The ketogenic diet (KD) is a very low carbohydrate, high fat diet that reduces glucose catabolism and enhances β-oxidation and ketogenesis. While research in female rodents is limited, research in male rodents suggests that ketogenic interventions initiated at midlife may slow age-related cognitive decline, as well as preserve muscle mass and physical function later in life. Aging is accompanied by cellular metabolic changes, and thus investigations into the system-wide effects of a KD will inform potential mechanisms related to the anti-aging effects of this diet. Given the dietary restrictions of a KD, such a diet can be difficult to follow. Thus, investigations into the impact of a ketone ester (EST) supplement to mimic some of the metabolic changes of the KD would provide evidence of its efficacy as an alternative for those unable to follow a KD. This dissertation aimed to investigate the effect of a 2-month KD on metabolism in the major metabolic organs of 14-month-old (middle-aged) female C57BL/6 mice, and additionally to determine whether 6 weeks of ketone ester supplementation provided to aged (24-month-old) male and female mice would impact memory, physical function, body composition and metabolism. Serum, liver, kidney, gastrocnemius muscle, as well as cortex and hippocampal brain regions of female mice at 16 months of age (middle-age) were collected after consuming a KD for 2 months, and targeted 1H-NMR metabolomics was conducted. Analysis of the serum metabolome revealed that the 2-month KD resulted in increased concentrations of fatty acid catabolism metabolites, as well as system-wide elevations in ketones. Metabolites involved in the glucose-alanine cycle, antioxidant defenses and energy reserve were altered in skeletal muscle. Other tissue specific alterations were detected, including distinct effects on hepatic and renal one-carbon metabolism, as well as region specific differences in metabolism across hippocampal and cortical parts of the brain. Effects of a 6-week EST supplemented diet on age-related behavioral outcomes and circulating metabolites in 24-month-old (advanced age) male and female C57BL/6 mice revealed post-prandial elevations in ketones and reductions in glucose. Interestingly, in EST mice, greater elevations in ketone concentrations were observed in females compared to males. Sex-specific effects on body composition, memory, and serum metabolites were also detected. In female EST supplemented mice, muscle mass was conserved, while liver weights and the incidence of age-related spleen hypertrophy were reduced compared to controls. In contrast, only body fat percentage was reduced in EST supplemented males compared to controls. In females, EST supplemented mice had improved spatial memory and alterations to serum metabolites related to gut microbial metabolism, distinct from findings in the serum of middle-aged female mice on a KD. This dissertation contributes novel findings related to the systemic metabolic profile of middle-aged female mice on a KD, which is especially important given the lack of metabolomic studies in females. These results also show that a KD produces metabolic changes to serum that are not fully mimicked by a ketone ester supplement. Our findings demonstrate the distinct sex effects of an EST on memory and body composition in aged mice, with the EST preventing age-related memory loss, decline in muscle mass, and splenic hypertrophy in females.