Any come to mind, given that blood transports nutrients all the time. And that might explain why these "vampires" feel so gratified upon ingestion, they seem to suffer some slight chronic deficiency. Now IIRC we don't really know "all micronutrients in existence" yet so it might not even show up on a blood panel. Wonder whether any of them ever tried pig or snake blood on a trip to China/Taiwan/Hongkong (or cattle blood while visiting a Maasai tribe) and felt similarly "energized"..