It’s not just the protein. Vitamin B12 is essential for proper brain function; the first thing my doc did when I complained about insomnia and anxiety was check my B12 level. The best sources of B12 are meat and dairy- a vegan diet without supplements will lead to deficiency.
You're right. The book I was talking about went into some detail about the other vitamins and minerals that were absent in the islander's diet - but I'm not a doctor or scientist, so much of it left my mind almost immediately after reading it.