I'm currently on a low glycemic index diet, following advice from Bredesen's book The End of Alzheimer's. So far as I know I don't have Alzheimer's, but I do have one copy of the genetic variant that makes it more likely and I have observed what I think is age-related cognitive decline, most notably the fact that I can no longer easily memorize poems. Given the theory behind Bredesen's book, I think his approach has a reasonable chance of helping even if the cause is normal aging rather than Alzheimer's.