MRI scan of brain
The NYT’s Well blog has the details. The brain’s hippocampus is a critical center for memory. Alzheimers disease is associated with a gene called apo-E4. Carriers of that gene who exercise regularly have less shrinkage of the hippocampus than non-exercisers.
To PROVE that regular exercise prevents dementia-related shrinkage of the hippocampus, you’d have to force some folks to exercise and stop others who wanted to exercise. A couple years later, scan their brains and compare the two groups. That study may never be done.
The Mediterranean diet also seems to prevent or forestall dementia.
Steve Parker, M.D.
Diabetic Mediterranean Diet 
This is great news for those who work out and exercise every day. People who don’t need to consider why a little exercise goes a long way and start moving.