People with Alzheimer's disease tend to have low levels of a brain protein called LR11 and about 15% of people with Alzheimer's carry a genetic mutation that reduces this LR11.
LR11 helps clear the brain of 'amyloid precursor protein', essential for production of the brain-gumming beta-amyloid plaque that clogs the brains of people with Alzheimer's disease.
In live mice and in cultures of human brain cells, the researchers found that the fish-oil compound DHA causes brain cells to make lots more LR11. This being the case, fish oil should be started AS SOON AS POSSIBLE and as EARLY as possible with diagnosis (or before if you think your pre-disposed!!)
from what I can gather, if you give equal amounts of Omega 3 coupled with equal amounts of Omega-6 fatty acids (which are found in corn, peanut and sunflower oils) this can be of benefit.
There doesn't appear to be any 'dosage' requirements, other than trying to keep a normal 'healthy' diet that include Omega 3 and 6.
Goodluck!