A series of scientific research studies since 2004 point to the possibility that Alzheimer's disease might be prevented, or at least ameliorated, by consumption of the chemicals abundant in apples and other fruits and vegetables.
The first of these studies was done by doctors at the University of Massachusetts at Lowell. They found that human brain cells (that they acquired as a byproduct of brain surgery) don't make the "tangling" proteins characteristic of Alzheimer's when they are exposed to apple juice concentrate. Obviously, you can't pour apple juice concentrate directly into your brain, but the University of Massachusetts research raised questions about whether apples and other fruit might prevent age-related memory loss and preserve brain health.
Just a year later, British scientists were reporting that glucosinolates, a family of compounds found in apples, oranges, and potatoes but especially in broccoli and dried broccoli sprouts, helped prevent destruction of the brain chemical acetylcholine. In Alzheimer's disease, too much destruction of acetylcholine (by the enzyme acetylcholinesterase) prevents signals from "jumping" across the synapses from one brain cell to another.
One of the best sources of glucosinolates, by the way, is the green under the skin of an old potato--but despite what you may read in some well-intentioned health articles, you really shouldn't eat the green flesh under an aging potato's skin. It has so many of these chemicals it can actually overstimulate the brain, and in pregnant women it can cause birth defects.
After the finding in 2005 that the brain-protective chemicals in apples also appear in oranges, potatoes, and broccoli, scientists at the University of Massachusetts then reported that mice given apple juice don't develop the "brain tangles" that cause a mouse's version of Alzheimer's. More importantly, the scientists recognized that reason was that apple juice helps the brain make SAM-e (S-adenosylmethionine). Other researchers had already found that the brain in Alzheimer's doesn't make enough SAM-e.
Put altogether, what does this tell us?
First of all, while it's certainly not proven, it just may turn out to be true that an apple a day can keep Alzheimer's away. And if it doesn't completely prevent Alzheimer's, it may slow it down.
Secondly, there are lots of brain-benefiting fruits and vegetables besides apples, and they aren't especially exotic or expensive. Oranges, broccoli, and potatoes also seem to help.
Thirdly, all these beneficial effects are tied to folic acid, which was named after the "foliage" in which it frequently occurs. Spinach, for instance, is a good source of folic acid, as are the yeasty Australian sandwich spread Marmite, beans, peas, liver, and any vitamin-fortified cereal.
Finally, folic acid is assisted by vitamin B-12. Elderly people can become deficient in vitamin B-12 as their digestion slows down. Vitamin B-12 is only extracted from food with the help of stomach acid, and reduced production of stomach acid, as well as taking antacids for GERD and heartburn, lead to vitamin B-12 deficiencies in people over 60. In a few cases, supplemental vitamin B-12 has made profound differences in behavior, for the better, in people who already have Alzheimer's.
You may also be interested in Which Orange Juice Has the Most Vitamin C?.