Diabetes
There are researchers calling Alzheimer's disease, Diabetes Type 3 because of the close relationship the two diseases demonstrate. Higher than normal gluscose levels affect the smooth muscle lining (endothelial lining) of all the arteries of the body. These cells do not use insulin to regulate their sugar intake. They absorb excessive glucose which weakens the blood vessel walls at the same time thickening the walls. They become brittle much like a plastic garden house left out in the sun. With a surge in (blood) pressure, the hose can no longer expand and will leak fluid out all along its length in a mist. Stiffed blood vessels may starve cells of needed oxygen and nutrients causing conditions like diabetic retinopathy (damage to the retina), diabetic nephropathy (kidney disease) and diabetic neuropathy (damage to nerves).
The tiniest blood vessels (like in the retina) are affected first, blood leakage to surrounding tissue causes inflammation and then cell death which is why uncontrolled or long-term diabetics may have finger and toe amputations. Consider that the blood vessels in the brain (which uses 30% of our blood flow) are far smaller than those of the fingers and toes. The retina is actually are part of the brain. The damage secondary to diabetes to the brain is massive, but occurs on a microvascular scale so small that the effects can not be seen on brain scans. However, old strokes are revealed on brain scans which lends support to a diagnosis of vascular dementia. If there have been old strokes, then new small ones are likely.
Neuroprotective treatments slow down the disease process. Keeping one's blood sugars as close to the normal range with as little fluctuation as possible, is step one. If there is high blood pressure, this is a huge risk factor–remember the garden hose example, above, that leaked with increased pressure. Thus, antihypertensive drugs and therapies (like diet control) are critical in diabetes to help protect the brain.