydrate and lipid metabolism, with early reductions during the weight‑loss phase followed by stabilization or compensatory responses during weight maintenance. These dynamic metabolic changes may relate to inflammation‑linked mechanisms. Further work is needed to distinguish CR‑specific adaptations from dietary influences and to clarify the functional significance of these metabolic responses for aging and long‑term metabolic health.