Aged mice exhibit widespread metabolic changes but preserved major fluxes

Metabolic dysregulation is a hallmark of aging. Here, we investigate in mice age-induced metabolic alterations using metabolomics and stable isotope tracing. Circulating metabolite fluxes and serum and tissue concentrations were measured in young and old (20-30 months) C57BL/6J mice, with young obese (ob/ob) mice as a comparator. For major circulating metabolites, concentrations changed more with age than fluxes, and fluxes changed more with obesity than with aging. Specifically, glucose, lactate, 3-hydroxybutryate, and many amino acids (but notably not taurine) change significantly in concentration with age. Only glutamine circulatory flux does so. The fluxes of major circulating metabolites remain stable despite underlying metabolic changes. For example, lysine catabolism shifts from the saccharopine toward the pipecolic acid pathway, and both pipecolic acid concentration and flux increase with aging. Other less-abundant metabolites also show coherent, age-induced concentration and flux changes. Thus, while aging leads to widespread metabolic changes, major metabolic fluxes are largely preserved.