Isn't aluminum natural?
It is! It's the third-most-common element found in the Earth's crust, and everyone is basically exposed to it all the time through food and water. In fact, you've got some in your system right now.
The problem is that it's a well-known neurotoxin, one that may be associated with Alzheimer's disease, and can hang out in your brain, parathyroid, and bones -- though it's important to note that no one has shown aluminum to be a direct cause of Alzheimer's. In any case, researchers say the connection shouldn't be ignored, because there may be a real link here, and why hoard aluminum if you don't have to?
Also, aluminum toxicity has been studied time and time again in laboratory animals as well as from human patient data. The nervous system, which of course includes the ever-important brain, seems to be sensitive to excess aluminum. There have also been studies that explore the brain's uptake of aluminum as it relates to other neurodegenerative diseases, such as ALS, and while no concrete link or connection has been made, it certainly bears further research.