I think there are two possible explanations for the disparity. 1) Genetics. 2) A leaky blood-brain barrier (which could also be influenced by genetics.) Multi

I think there are two possible explanations for the disparity. 1) Genetics. 2) A leaky blood-brain barrier (which could also be influenced by genetics.) Multiple studies have shown a link between leaky a blood-brain barrier and the same neurodegenerative diseases that increased levels of glutamate are linked to. Things as simple as high blood sugar can contribute to a leaky blood-brain barrier, so it could be more common than we imagine. Tests are difficult to administer, so it's hard to estimate how common degrees of blood-brain barrier leakage are in populations.