Ketone body metabolism declines with age in mice in a sex-dependent manner

Understanding how our cells maintain energy homeostasis has long been a focus of aging biology. A decline in energy metabolism is central to many age-related diseases such as Alzheimers disease, heart failure, frailty, and delirium. Intervening on pathways involved in energy homeostasis can extend healthy lifespan. When the primary energy substrate glucose is scarce, mice and humans can partially switch cellular energetic needs to fat-derived ketone bodies (i.e., beta-hydroxybutyrate (BHB), acetoacetate, acetone). Aging is associated with glucose intolerance and insulin insensitivity, yet, surprisingly, what role ketone body metabolism might play in compensating for impaired glucose utilization in age-related diseases is understudied. Here, we investigate how endogenous ketone body production and utilization pathways are modulated by age across the lifespan of male and female C57BL/6N mice (3 mo old, 12 mo old, 22 mo old). We show how different ages have different metabolic and gene expression responses to 1-week ketogenic diet (KD). We hypothesized that there would be a compensatory ketogenic response with age but instead saw declines in plasma BHB concentrations under fasting and non-fasting conditions with strong sexual dimorphism. Under KD, both sexes increased BHB concentrations at all ages, but only males showed strong gene expression induction. We also observed tissue-specific changes with age in baseline ketone metabolism, and surprising induction of extrahepatic ketogenic genes under KD. We found significant residual blood concentrations of BHB in KD even after a knockout of liver BHB production. Overall, these findings show that older mice have impaired non-fasting ketogenesis but are capable of increasing their ketogenic capacity under stimulation (i.e., KD) to meet energetic demands in aging. Therapies to augment non-fasting ketogenesis or provide exogenous ketones may be useful to improve energy homeostasis in diseases and conditions of aging.