I'm making a vampire human subspecies that is hundreds of times stronger than humans and also is immortal, but I can't seem to find a suitable energy source.
First, vampires are humans infected by a virus which alters their biological makeup, so nuclear fusion is not viable, well it is an option, but it would make the poor guy explode.
So, the vampire would have some of its cells would be upgraded from human cells to produce more energy, allowing it to be smarter, faster and stronger and to be cancer and senescence-free and these cells wont agr. Secondly, the more blood they drink, the more cells get infected, because the vampire needs a certain protein from human blood to make more viruses to infect and alter more cells.
These cells would die very quickly, around every few weeks because their metabolism would produce so much energy, their cell membranes might tear and mitochondria die. They would be replaced by normal human cells and wont be able to make more energy. They would need more of the viruses, which would be made from that special human protein they cannot make.
So, what would power them?