Human epidemiologic studies provide convincing evidence that dietary patterns practiced during adulthood are important contributors to age-related cognitive decline and dementia risk. Diets high in fat, especially trans and saturated fats, adversely affect cognition, while those high in fruits, vegetables, cereals, and fish are associated with better cognitive function and lower risk of dementia. While the precise physiologic mechanisms underlying these dietary influences are not completely understood, modulation of brain insulin activity and neuroinflammation likely contribute. Not surprisingly, deficits in cognitive functions, especially those dependent on the medial temporal lobes, are apparent in type 2 diabetes mellitus (T2DM). Special care in food selection at meals should be exercised by those with T2DM since ingestion of rapidly absorbed, high-glycemic index carbohydrate foods further impairs medial temporal lobe function, with food-induced increases in oxidative stress and cytokine release likely explaining the association between food ingestion and reduction in cognitive function in those with T2DM.