This is a very interesting article that suggest Alzheimer starts outside the brain.
These researchers have identified a series of breakdowns throughout the body that appear to induce the nerve damage responsible for Alzheimer’s symptoms. Alzheimer’s disease develops as neurons in key areas of the outer parts of the brain die. This loss of function is linked with a build-up of a protein called beta-amyloid and the deformation of another protein called tau.
Beta-amyloid proteins are produced when a precursor protein found in the membranes of many different tissues is clipped in two, so technically can be produced anywhere in the body. More importantly, they can cross the near-impenetrable wall of cells separating blood vessels from the brain’s precious grey matter. “The blood-brain barrier weakens as we age,” says Song. “That might allow more amyloid beta to infiltrate the brain, supplementing what is produced by the brain itself and accelerating the deterioration.”