This outcome of this study will elucidate how the phenotype of the individual modulates the KE metabolic effect. Most studies of KE have been in homogenous populations, usually young, male athletes. However, two striking experiments using identical, body weight adjusted KE doses in healthy and obese individuals found that BHB area under the curve (AUC) and removal was reduced by obesity and poor metabolic health. Similarly, ketone infusion experiments found that diabetes, obesity, and insulin resistance alter BHB metabolism. It is important to determine how obesity affects KE 'sensitivity' (i.e., breakdown and oxidation) because the increasing prevalence of obesity as a function of age. Age may be another important source of variation in ketone metabolism. The genes that control the ketone system are regulated by a cascade of transcription factors and hormones including PPARα and FGF21, which are themselves known to be affected by aging and dietary status, and the cellular protein sensor target of rapamycin (TOR). Aberrant hyperactivation of TOR with aging may reduce ketogenesis, while it was observed that a long-term ketogenic diet specifically up-regulated PPARα activity. Preliminary work revealed substantial changes across mouse lifespan in the expression of ketone-related genes in the liver such as Hmgcs2 (rate limiting for ketone production) and Bdh1 (rate limiting for BHB oxidation) between young, middle-aged, and old mice, with a nadir of gene expression in middle age before increasing again late in life. Substantial age differences were found in response to matched doses of oral KE in mice and in rats. These data may have important implications for treating people of different ages and for translating KE technologies into the Department of VA. Therefore, this project plans to study individual responses to KE ingestion across the lifespan, against the background of varying metabolic health