It is known that obesity raises the risk of suffering Alzheimer's (as well as numerous other fatal age-related conditions); the open question is the biochemical mechanisms linking the two. One research team proposes the following: "They found a strong correlation between body mass index and high levels of beta-amyloid, the sticky protein substance that builds up in the Alzheimer's brain and is thought to play a major role in destroying nerve cells and in cognitive and behavioral problems associated with the disease. We looked at the levels of beta-amyloid and found a relationship between obesity and circulating amyloid. That's almost certainly why the risk for Alzheimer's is increased." Given that scientists are still arguing over the role of beta-amyloid in Alzheimer's, this is little too much certainty, but the mechanism is going to be something like this.