Diet and Alzheimer's Disease: What the Evidence Shows

Martha Clare Morris, ScD

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In This Article

Dietary Fat Composition

Another promising area of study involves the effect of dietary fat composition on the risk of Alzheimer's disease. The composition of fat in the diet is known to affect blood cholesterol levels. In metabolic studies, diets with a high ratio of saturated fat to polyunsaturated or monounsaturated fats resulted in a poor blood cholesterol profile, characterized by high levels of low-density lipoprotein and low levels of high-density lipoprotein cholesterol.[12] Consumption of transunsaturated fat, obtained from partially hydrogenated vegetable oils in commercially baked products, is particularly hypercholesterolemic.[13]

Although the biochemical mechanism is not yet identified, cholesterol appears to be an important component in Alzheimer's disease and is involved in both the generation and deposition of A-beta.[14] One of the more important genetic risk factors for Alzheimer's disease, the apolipoprotein E-ε4 allele (APOE-ε4), is the principal cholesterol transport in the brain.

Several lines of evidence support the theory that an elevated blood cholesterol level is related to the development of Alzheimer's disease. In experimental models, animals fed high-fat and high-cholesterol diets exhibited impaired learning and memory performance compared with animals on control diets and also demonstrated more A-beta deposition in the brain, greater loss of neurons, and other Alzheimer's disease-related neuropathology.[15,16] One study of 444 Finnish men found that an elevated blood cholesterol level (> 6.5 mmol/L) in midlife was associated with 3 times the risk of developing Alzheimer's disease in late life.[17]

Two recent studies of patients who had been prescribed statin drugs found a significantly lower risk of Alzheimer's disease compared with similar patients who were not prescribed these medications.[18,19] Whether the observed reduction in Alzheimer's disease resulted from cholesterol lowering or some other property of these medications remains to be seen as the findings of related studies emerge.

The 3 prospective dietary studies conducted in Chicago,[20] New York,[21] and Rotterdam[22] also examined the relation of dietary fat intake to the development of Alzheimer's disease. The Chicago study reported the strongest evidence of an association. High intake of saturated fat doubled the risk of Alzheimer's disease, and even moderate intake of trans fat increased the risk by 2 to 3 times.[20] By contrast, higher intake of both polyunsaturated and monounsaturated fats was associated with lower risk of developing Alzheimer's disease.

The New York study found evidence of a greater 4-year risk of Alzheimer's disease for those with higher intakes of total fat and saturated fat but no evidence of an association with the intake of polyunsaturated fat.[21] Investigators for the Rotterdam study also found an increased risk of disease with higher intakes of total fat, saturated fat, and cholesterol after 2 years of follow-up,[22] but none of the dietary fats was associated with Alzheimer's disease after 6 years of follow-up.[23] Further study will be required to understand the inconsistent findings across studies and to determine whether the composition of fat in the diet is causally related to risk of Alzheimer's disease.

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