The study that I just posted (Study: High Saturated Fat Intake Associated With Worse Memory and Cognition) was from 2012. It was a large epidemiological study out of Harvard. Here’s a recent one, smaller and more controlled. Same finding: saturated fat intake is associated with poorer memory:
Nutrient Intake And Cerebral Metabolism In Healthy Middle-Aged Adults: Implications For Cognitive Aging, Nutritional Neuroscience, Online May 2016
Methods: Thirty-six adults recorded dietary intake for 3 days prior to completing cognitive testing and a proton magnetic resonance spectroscopy (1H-MRS) scan.
Results: Regression analyses revealed that high intake of polyunsaturated fatty acids (PUFAs) was associated with lower cerebral Glu/Cr (P = 0.005), and high intake of saturated fat (SFA) was associated with poorer memory function (P = 0.030) independent of age, sex, education, estimated intelligence, total caloric intake, and body mass index.
This was a relatively young population, average age was 54. They were healthy. An older population with higher prevalence of disease may be even more vulnerable to the effects of saturated fat, as these authors suggest.
They also found that high intake of PUFAs was associated with lower glutamate (Glu) in the brain, and:
Maintaining low cerebral Glu in midlife may be protective against future cognitive decline, as accumulation of extracellular Glu is neurotoxic and may increase brain vulnerability to neurodegenerative diseases.
Unsaturated fats are found primarily in plant foods.