An apple a day. That age-old advice still makes a lot of sense. Apples contain an ample dose of fiber (two-thirds of which is in the peel), some potassium, and a modest amount of vitamin C. One medium apple is also a good source of flavonoids, which protect nerve cells from oxidative stress, suggesting that this fruit may help prevent Alzheimer's and other degenerative brain diseases. Enjoy them in their natural state or in one of these tasty dishes.