APOE4 is the gene tied to elevated Alzheimer risk. About 25% of the population has APOE4, but most people never get tested. Having APOE4 raises your dementia risk, as well as cardiovascular risk -- how much depends on how many copies you have (1 or 2), and, amongst other things, what you eat. There is a study now that shows that, if you have a copy of APOE4, eating MORE MEAT actually lowers your risk of Alzheimer by as much as half! This runs contrary to most historical advice which says meat is bad. It's important to note that the comparison is between groups WITH APOE4, not compared to the general population. If you get the maximum benefit, though, with one copy of the APOE4, you're back at average risk. 

YOU NEED TO KNOW YOUR APOE STATUS! If you don't get tested, you can't know. It matters.