Immune-cell metabolomics in frailty reveals mitochondrial dysregulation potentially linked to functional vitamin B12 deficiency

Frailty is a multifactorial condition of reduced physiological reserve and increased vulnerability to adverse health outcomes, in which nutritional deficiencies play a central but underestimated pathophysiological role. While most metabolomics studies in frailty focused on plasma or serum, the intracellular metabolism of immune cells remains unexplored. This study applied untargeted metabolomics and lipidomics to peripheral blood mononuclear cells (PBMCs) and their subpopulations, T lymphocytes (CD3+) and monocytes (CD14+), isolated from elderly subjects to identify metabolic variation associated with frailty.

In 82 subjects aged>65 years frailty and multimorbidity were assessed according to Fried criteria, and considering the presence of at least two chronic diseases, respectively. Participants were categorized as Non-Frail (NFWoM), Frail without multimorbidity (FWoM), and Frail with multimorbidity (FrailtyWM). Cell populations were isolated from whole blood by density gradient centrifugation and immunomagnetic separation. Metabolites and lipids were extracted using methanol-based protocol and analyzed by UHPLC-Q-TOF.

CD14+ captured the most robust metabolic signatures of frailty. FWoM showed increased glycolytic intermediates, energy metabolites, acylcarnitines, and antioxidant-related compounds, alongside enrichment of glycolysis, TCA cycle, and β-oxidation, with respect to NFWoM. Methylmalonic acid (MMA) was markedly elevated in FWoM, suggesting a metabolic pattern compatible with vitamin B12 deficiency via impaired methylmalonyl-CoA mutase. Together with increased 2-hydroxyglutarate and 1-methylnicotinamide, MMA may also reflect mitochondrial dysfunction and pseudohypoxic, pro-inflammatory phenotype. FrailtyWM group showed a similar but attenuated profile. Lipidomics revealed in both frailty groups increased concentration of structural phospholipids and plasmalogens, and depletion of triacylglycerols and fatty acids, indicating membrane remodeling, antioxidant adaptation, and enhanced lipid catabolism.

This study provides the first integrated metabolomic and lipidomic characterization of immune cell subpopulations in frailty, identifying CD14+ as the most informative cell type, and MMA as potential marker linking vitamin B12 deficiency, mitochondrial and metabolic decline, to be explored in future clinical studies.