Dementia encompasses a group of neurodegenerative diseases marked by cognitive decline, memory loss, and diminished ability to perform daily activities. Alzheimer’s disease is the most common form, followed by vascular and frontotemporal dementia. Globally, dementia affects 7% of adults over 50. Its prevalence is increasing and is projected to double in Europe by 2050. As no cure currently exists, prevention through modifiable lifestyle factors is essential. The Lancet Commission has identified 12 modifiable risk factors for dementia, including low educational attainment, hypertension, hearing loss, smoking, obesity, depression, physical inactivity, diabetes, social isolation, excessive alcohol consumption, traumatic brain injury, and air pollution. While diet influences many of these factors, poor diet quality is not yet officially recognized as a direct risk factor due to limited evidence. Nonetheless, healthy dietary patterns—such as the Mediterranean diet—have been associated with a reduced risk of cognitive decline and dementia, although the underlying mechanisms remain unclear. Choline is a nutrient of particular interest. It is obtained through the diet and synthesized in small amounts by the body, though endogenous production is insufficient to meet human needs. Choline plays a critical role in brain function as a precursor to acetylcholine, a neurotransmitter essential for cognition. While maternal choline intake has been linked to improved neurodevelopment in children, evidence of its benefits in older adults is limited. Some studies suggest that higher intake of choline or phosphatidylcholine may enhance memory or reduce dementia risk, but findings are inconsistent. Dietary fat quality also influences health, particularly cardiovascular health. High intake of saturated fatty acids increases disease risk, while polyunsaturated fatty acids are protective, primarily by improving blood lipid profiles. Fat quality may also impact dementia risk through its effects on blood lipids and genetic factors such as the ApoE4 allele, which is associated with Alzheimer’s disease. However, further research is needed to clarify these relationships. The aim of the current study is to investigate the associations between dietary choline and fat quality with cognitive function and dementia risk in two large Swedish population-based cohorts: the Swedish Mammography Cohort (SMC) and the Cohort of Swedish Men (COSM), both part of SIMPLER (Swedish Infrastructure for Medical Population-based Life-course and Environmental Research) project. These cohorts provide extensive data on diet, lifestyle, health, and biomarkers. Participants completed food frequency questionnaires, allowing for the assessment of dietary fat and choline intake. Cognitive tests were conducted in a sub-sample, and dementia diagnoses will be obtained from national health registries. This research will contribute valuable insights into the role of dietary choline and fat quality in cognitive health and support the development of evidence-based dietary recommendations aimed at preventing dementia.