Background: Dietary fat intake is a compelling focus for prevention of late-life cognitive decline; it is readily modifiable and may have significant impact on both general health and cognitive function.They found that monounsaturated fat was related to less decline than saturated fat. When people think "monounsaturated fat" they think olive oil. Unfortunately, olive oil contains saturated fat. Two tablespoons of olive oil contain the same amount of saturated fat as 2 eggs or a cubic inch of cheddar cheese.
Methods: We examined prospectively the relations of major dietary fatty acids (FAs) (saturated [SFA], mono-unsaturated [MUFA], total poly-unsaturated [PUFA], trans-unsaturated); n-6 and n-3 (omega-3) FAs; and the n-6:n-3 FA ratio to late-life cognition. Among 6,183 Women's Health Study participants aged 65-95 years, we measured baseline fat intake using a validated semi-quantitative food frequency questionnaire and assessed cognition 3 times over 5 years.
Conclusions: Higher saturated fat intake is associated with worse 5-year cognitive decline, while higher MUFA intake is related to less decline. Higher omega-3 intake is associated with better episodic memory over time, while higher n-6:n-3 ratio is associated with decline. These results strongly support the hypothesis that the balance of different fat types is relevant to cognitive aging.
Foods that are naturally high in monounsaturated fat and also low in saturated fat include vegetable greens, rice, corn, chickpeas - foods that are naturally low in fat.
Update: Here's another study, except in women with type 2 diabetes. Women, by the way, have a higher prevalence of both type 2 diabetes and cognitive decline than men. Same finding, saturated fat leads to cognitive decline:
Dietary Fat Intake and Cognitive Decline in Women With Type 2 Diabetes, Diabetes Care, 2009
We evaluated cognitive function in 1,486 Nurses' Health Study participants, aged ≥70 years, with type 2 diabetes.You can't say that their diabetes led to their cognitive decline because the researchers analyzed diet before and after diabetes diagnosis and found the same link. (You might say that it contributed though.) You might also say, as they did, that the same processes that caused the diabetes also caused the mental decline:
Conclusions: In women with type 2 diabetes, we found that higher intakes of saturated and trans fat were related to substantially worse cognitive decline. ... The magnitude of these results was considerable, and the associations we observed were equivalent to the cognitive effects we find for 6–7 years of aging in these women.
Importantly, these relations were similar before and after diabetes diagnosis.
To our knowledge, this is the first large-scale prospective study to examine dietary fat intake in relation to cognitive decline among type 2 diabetic subjects.
"Higher intakes of saturated and trans fat and lower intakes of mono- and polyunsaturated fat can contribute to insulin resistance and an atherogenic lipid profile. Moreover, insulin resistance, high insulin levels, and cholesterol are all implicated in β-amyloid accumulation in the brain—the pathologic hallmark of Alzheimer's disease."