From ScienceDaily, indicators as to why it is that exercise improves resistance to Alzheimer's - by increasing blood flow to the brain: "there's a great deal of evidence to suggest that Alzheimer's disease is a problem having much to do with the vascular plumbing. ... . It's not simply that reduced blood supply hurts brain cells by causing a shortage of oxygen and other nutrients. Rather, deterioration of blood flow seems to gum up the brain's ability to remove toxic amyloid beta. Normally, amyloid is picked up efficiently by blood vessels that then whisk the toxic trash away. But in Alzheimer's disease, the system no longer is able to keep up with the body's production of the substance. The molecular trash accumulates, and [researchers] believe the buildup kills brain cells." The researchers here show that two proteins linked to the cardiovascular system change both blood flow and the ability to clear amyloid beta. It's worth remembering that Alzheimer's is a multifaceted, complicated condition with numerous contributing issues: see for example the convincing evidence for involvement of degeneration in the choroid plexus, diabetes-like lifestyle conditions, a herpes simplex virus, and so forth.