Most fats seem to be protective against Alzheimer disease

In 1989-99, an association was found, between dietary fat composition and cognitive performance in later adult years: the higher intake of monounsaturated and polyunsaturated fats and the lower intake of saturated fat -- the higher cognitive performance. Another, epidemiologic study conducted in 1997 suggested that high intake of total fat, saturated fat, and dietary cholesterol may increase the risk of dementia.

However, researchers at St Luke's Medical Center, Chicago, Ill found increased risk of Alzheimer's disease among people with high intakes of saturated and trans-unsaturated fats and decreased risk with high intakes of polyunsaturated and monounsaturated fats. Consumption of vegetable fat and a high ratio of polyunsaturated to saturated fats were also protective, whereas total fat, animal fat, and dietary cholesterol had no association with Alzheimer disease.

Sources

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