Here’s a very thoughtful paper that I hadn’t stumbled on before with updated recommendations for Vitamin D with regards to cognition from an international expert panel:
Unfortunately upper threshold levels aren’t addressed in the paper, but all of the experts agreed that hypovitamintosis D (defined as <30 ng/dL or 75 nmol/L) increases the risk for cognitive decline. All (with the exception of one) agreed that supplementing with Vitamin D should be a part of addressing cognitive decline. Unfortunately, preventing cognitive decline in high risk folks, like us, wasn’t addressed... but one can safely presume that staying above the hypovitamintosis D threshold would be a good idea . FWIW, this concurs nicely with Dr. Bredesen's clinical observations.
Martha & I got to hear Dr. Annweiller present in NYC last spring. It was hard to get beyond his darling French accent. Nice to actually read the paper
I just stumbled upon an excellent (full-text) review paper re. vitamin D and cognition that nicely summarizes what is currently known and unknown about this correlation. The authors points towards specifics that are still badly in need of elucidation. One of the authors, Cedric Anweiller, presented in NYC on this topic. Because of his very strong accent, Martha and I didn't understand a word of his presentation, but we still enjoyed it immensely . Something about that French accent...
. FWIW, this concurs nicely with Dr. Bredesen's clinical observations. 
. Something about that French accent...