The ketogenic diet has been around for 80 years, and it works. It has been used to get kids with epilepsy off of drugs through diet alone. It has been shown to help children with seizures.
An extremely high-fat (ketogenic) diet has been found to improve cognitive ability in Alzheimer's patients. These and other observations described below lead me to conclude that both a low-fat diet and statin drug treatment increase susceptibility to Alzheimer's.
Listen to a panel discussion with five medical doctors, Gary Taubes, and Robb Wolf discuss coconut oil and Alzheimer's Disease.
There are long-term studies on the effects of a high fat diet, and one of them is on the US Government's website.