Consumption of sucrose-rich and fat-rich foods poses an in increased risk of obesity, and has been shown to be associated with cognitive decline, reduced cortical volume, and dementia. The goal of this thesis was to determine the effect of diet on metabolic measures, gut microbiota, brain macrostructure, cognitive function, and dementia biomarker expression in middle-aged female mice. In Chapter 2, high sucrose diet exposure did not alter metabolism, but increased liver triglyceride concentration and altered gut microbiota relative abundance. We also show that sucrose fed mice experienced a significant decrease in the volume of the parietal temporal cortex, and a deficit in cognitive flexibility. In Chapter 3, high fat diet exposure induced significant differences in weight, glucose sensitivity and plasma leptin levels, without altering the expressions of phosphorylated tau. Together, these studies demonstrate that diet, without affecting metabolic measures, can alter cortical volume and cognitive function.