Well VTR is a roleplaying game. It's similar to Vampire the Masquerade, but different setting and somewhat different mechanics. I guess it's best explained as "nutrients." Animal blood and blood from e.g. blood bags gives less Vitae (magic blood points resource) than blood harvested from living humans. And as the character becomes more powerful, eventually that "lesser" blood can't actually give them Vitae.

The vampiric curse in VTR is explicitly stated to be supernatural, though, so there's not a necessary scientific explanation for it. The curse imparts the Beast, which is the predator in all vampires.