Very well explained - up to your usual high standard!

That said, I wonder how the BBB of people with an APOe4 allele might behave.

For those unaware, people with that allele typically have a BBB that doesn't pass lipids well. The CNS needs lipids, so, long term (decades), this can lead to dementia. However, DHA (the lipid the CNS needs) *is* passed in phospholipid form (different pathway ). Fish roe (e.g. caviar) is a particularly good source of this kind of DHA.

So, at least for some of us, caviar is medicinal - but needing it might turn out to be an advantage here...