The aged amygdala's unique sensitivity to refined diets, independent of fat or sugar content: A brain region and cell type-specific analysis.

Western-style diets, high in saturated fats and refined carbohydrates and low in dietary fiber, are strongly linked to cognitive decline, particularly in aging. However, the specific macronutrient contributions and mechanisms underlying these effects remain unclear. Here, we investigated how short-term exposure to refined-ingredient diets (RDs) varying in fat and sugar content impacts memory, mitochondrial function, and metabolic signaling in young adult and aged male rats. A key finding was that amygdala-dependent memory was broadly impaired in aged rats across all RDs, regardless of fat or sugar content, suggesting a unique vulnerability of the aging amygdala to refined dietary ingredients. In contrast, hippocampal-dependent memory impairments were observed only in aged rats fed a high-fat, low-sugar RD. Functional mitochondrial assays revealed significant RD-induced reductions in oxygen consumption in amygdalar and hippocampal mitochondria isolated from aged rats. Cell-type-specific analyses identified aged microglia as particularly susceptible, showing widespread suppression of mitochondrial respiration with limited metabolic flexibility. Astrocytes and synaptic mitochondria exhibited more region- and age-specific effects. All RDs lacked dietary fiber, and consistent with prior findings, butyrate, a microbial-derived short-chain fatty acid, was rapidly and robustly depleted in both gut and circulation, especially in aged animals. Proteomic and phosphoproteomic analyses identified diet-induced disruptions in mitochondrial proteins and synaptic signaling pathways, including complex I subunits and glutamate receptor signaling. Together, these findings reveal that the aged amygdala is especially sensitive to refined diet exposure and highlight microbial, metabolic, and inflammatory pathways that may underlie diet-induced cognitive decline.