The British Medical Journal have an article at the moment that concludes that tackling diabetes and depression may be key to preventing this disease.
I suppose that although this sound a bit odd, the old song that went "The thigh bone is connected to the knee bone.." etc. must be true and it makes sense that all of our systems have some sort of interdependence, so that one disease might trigger another, for example. I can see the link between depression and Alzheimer's because they are both neurological disorders, and I suppose since diabetes is a systemic disease, that explains the link to Alzheimer's. Whatever the link however, one thing is clear, we should try to keep ourselves as healthy as possible.