Data from the Women's Health Study, involving 6,183 older women (65+), has found that it isn’t the amount of fat but the type of fat that is associated with cognitive decline. The women were given three cognitive function tests at two-yearly intervals, and filled out very detailed food frequency surveys at the beginning of the study.
Women who consumed the highest amounts of saturated fat (such as that from animals) had significantly poorer cognitive function compared to those who consumed the lowest amounts. Women who instead had a high intake of monounsaturated fats (such as olive oil) had better cognitive scores over time. Total fat, polyunsaturated fat, and trans fat, were not associated with cognitive performance.
The findings are consistent with research associating the Mediterranean diet (high in olive oil) with lower Alzheimer’s risk, and studies linking diets high in saturated fats with greater cognitive decline.
 . Dietary fat types and 4‐year cognitive change in community‐dwelling older women. Annals of Neurology [Internet]. 2012 . Available from: http://onlinelibrary.wiley.com/doi/10.1002/ana.23593/abstract