As evidence of this effect are the results of a study conducted by researchers at Brigham and Women's Hospital, published online May 17 in the journal Annals of Neurology. Women in the study who ate the most saturated fats from foods such as red meat and butter performed worse on tests of thinking and memory than women who ate the lowest amounts of these fats.

The exact reason for the connection between diets high in saturated and trans fats and poorer memory isn't entirely clear, but the relationship may be mediated by a gene called apolipoprotein E, or APOE. This gene is associated with the amount of cholesterol in your blood, and people with a variation of this gene, called APOE e4 are at greater risk for Alzheimer's disease. "About 65% of individuals who wind up with dementia due to Alzheimer's disease in their 60s and 70s have that gene," says Dr. Gad Marshall, assistant professor of neurology at Harvard Medical School.

How does the APOE e4 gene contribute to dementia? Researchers aren't exactly sure, but they have discovered that people with this genetic variation have a greater number of sticky protein clumps, called beta-amyloid plaques, in the brain. These plaque deposits, which are associated with the destruction of brain cells, are a hallmark of Alzheimer's disease.

The connection is a little clearer when it comes to memory loss that's related to blood vessel damage. The buildup of cholesterol plaques in brain blood vessels can damage brain tissue, either through small blockages that cause silent strokes, or a larger, more catastrophic stroke. Either way, brain cells are deprived of the oxygen-rich blood they need to function normally, which can compromise thinking and memory.