The relationship between key energy metabolites and brain health is not well understood. We investigated the association between circulating ketone bodies, pyruvate, and citrate with cognitive decline, structural brain characteristics, and risk of dementia. We measured ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone), pyruvate, and citrate species using NMR in plasma samples from 1,850 older adults in the Cardiovascular Health Study collected in 1989-90 or 1992-93. Cognitive decline was assessed using the modified Mini-Mental State Examination and the Digit Symbol Substitution Test. Dementia was adjudicated by a committee of experts through comprehensive evaluations including cognitive tests, medical records, and interviews with the next of kin. Dementia-related mortality was confirmed by a committee using death certificates and other clinical data from hospitalization. Multivariable linear mixed models were used to assess 9-year cognitive decline, while multivariable Cox regression models evaluated 6-year dementia incidence and 22-year dementia-related mortality. White matter lesions and ventricular size were measured using MRI in 1992-94 and were analyzed using multivariable linear regression models. Higher plasma levels of ketones, particularly β-hydroxybutyrate, were associated with faster cognitive decline (β, -0.10; 95% CI, -0.15 to -0.05; P<.001) and dementia-related mortality (HR per SD, 1.29; 95% CI, 1.07 to 1.56; P=0.023). Higher pyruvate concentrations were associated with slower cognitive decline, smaller ventricular size, lower dementia risk (HR per SD, 0.87; 95% CI, 0.77 to 0.97; P=0.013; P=0.073), and lower dementia mortality. Higher citrate levels were associated with less cognitive decline and lower dementia risk. In adults aged 65 years and older, circulating ketone bodies are associated with faster cognitive decline and higher dementia mortality, while pyruvate and citrate are associated with lower dementia risk.