It's been shown that alzheimer's is caused by the build-up of beta amyloid plaque, which are abnormally formed proteins. These plaques are in turn believed to be caused by small (also abnormal) fragments of amyloid precursor proteins (APP). These fragments result when a signaling protein on the cell's surface is cleaved incorrectly by various enzymes. It's been shown that vitamin E can reduce the presence of these APP fragments and protect brain neurons from glutamate toxicity. It's important to take a "natural" vitamin E source. It's been shown that the alpha, gamma, delta, etc.. forms all exert an effect.
Selenium is also an important factor, all alzheimer's patients tested had low levels of selenium.
Most noteworthy is that pigs get a "form" of alzheimer's called encephalomalacia and the historic vetrinary treatment was vitamin E and selenium. Encephalomalacia in pigs is generally associated with a virus, but that may be a result of deficient levels of selenium, an extremely important mineral for the immune system.
Dosage: Vitamin E 1500iu daily, Selenium 400mcg daily.
400mcg is approx 330% of the RDA, and 1500 IU is about 5000% of the RDA. They are both safe at these dosages. Megadosing with vitamin E was only found to be problematic when supplementing with just one form. When the various forms alpha, gamma, delta... of vitamin E are taken together in fairly large dosages there are only benefits.
Take a good multivitamin/mineral supplement (the best and most complete you can find) along with the vitamin E and selenium.